Ana Maria Rey

JILA Collaboration Makes Cavity Quantum Electrodynamics into a Team Sport

Steven Burrows — Thu, 27 Nov 2025 19:01:01 +0000

JILA Collaboration Makes Cavity Quantum Electrodynamics into a Team Sport Steven Burrows Thu, 11/27/2025 - 12:01

Bailey Bedford / Freelance Science Communicator

Researchers used laser light to trigger a rapid sequence where atoms absorb and emit photons, shifting between energy states. Each emitted photon, now a different color, bounces through the cavity and drives the next atom’s transition, enabling a rare three-body interaction. Image Credit: Steven Burrows / JILA

Reality is the result of countless interactions. Everything in daily life, from a grain of dust floating in the air to a neuron firing in a brain, is the result of myriads of atoms and other quantum particles interacting.

Often, we get by with ignoring interactions and seeing just the big picture result. However, physicists learn a lot by digging down to the foundation and studying how the interactions between particles play out. In the past, researchers have mostly simplified things by focusing on interactions between two objects at a time—two-body interactions. However, reality isn’t always so simple. Sometimes three or more particles interact in fundamentally different ways than groups of interacting pairs would.

For the past several years, an experimental research group led by JILA Fellow James Thompson and a theoretical research group led by JILA Fellow Ana Maria Rey have been working together to study quantum interactions using cavity quantum electrodynamics (cavity QED)—the science of how light contained in reflective cavities interacts with quantum particles, like individual atoms. Recently, they tackled many-body interactions with a new experiment, described in an article published in the journal Science. In the experiment, they successfully created interactions that require the participation of either three or four atoms to achieve the observed results.

“Nature’s forces act between pairs, but when many particles come together, new interactions can emerge,” says Rey, who is also a National Institute of Standards and Technology (NIST) Fellow and a University of Colorado Boulder Physics professor. “Controlling these multi-body interactions opens the door to faster, richer and more powerful quantum matter.”

The new experiment took their research from looking at situations where all interactions are essentially the result of atoms playing two-player sports to a more complex world where atoms participate in team sports. Instead of two tennis players hitting a ball back and forth, the experiment introduces a baseball team where the ball gets thrown between several players. The change expands their ability to form quantum connections between the players.

“This is a whole new path to generate quantumy-stuff called entanglement that will improve quantum sensors for navigation, atomic clocks and maybe even detect exotic things like dark matter or gravitational waves,” says Thompson, who is also a NIST physicist and University of Colorado Boulder Physics professor.

The experiment used rubidium atoms as the players, and their games—interactions—were carried out by tossing around light. The researchers used cavities as the playing field and supplied around a thousand atoms to form small teams. The researchers controlled the colors of light they sent into the cavity and how different colors of light behaved in the cavity, which helped them set the rules of the game.

The researchers focused on the quantum states of the atoms defined by the movement of the atoms through the chamber. Thanks to conservation laws, the atoms couldn’t just change their speeds and run around the experiment in any random way; to change states, they needed to receive or release exactly the right amount of energy and momentum. The researchers set up the experiment so the only way the atoms could change states was by catching or releasing photons—individual particles of light that carry specific amounts of energy and momentum.

Since the atoms were in a frictionless vacuum, they didn’t stay in place like a pitcher on the mound when they caught or threw a photon. Instead, it was like the atoms were a baseball team forced to play on ice or were astronauts playing the game of catch while floating in the middle of a spacewalk: Every catch and throw gave them a shove.

The quantum nature of the atoms meant they were only stable in certain specific states, and each atom could only catch a photon if there was an appropriate state for it to move into afterward. This allowed the researchers to carefully design interactions by choosing what colors of light were in the cavity.

They focused on moving atoms between two stable states, and they made sure the cavity didn’t contain light that could simply knock an atom between the states with a single caught photon (the resulting interactions would be boring). Instead, they created a playing field where atoms had to coordinate a specific play—string of interactions—to move between stable states. They ensured that each game started and ended with photons whose energy differed by exactly enough to move three atoms between states.

To start the play, the researchers flooded the cavity with light that could push the atoms to an energetic state that they couldn’t stay in for long. Each time an atom caught a photon, it immediately threw out a photon to return to a lower energy state. Sometimes it threw out a photon just like the one it caught and returned to its original state. Other times, it instead tossed out a weaker photon and kept a little bit of its new energy and momentum. The only allowed option was keeping exactly enough to settle into the second stable state.

This released photon was a new color and was free to bounce around the cavity and quickly be caught by another atom. Similar to the first step, catching the light temporarily shoved the second atom to an unstable state before it, in turn, tossed off another photon. Again, the second atom sometimes kept enough energy and momentum to join the first atom in the new state. The process continued with a third atom joining the first two by catching the new photon and throwing out another weakened photon.

To ensure the chain of events, the researchers set up their cavity to encourage the presence of the initial light and the final photons released in this game of catch while being inhospitable to other undesired colors of light. The dynamics of the light in the cavity and the rubidium atoms’ available quantum states meant the whole play had to happen quickly or not at all.

“We build very strict rules in our system that all three processes have to happen at the same time in order for momentum and energy to be conserved,” says Chengyi Luo, the co-lead author of the paper.

The researchers confirmed the atoms moved between states following the prescribed three-body interactions, and they went a step further. They illustrated the adaptability of the approach by increasing the amount of energy and momentum available to fuel four-body interactions, adding another player to each game of catch. Their observations showed the atoms teaming up into a smoothly running machine and moving in groups of four to the new state.

These demonstrations are just the first steps in exploring many-body interactions with this approach.

“There are a lot of things people need to figure out about how we're going to explore these multibody interactions to make them useful,” Rey, says. “We just saw them, but there are a lot of new behaviors and capabilities to be explored. For example, we think they can be used to emulate exotic superconductors where four electrons team up instead of two electrons like in normal superconductor, producing a new kind of supercurrent that may contribute to high temperature superconductors.”

In the future, experiments should be able to use different quantum states, induce interactions between even larger numbers of particles and make the interactions do practical work. The ability to involve more particles in each interaction provides a new set of tools for researchers. As the technique is explored and refined, it has potential applications in a variety of areas including quantum simulation, quantum computing and quantum sensing.

“I think it's interesting that there's this new way to change the quality of the communication that can happen between all these atoms,” Thompson says. “You really fundamentally change what that communication looks like. It's just an open physics question, like, ‘Well, how good can it be?’ and going further ‘Can we build new quantum states to simulate and explore the universe around us?’”

For the past several years, an experimental research group led by JILA Fellow James Thompson and a theoretical research group led by JILA Fellow Ana Maria Rey have been working together to study quantum interactions using cavity quantum electrodynamics (cavity QED)—the science of how light contained in reflective cavities interacts with quantum particles, like individual atoms. Recently, they tackled many-body interactions with a new experiment, described in an article published in the journal Science. In the experiment, they successfully created interactions that require the participation of either three or four atoms to achieve the observed results.

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JILA Joins DOE’s Quantum Systems Accelerator for Next Phase of Quantum Innovation

Steven Burrows — Tue, 04 Nov 2025 19:18:36 +0000

JILA Joins DOE’s Quantum Systems Accelerator for Next Phase of Quantum Innovation Steven Burrows Tue, 11/04/2025 - 12:18

Steven Burrows / JILA Science Communications Manager

A round glass cell (centre, in black frame) is designed to hold a gas of molecules cooled to 50 billionths of a Kelvin. Credit: Ye Group/Steven Burrows/JILA

The U.S. Department of Energy (DOE) has announced a $625 million investment to advance the next phase of the National Quantum Information Science Research Centers, a cornerstone of the National Quantum Initiative. This funding will support five centers dedicated to accelerating quantum technologies that promise transformative impacts on science, industry, and national security.

Among these centers, the Quantum Systems Accelerator (QSA)—led by Lawrence Berkeley National Laboratory—will continue its mission to develop practical quantum systems that can solve real-world problems. QSA brings together leading institutions to tackle challenges in quantum computing, sensing, and networking, aiming to bridge the gap between theoretical advances and deployable technologies.

JILA is proud to remain a key partner in QSA through the Q-SEnSE Institute, which focuses on quantum sensing and precision measurement. These capabilities are essential for applications ranging from navigation and timing to probing fundamental physics. JILA Fellow Jun Ye will lead the JILA effort, supported by senior investigators and JILA Fellows Cindy Regal, Adam Kaufman, Ana Maria Rey, James Thompson, Murray Holland, and Xun Gao—a team internationally recognized for pioneering work in quantum optics, atomic physics, and many-body systems.

“JILA is proud to remain a key partner in QSA. Through our work in both QSA and Q-SEnSE, JILA plays a leading role in advancing quantum innovations at the national and international levels,” remarked Inese Berzina-Pitcher, Executive Director for Q-SEnSE.

The next five years of QSA will focus on building scalable quantum platforms, advancing error correction, and integrating quantum devices into scientific workflows. JILA’s expertise in ultracold atoms, optical lattices, and quantum simulation will play a critical role in these goals.

For more details, read the official announcements:

Energy Department Announces $625 Million to Advance the Next Phase of National Quantum Information Science Research Centers

The Quantum Systems Accelerator Embarks on Next Five Years of Pioneering Quantum Technologies for Science

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Smoother Ticking Through Topology

Steven Burrows — Wed, 06 Aug 2025 17:06:13 +0000

Smoother Ticking Through Topology Steven Burrows Wed, 08/06/2025 - 11:06

Steven Burrows / JILA Science Communications Manager

Imagine walking a tightrope in a windstorm. Every gust threatens to knock you off balance. Now imagine that the rope itself is designed in such a way that it naturally resists the wind, keeping you steady even when conditions are less than ideal. That’s the kind of protection topological physics offers to quantum systems.

In the world of atomic clocks, the “tightrope” is the delicate quantum state of atoms trapped in a lattice of laser light. These states are exquisitely sensitive to time and frequency, which is what makes optical lattice clocks so precise. But they’re also vulnerable to noise—tiny fluctuations in laser intensity, temperature, or magnetic fields can nudge the atoms off their ideal path, degrading the clock’s performance.

The new study, led by JILA postdoctoral researchers Tianrui Xu and Anjun Chu of Ana Maria Rey’s group, together with postdoc Kyungtae Kim of Jun Ye’s group, and in collaboration with James Thompson and JILA visiting Fellow Tillman Esslinger, proposes a way to stabilize these quantum states using the principles of topological phases of matter, a branch of physics that deals with properties that remain unchanged under continuous deformations. In these systems, the quantum states are 'knotted' in its quasimomentum space. Such 'knotting', quantified by the 'topological invariants' of the wavefunctions, remains constant even when the system is perturbed in ways that it does not change the global symmetry of the system.

Rey explains, “while discovering topological phases has been a big deal in condensed matter and atomic physics, what could be even more exciting is figuring out how these systems can help us build better sensors. For example, the quantum Hall effect, a topological state, has already been super useful—it has helped us measure the Hall resistance with incredible precision, which in turn lets us pin down constants like the fine-structure constant and even the elementary electron charge with great accuracy. Right now, atomic clocks are limited by laser noise. Even when we do clever tricks like comparing two clocks at once, we are still not hitting the theoretical limit of how precise they should be. So, the question is, can we use topology to improve state-of-the art clocks? This was the main question we tried to answer, and we found out that indeed it is possible.”

Artistic rendering of topological protection of an optical lattice clock.

Image Credit: Steven Burrows / JILA.

From Quantum Simulation to Quantum Sensing

Over the past decade, JILA has been at the forefront of both quantum simulation and quantum sensing. In one line of research, scientists use ultracold atoms to simulate exotic phases of matter, such as those found in high-temperature superconductors or topological insulators. In another, they build some of the world’s most accurate clocks and sensors, capable of detecting tiny changes in gravity or time dilation across millimeters.

Until now, these two fields have largely developed in parallel. But the new study brings them together, showing that the same topological phases explored in quantum simulators can also enhance the performance of quantum sensors.

At the heart of the proposal is the Su-Schrieffer-Heeger (SSH) model, a simple yet powerful model that provides the essential intuitions of topological phases and phase transitions. Originally developed to describe electrons in polyacetylene, a type of polymer, the SSH model describes a chain of sites with alternating strong and weak connections, leading to topological edge states and the corresponding topological bulk properties, both protected by symmetry.

JILA researchers propose implementing this model in a one-dimensional optical lattice clock (OLC) tilted by gravity or by an applied force. In this setup, known as Wannier-Stark OLC, atomic tunneling between lattice sites can be controlled by laser drives, as demonstrated by the collaboration of the Rey group and Jun Ye’s lab in recent years. In this study, the Rey group proposes to use two laser tones to create a hybrid synthetic lattice combining atomic internal states and the position of the atoms, leading to a natural realization of the SSH model.

One of the key innovations in the study is a new spectroscopic protocol that leverages the topological properties of the SSH model. In conventional Rabi spectroscopy, atoms are driven between two states using a single laser tone, and the resulting oscillations are used to measure the transition frequency between the two states. However, this method is sensitive to noise in the laser amplitude, which can distort the signal.

In the SSH-based protocol, the resulting dynamics of the atomic wavefunctions under two laser tones depends on the 1D topological invariant of the SSH model known as the winding number—a quantity that is robust against many types of noise.

JILA researchers have shown that the winding number can be measured by tracking the displacement of the atomic wavefunction over time. This protocol can thus in-turn be used as a spectroscopic probe for atomic transition frequencies. Numerical simulations show that this “SSH spectroscopy” is less sensitive to both global and local amplitude noise than the traditional Rabi spectroscopy. In particular, the statistical noise scales more favorably with the number of atoms, suggesting that the protocol could be especially useful in future clocks that interrogate millions of atoms simultaneously.

Measuring Gravity with Quantum Pumps

The study also explores how topological physics can enhance matter-wave interferometry, a technique used to measure forces like gravity by splitting and recombining atomic wave packets. In a typical matter-wave interferometer, atoms are pushed apart using a sequence of laser pulses, allowed to evolve in different gravitational potentials, and then brought back together to measure the accumulated phase difference.

However, imperfections in the laser pulses can introduce errors, limiting the sensitivity of the device. To address this, the Rey group proposes using a technique known as Thouless pumping—an adiabatic process in which particles are transported across a lattice by slowly varying the system’s parameters.

In their proposed “topological pumping protocol,” atoms are adiabatically moved apart and then recombined using a sequence of laser-driven transitions that trace out a topologically-nontrivial path in parameter space. Here, a topological non-trivial path means a trajectory that is robust to changes in system parameters. This method is inherently robust to many types of experimentally-relevant noise and can achieve larger separations with lower uncertainty than conventional pulse sequences.

Simulations show that the topological protocol outperforms traditional methods in terms of both signal strength and noise resilience, especially when the number of atoms is large. This could pave the way for new types of interferometers capable of measuring gravitational gradients or testing fundamental physics with reduced sensitivity to experimental noise.

Topologically Enhanced Clocks

The proposed protocols are designed to be compatible with existing optical lattice clock platforms, such as those developed in Jun Ye’s lab at JILA. These clocks already operate with extraordinary precision—recent experiments have achieved fractional uncertainties below 10⁻¹⁸—but further improvements are needed to reach the standard quantum limit (SQL), the ultimate sensitivity allowed by quantum mechanics for uncorrelated atoms.

By reducing classical noise, the topological approach offers a practical path toward SQL-limited performance. It also opens the door to new applications, such as measuring gravitational redshifts over even shorter distances or detecting tiny variations in fundamental constants.

Moreover, the study suggests that the same principles could be extended to more complex systems, including higher-dimensional lattices or atoms with multiple internal states. This could enable the exploration of richer topological phases and their potential benefits for quantum sensing and metrology.

In the quantum world, precision often comes at the cost of fragility. But topology offers a way to have both precision and protection. By embedding topological structures into the architecture of optical lattice clocks, the Rey group has outlined a strategy for making these devices more resilient to noise, more sensitive to signals, and more versatile in their applications.

As JILA researchers continue to refine these protocols and bring them into the lab, we may soon see a new generation of quantum sensors that are not only more accurate, but also more robust, thanks to the hidden geometry of quantum states.

This research is supported by the U.S. Air Force Office of Scientific Research, the U.S. Department of Energy Office of Science, National Quantum Information Science Research Centers, Quantum Systems Accelerator, NIST, NSF, Swiss National Science Foundation, Heising-Simons Foundation, Simons Foundation, and Sloan Foundation.

In a new theoretical study, physicists at JILA and the University of Colorado Boulder have proposed a way to make the most precise clocks in the world even more robust—by weaving in the strange, protective properties of topological physics. Their work, published in PRX Quantum, explores how a class of quantum states known as symmetry-protected topological (SPT) phases could be used to improve the performance of optical lattice clocks, a cornerstone of modern precision measurement.

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The Pursuit of Perfect Timekeeping — Understanding Superexchange Interactions in Atomic Clocks

Steven Burrows — Mon, 19 May 2025 17:58:56 +0000

The Pursuit of Perfect Timekeeping — Understanding Superexchange Interactions in Atomic Clocks Steven Burrows Mon, 05/19/2025 - 11:58

Steven Burrows / JILA Science Communications Manager

Tunable Superexchange interactions in a 3D optical clock. Image credit: Steven Burrows / JILA

The challenge of creating the world’s most precise clock is that that even the slightest deviations limit the precision. Atomic clocks, which rely on the coherent evolution of atomic states, are the most accurate timekeeping devices known to humanity. However, achieving this level of precision requires a deep understanding of the interactions between atoms, especially as many atoms are packed in a dense ensemble to increase the signal strength.

In a recent study published in Science, by JILA and NIST Fellows and University of Colorado Boulder physics professors Jun Ye and Ana Maria Rey, interactions between atoms are explored in depth, focusing on superexchange processes that occur in a three-dimensional optical lattice.

The Role of Superexchange Interactions

Superexchange interactions are second-order tunneling processes between nearest neighbor atomic spins. These interactions are central to understanding magnetic phenomena such as antiferromagnetism and superconductivity. In the context of atomic clocks, superexchange interactions can influence the coherence time and precision of the clock.

To grasp the concept of superexchange, consider a relay race where the baton is passed through several intermediaries before reaching the final runner. Similarly, in superexchange interactions, atoms exchange spins through virtual tunneling processes, leading to coherent spin dynamics. This interaction is ordinarily quite weak, and would have been ignored had the researchers not been pursuing the best possible clock precision.

In the study, researchers used a degenerate Fermi gas of nuclear spin-polarized ⁸⁷Sr atoms loaded into a three-dimensional optical lattice. By tuning the lattice confinement and applying imaging spectroscopy, they mapped out favorable atomic coherence regimes. The clock laser prepared each atom in a coherent superposition of the two electronic states, which can be considered as a pseudo-half spin. The propagation effect of the clock laser introduced a spin-orbit coupling phase, transforming the Heisenberg spin model into one with XXZ-type spin anisotropy.

William Milner, first author on the paper, explains, "You want to use as many atoms as possible and get the best precision. As you pack them into this 3D lattice, they can start to interact. These atoms can talk to each other, so you can no longer think of them as isolated atoms."

The experimental setup involved a highly filled Sr 3D lattice, where atoms were confined in the ground band of the lattice. The researchers employed Ramsey spectroscopy to measure atomic coherence and observe superexchange interactions. This technique allowed them to directly probe the coherent nature of superexchange interactions over timescales of multiple seconds.

Balancing Interactions

One of the key findings was the identification of regimes where atomic coherence is maximized. By varying the lattice confinement, researchers observed how both s- and p-wave interactions contribute to decoherence and atom loss. These interactions can be balanced to achieve optimal coherence times, which are crucial for the precision of optical lattice clocks. Imagine balancing a seesaw with two children of different weights. To achieve equilibrium, you need to adjust their positions carefully. Similarly, in the optical lattice, researchers balanced s- and p-wave interactions to minimize decoherence.

However, at deep transverse confinement, coherent superexchange interactions were directly observed, tunable via on-site interaction and site-to-site energy shift. Milner elaborates, "In this regime is where you get these superexchange interactions. These higher order interactions occur because the atoms can't move around, but they can virtually jump onto a site and then jump back, with spin exchanged."

The study provided direct observations of superexchange dynamics, which were manifested in oscillations of the Ramsey fringe contrast persisting over a timescale of several seconds. These observations were well captured by an anisotropic lattice spin model, breaking the Heisenberg SU(2) symmetry due to the spin-orbital coupling phase. Additionally, the experiments showed the direct tunability of the interactions via lattice strength and potential gradients.

Enhancing Clock Performance

Optical lattice clocks are advancing the fields of fundamental physics, metrology, and quantum simulation. By controlling superexchange interactions, researchers can enhance the performance of these clocks, leading to more precise timekeeping and new insights into quantum magnetism and spin entanglement.

Just as a finely tuned orchestra produces a flawless performance, a well-controlled optical lattice clock can achieve unprecedented precision. The experiment demonstrated that by tuning the lattice confinement and controlling superexchange interactions, researchers can optimize the coherence time of the clock. This has the potential to further advance timekeeping and enable new applications in quantum technologies.

Milner notes, "By changing the confinement, you can make it so these superexchange interactions are very small and pretty much negligible. On the other hand, there's promise that you can use these interactions to create entangled states, which should give you even better precision."

Stefan Lannig, a postdoc in the Ye group, adds, "We want to trap the atoms in the 3D lattice to get the highest atom number for the best precision, but in a sample as small as possible. This helps us get rid of background effects and achieve optimal performance."

Looking ahead, the research opens new avenues for exploring quantum magnetism and spin entanglement using optical lattice clocks. By leveraging the coherent nature of superexchange interactions, scientists can probe deeper into the quantum dynamics of many-body systems. This could lead to breakthroughs in understanding fundamental physics and developing advanced quantum technologies.

This study by the Ye group represents a significant step forward in the field of atomic clocks and quantum metrology. By unraveling the complexities of superexchange interactions, researchers have laid the groundwork for enhancing the precision and performance of optical lattice clocks. Researchers at JILA are orchestrating the interactions of atoms to unlock the secrets of time itself, pushing the boundaries of what is possible in the quest for perfect timekeeping.

This research is supported by U.S. Department of Energy Center of Quantum System Accelerator, National Science Foundation QLCI, JILA Physics Frontier Center, V. Bush Fellowship, and NIST.

In a recent study published in Science, by JILA and NIST Fellows and University of Colorado Boulder physics professors Jun Ye and Ana Maria Rey, interactions between atoms are explored in depth, focusing on superexchange processes that occur in a three-dimensional optical lattice.

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Where Motion Meets Spin: A Quantum Leap in Simulating Magnetism

Steven Burrows — Thu, 24 Apr 2025 18:11:43 +0000

Where Motion Meets Spin: A Quantum Leap in Simulating Magnetism Steven Burrows Thu, 04/24/2025 - 12:11

Kenna Hughes-Castleberry / JILA Science Communicator

The strange behaviors of high-temperature superconductors—materials that conduct electricity without resistance above the boiling point of liquid nitrogen—and other systems with unusual magnetic properties have fascinated scientists for decades. While researchers have developed mathematical models for these systems, much of the underlying quantum dynamics and phases remain a mystery because of the immense computational difficulty of solving these models.

In a new study published in Science, researchers from JILA, led by JILA and NIST Fellows and University of Colorado Boulder physics professors Jun Ye and Ana Maria Rey and JILA and ��Ҵ�ý�ƽ�� physics professor John Bohn, used ultracold molecules to realize these models with an unprecedented level of control. Their work bridges the fields of atomic, molecular, and optical (AMO) physics with condensed matter physics, opening new doors for quantum simulations and advances in quantum technologies.

“It is very exciting that experiments with polar molecules are now reaching the point where these models can be implemented in the lab,” Rey says. “While currently, we are exploring dynamics at low filling fractions where theory effort can still have some predicting capabilities, very soon experiments will reach dense regimes intractable by theory, fulfilling the dream of quantum simulation.”

A Decade in the Making

JILA has long been celebrated as a hub where experimentalists and theorists collaborate to tackle some of the most challenging questions in physics. Indeed, over two decades of collaboration among JILA researchers, first with the experimentalists Ye and the late JILA Fellow Deborah Jin, later joined by theory colleagues Rey and Bohn, pioneered ultracold molecule research and laid the foundation for this work.

In this study, the researchers from Ye’s experimental group collaborated with theorists in Rey’s and Bohn’s groups to understand the data from several new experiments exploring different regimes of molecular motion and dipolar interactions.

“We wanted to understand how motion and magnetism are coupled in quantum systems,” says Annette Carroll, a JILA graduate student in the Ye laboratory and the paper’s first author. “The molecules offer a unique platform to study this interplay, thanks to their long-range dipolar interactions.”

Molecules sparsely occupy a deep 3D optical lattice. Molecules interact with induced dipole moments and transition dipole moments represented by squiggly lines between lattice sites. Lowering the lattice depth in the horizontal direction allows tunneling between sites within layers. Image Credit: Steven Burrows / JILA

These dipolar interactions were key to the experiment’s success. While neutral atoms have been widely used in quantum studies due to their ease of cooling and control, their typical short-range interactions often limit their ability to simulate magnetism. Ultracold molecules, with their natural long-range dipolar interactions, offer a richer platform for exploring exotic quantum phases but are more complicated to control.

Focusing on Framework

In the experiment, an array of ultracold potassium-rubidium molecules were used to emulate the behavior of electrons in a solid state crystal. Electrons tunnel between nearby ion cores in a crystal at a rate “t”.

To imitate the fact that electrons are like tiny magnets, which can point in two directions, spin up or spin down, molecules were prepared in two accessible internal (rotational) states. Electrons are charged particles and see each other at a distance, but due to the ion cores and other electrons in the system, they strongly screen each other, and effectively, one electron only sees another electron when they are at the same lattice site. In this setup, two nearby molecules (simulating electrons), one with spin up and one with spin down, can flip their spins but to do that, for example, the spin up electron needs to hop into the site where the down electron is, interact just for a glimpse to reduce the large energy cost to be at the same site, and then hop back to its original site now as a spin down.

This process is called superexchange and happens at a rate “J.” The behavior of electrons hopping and exchanging their spins is called “t-J” model and it is believed to have all the necessary ingredients to explain the emergence of high temperature superconductivity. But, this is not yet well understood.

“Polar molecules have the advantage that they carry a dipole moment, and this means that two molecules can exchange the spins far from the distance without needing to move where the other is. This has great consequences,” elaborates Rey. “It allows us to simulate the ‘t-J’ model in a broader parameter regime since the exchange rate J can be controlled in the lab. It opens exciting opportunities for the exploration of magnetism and superconductivity in new regimes.”

“The t-J model captures the interplay between motion and spin interactions,” adds Sean Muleady, a former JILA graduate student in Rey’s theory group now at the Joint Center for Quantum Information and Computer Science (QuICS) and the Joint Quantum Institute (JQI), who was also involved in this study. “These dynamics are critical to understanding phenomena like magnetism in strongly-correlated systems and, in certain regimes, even superconductivity. But studying these effects in real materials is notoriously difficult.”

To overcome these challenges, Rey, Muleady, and postdoctoral researcher David Wellnitz worked with Bohn and his graduate student Reuben Wang to develop mathematical tools to simulate the spin dynamics of moving dipolar particles within different lattice arrangements set up by the researchers within Ye’s experimental group.

“Using dipolar interactions adds an entirely new dimension,” says Bohn. “This is a more generalized version of the t-J model, incorporating features that condensed matter physicists could only theorize about.”

Combining Theory and Experiment

For the researchers in Ye’s laboratory, the team focused on ultracold potassium-rubidium molecules trapped in an optical lattice—a grid of laser light designed to confine the molecules to specific locations. This lattice structure served as a simulated crystal, mimicking the confinement of electrons in real materials. By applying electric fields, the researchers precisely controlled the strength and nature of the molecules’ dipolar interactions and, by tuning the strength of the optical lattice, tuned their ability to move within the lattice.

The experimentalists studied the dynamics between two distinct motional extremes: one where the molecules were “frozen” in place and another where they could move freely within two-dimensional planes without any transverse lattice confinement. By tuning the transverse lattice depth between these two extremes, the researchers explored a large range of behaviors governed by the t-J model, from interactions between frozen spins to dynamic coupling between spin and motion. In all setups, the researchers prepared the molecules in a superposition of rotational states, simulating magnetic spins all pointing in the same direction, and measured how quickly the spins lost their initial magnetization because of their interactions.

Interpreting these behaviors, however, required an equally flexible theoretical approach. Two theoretical groups, led by JILA Fellows Ana Maria Rey and John Bohn, collaborated to combine their unique expertise. Rey’s group specialized in lattice-based models, while Bohn’s group brought insights into molecular collisions and scattering processes.

“These were two very different schools of thought,” says Muleady. “Bringing them together was critical because the experiment operates in a middle ground that neither approach alone could fully describe.”

The collaboration resulted in novel theoretical frameworks that bridged the gap between frozen and dynamic motional regimes, enabling a comprehensive interpretation of the experimental data.

Connecting Magnetism and Motion

Through their collaboration, the team made several significant discoveries, including that the spins stayed aligned much longer at a particular electric field when the interaction between the spins is independent of their orientation. Observing coherence in this context is crucial because the spins maintain their alignment over time, which is rare. Long coherence times are important for preserving quantum entanglement, a behavior where particles’ quantum states are interdependent.

“At this special point, the spins of the molecules align perfectly, leading to slower decay of quantum coherence than at any other point,” explains Cal Miller, a JILA graduate student in the Ye group. “This is something that had been theorized but never observed in an experiment until now.”

This finding confirmed theoretical predictions about the behavior of spin systems and demonstrated the precise tunability of interactions between molecules.

However, the experimentalists observed other dynamics that required new theoretical modeling. The researchers systematically explored how the coherence between the spins depends on molecular motion, developing for the first time a model of how collisions between molecules allowed to move freely within 2D layers lead to the decoherence of the spins.

“At first, we couldn’t explain why the decoherence behaved this way,” explains Junyu Lin, a postdoctoral researcher in Ye’s group. “It took many discussions. Finally, when we saw the model from Reuben and John, and it matched our data, we thought: ‘Oh, that’s the mechanism.”

Moreover, when the molecules were allowed to move freely, the researchers observed a striking new phenomenon in the spin alignment.

“We saw a fascinating ‘stretched exponential’ behavior in the decay of spin alignment,” says Wang. “It’s a result of the molecules’ motion and their spin alignment—a combination that’s difficult to describe using traditional methods.”

The key understanding from the work is how motion, which can be regulated by optical lattices, affects the magnetization dynamics of strongly interacting dipoles. The researchers observed more complex spin orientation dynamics by allowing the molecules to move. The coupling between spin and motion modifies the rate at which interacting spins evolve.

Pushing New Frontiers in Experiment and Theory

Understanding these experimental discoveries would not have been possible without the team's new advances in theoretical modeling.

“This project pushed our tools to the limit,” explains Wellnitz. “We had to develop new methods to bridge the gap between systems where molecules are frozen and those where they’re moving freely.”

The collaboration also highlighted the challenges and rewards of interdisciplinary research within theoretical physics.

“One of the most exciting parts of this work was finding a shared language between the different theoretical approaches,” says Muleady. “Each group brought something unique to the table, and the experiment provided a real-world test for our models.”

For the experimentalists, these results may bring new interest to the t-J model from multiple different subfields of physics.

“While the condensed matter community is already interested in this model, I think the AMO community will also be more interested in our work because we’re approaching things differently,” adds Lin.

While the results of this study have uncovered vital information about the rich dynamics of long-range interacting spin systems, the researchers are already looking toward the project’s next steps.

For the experimentalists, future work will focus on achieving even colder temperatures and higher densities of molecules.

“We’re working toward regimes where the molecules’ interactions are strong enough to create new quantum phases,” says Carroll. “These are the conditions where we might observe rich phenomena like superfluidity.”

For others, the results of this project suggest major implications for the future development of quantum devices.

“By advancing our understanding of spin-motion coupling, this work could inform the design of new quantum technologies,” notes Wellnitz. “It’s an exciting time to be in this field.”

This research is supported by the National Science Foundation, the US Department of Energy's Office of Science, National Quantum Information Science Research Centers, the Quantum Systems Accelerator, the Air Force Office and Office of Science and Research, and the JILA Physics Frontier Center.

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Quantum Teleportation Gets an Ionic 2D Upgrade

Steven Burrows — Fri, 11 Apr 2025 18:21:54 +0000

Quantum Teleportation Gets an Ionic 2D Upgrade Steven Burrows Fri, 04/11/2025 - 12:21

Kenna Hughes-Castleberry / JILA Science Communicator

Teleporting quantum information stored in collective spin states of ions within a two-dimensional crystal. Image credit: Steven Burrows / JILA

Quantum entanglement is one of the most well-studied phenomena in quantum physics. Einstein called it “spooky action at a distance,” as it enables particles to be deeply connected—such that measuring one instantly reveals information about the other, regardless of the distance between them. For decades, quantum entanglement has been used to design protocols for studying other physical processes, including quantum teleportation, which allows the transfer of quantum states without physically moving particles.

While quantum teleportation has been experimentally demonstrated in various settings, including individual photons and ions, extending this protocol to many-body systems—composed of many interacting particles—has remained a significant theoretical challenge. In contrast to isolated particles, many-body systems feature complex interdependencies where quantum information is shared across the entire ensemble. These collective behaviors give rise to rich dynamics and entanglement structures that are essential for quantum technologies but also introduce a level of complexity that makes teleportation more difficult to design and implement.

Now, in a recent study published in Physical Review Research, researchers at JILA—led by JILA and NIST Fellow and University of Colorado Boulder physics professor Ana Maria Rey and her team, along with Klaus Molmer from the Neils Bohr Institute and John Bollinger from NIST—have developed a new protocol for teleporting quantum information stored in collective spin states of ions within a two-dimensional crystal. This approach bridges concepts from atomic physics, quantum optics, and quantum information science, opening new avenues for building modular, scalable systems for quantum information processing.

“Fundamentally, we generate Einstein-Podolsky-Rosen correlations—entanglement—between collective spin systems, using tools experimentally accessible in trapped ions,” explains theorist Muhammad Miskeen Khan, who recently completed his postdoc from JILA and is now a postdoctoral researcher at Saint Louis University. “Then came up with a protocol that used that entanglement as a resource to teleport many-body collective spin states between energetically distinct ensembles.”

“Our proposed teleportation protocol leverages both phonon-mediated collective spin-spin interactions among three energetically separated spin-ensembles in a two-dimensional trapped ions crystal together with measurements and local operations on these ensembles,” Rey says. “We have taken advantage of different nuclear spin levels accessible in the system. Although in the current proposal the suggested protocol depends on spin ensembles that are energetically separated but spatially overlapping, future advancements could be gained from implementing our proposed protocols in spatially separated ensembles, for example, using 3D ion crystals.”

From Individual Ions to Collective Spin Ensembles

In quantum experiments, spin refers to the internal angular momentum of particles such as electrons or atomic nuclei. Like each goose in a flock facing one direction so that the whole flock is facing the same direction, in a system of many atoms, their spins can be treated as a collective spin—a combined property that captures the net behavior of the ensemble. Such collective spin states are helpful in precision measurements and quantum computing, especially when they exhibit quantum correlations such as squeezing or entanglement.

The JILA team designed a teleportation protocol in which these collective states—rather than individual ions—are transferred between ion subgroups within a Penning trap, a system used to trap large systems of ions via a set of electrodes and a strong magnetic field. The system they studied consists of a 2D array of trapped beryllium ions in a crystal, which vibrate coherently through shared vibrations, or phonon modes, which can be used to individually manipulate nuclear and electronic spin states of these ions by driving them with lasers and microwave fields. Instead of spatially separated ion ensembles, the researchers separated the ensembles by using distinct nuclear spin levels (or internal ion levels) within the crystal, which have large energy splittings in the strong magnetic field used in the Penning trap to confine the ions. The investigators used three levels to emulate three independent quantum subsystems: Alice, Bob, and Charlie.

First, Alice and Bob—two ion groups—are linked together through a phonon mode of the entire crystal. This phonon mode acts like a mediator, allowing the spins in Alice and Bob to become entangled and form a correlated quantum state.

Next, the third sub-ensemble or nuclear spin energy level, Charlie, holds a quantum state that the researchers want to teleport. This state is gently combined with Alice’s state to mix their information coherently without measuring it directly—like blending two sound waves before analyzing them.

Finally, both Alice and Charlie quantum states are measured. These measurements don't reveal the full quantum state but provide enough information, which is sent to adjust Bob’s group to end up in the same quantum state that Charlie originally had. The result is that Charlie’s state appears in Bob’s ions, effectively teleporting the quantum information across the system.

The protocol, adapted from continuous-variable teleportation schemes in quantum optics, was successfully numerically simulated in systems containing up to 300 ions per ensemble. The simulation also showed the possibility of achieving a high-fidelity teleportation of classical and non-classical spin states, including spin-coherent, spin-squeezed, and Dicke states, under current experimental conditions. Spin-squeezed and Dicke states are particularly interesting because they exhibit entanglement and are helpful for quantum-enhanced sensing, quantum information processing, and computation.

Demonstrating the teleportation of these states suggests that the protocol could one day serve as a mechanism for distributing entanglement and quantum information processing in quantum networks of ions.

“Typically, teleportation circuits are applied to spin-coherent states, which are relatively easy to prepare and simulate,” Khan notes. “We showed that the same protocol can be extended to non-trivial, entangled states, which had not been clearly demonstrated before in a collective spin setting.”

This theoretical protocol is also experimentally realistic. The proposed platform leverages existing techniques in Penning traps, where ion crystals are cooled, entangled, and interrogated with a high degree of control. The phonon modes in the trap are long-lived, robust to noise, and capable of coupling hundreds of ions simultaneously, making them ideal for entanglement distribution.

Protocols with Many Applications

Teleportation protocols like these could serve as building blocks for special types of quantum devices, where quantum information is moved from one register to another without physical transport. It also lays the groundwork for distributed quantum sensing, where entangled states are shared across separate sensors to improve measurement precision.

Beyond practical applications, this work could be the starting point for implementing schemes relevant to simulating quantum gravity in the lab and quantum information scrambling.

“Currently, we are working on some quantum information protocols implementable in the NIST Penning trap inspired by black hole physics,” JILA graduate student Edwin Chaparro says. “Black holes are believed to be the fastest-scrambling objects in nature. We want to leverage the capability to generate scrambling in ion arrays to perform quantum teleportation and information recovery protocols and quantum metrology applications.”

Looking ahead, the researchers hope to extend the protocol to spatially separated systems, potentially using a new generation of trapped ion crystals that live in three special dimensions instead of two. They also plan to explore how similar schemes could be implemented in other physical platforms, such as neutral atom arrays or polar molecules.

This work was supported by the U.S. Department of Energy’s Office of Science, National Quantum Information Science Research Centers, the Quantum Systems Accelerator, the JILA Physics Frontier Center, and NIST.

Researchers at JILA, led by Ana Maria Rey, developed a new protocol for teleporting quantum information in collective spin states of ions within a two-dimensional crystal. This involves entangling ion groups through phonon modes and using measurements to transfer quantum states. The protocol, successfully simulated with up to 300 ions, shows potential for quantum networks and distributed quantum sensing.

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Sneaky Clocks: Uncovering Einstein’s Relativity in an Interacting Atomic Playground

Steven Burrows — Wed, 05 Mar 2025 19:43:14 +0000

Sneaky Clocks: Uncovering Einstein’s Relativity in an Interacting Atomic Playground Steven Burrows Wed, 03/05/2025 - 12:43

Kenna Hughes-Castleberry / JILA Science Communicator

For over a century, physicists have grappled with one of the most profound questions in science: How do the rules of quantum mechanics, which govern the smallest particles, fit with the laws of general relativity, which describe the universe on the largest scales?

The optical lattice clock, one of the most precise timekeeping devices, is becoming a powerful tool used to tackle this great challenge. Within an optical lattice clock, atoms are trapped in a “lattice” potential formed by laser beams and are manipulated with precise control of quantum coherence and interactions governed by quantum mechanics. Simultaneously, according to Einstein’s laws of general relativity, time moves slower in stronger gravitational fields. This effect, known as gravitational redshift, leads to a tiny shift of atoms’ internal energy levels depending on their position in gravitational fields, causing their “ticking”—the oscillations that define time in optical lattice clocks—to change.

By measuring the tiny shifts of oscillation frequency in these ultra precise clocks, researchers are able to explore the influences of Einstein’s theory of relativity on quantum systems. While relativistic effects are well-understood for individual atoms, their role in many-body quantum systems, where atoms can interact and become entangled, remains largely unexplored.

Making a step forward in this direction, researchers led by JILA and NIST Fellows and University of Colorado Boulder physics professors Jun Ye and Ana Maria Rey—in collaboration with scientists at the Leibnitz University in Hanover, the Austrian Academy of Sciences, and the University of Innsbruck—proposed practical protocols to explore the effects of relativity, such as the gravitational redshift, on quantum entanglement and interactions in an optical atomic clock. Their work revealed that the interplay between gravitational effects and quantum interactions can lead to unexpected phenomena, such as atomic synchronization and quantum entanglement among particles. The results of this study were published in Physical Review Letters.

“One of our key findings is that interactions between atoms can help to lock them together so that now they behave as a unified system instead of ticking independently due to the gravitational redshift,” explains Dr. Anjun Chu, a former JILA graduate student, now a postdoctoral researcher at the University of Chicago and the paper’s first author. “This is really cool because it directly shows the interplay between quantum interactions and gravitational effects.”

“The interplay between general relativity [GR] and quantum entanglement has puzzled physicists for years,” Rey adds. “The challenge lies in the fact that GR corrections in most tabletop experiments are minuscule, making them extremely difficult to detect. However, atomic clocks are now reaching unprecedented precision, bringing these elusive effects within measurable range. Since these clocks simultaneously interrogate many atoms, they provide a unique platform to explore the intersection of GR and many-body quantum physics. In this work, we investigated a system where atoms interact by exchanging photons within an optical cavity. Interestingly, we found out that while individual interactions alone can have no direct effect on the ticking of the clock, their collective influence on the redshift can significantly modify the dynamics and even generate entanglement among the atoms which is very exciting.”

An optical lattice clock embedded in the curved spacetime formed by the earth’s gravity. Dynamical interplay between photon-mediated interactions and gravitational redshift can lead to entanglement generation and frequency synchronization dynamics. Image credit: Steven Burrows / JILA

Distinguishing Gravitational Effects

To explore this challenge, the team devised innovative protocols to observe how gravitational redshift interferes with quantum behavior. The first issue they focused on was to uniquely distinguish gravitational effects in an optical lattice clock from other noise sources contributing to the tiny frequency shifts. They utilized a technique called a dressing protocol, which involves manipulating the internal states of particles with laser light. While dressing protocols are a standard tool in quantum optics, this is one of the first instances of the protocol being used to fine-tune gravitational effects.

The tunability is based on the mechanism known as mass-energy equivalence (from Einstein's famous equation E=mc²), which means that changes in a particle’s internal energy can subtly alter its mass. Based on this mechanism, an atom in the excited state has a slightly larger mass compared to the same atom in the ground state. The mass difference in gravitational potential energy is equivalent to gravitational redshift. The dressing protocol provides a flexible way to tune the mass difference, and thus the gravitational redshift, by controlling the particles to stay in a superposition of the two internal energy states. Instead of being strictly in the ground or excited state, the particles can be tuned to occupy both of the states simultaneously with a continuous change of occupation probability between these two levels. This technique provides unprecedented control of internal states, enabling the researchers to fine-tune the size of gravitational effects.

In this way, the researchers could distinguish genuine gravitational redshift effects from other influences, like magnetic field gradients, within the system.

“By changing the superpositions of internal levels of the particles you're addressing, you can change how large the gravitational effects appear,” notes JILA graduate student Maya Miklos. “This is a really clever way to probe mass-energy equivalence at the quantum level.”

Seeing Synchronization and Entanglement

After providing a recipe to distinguish genuine gravitational effects, the researchers explored gravitational manifestations in quantum many-body dynamics. They made use of the photon-mediated interactions generated by placing the atoms in an optical cavity.

If one atom is in an excited state, it can relax back to the ground state by emitting a photon into the cavity. This photon doesn’t necessarily escape the system but can be absorbed by another atom in the ground state, exciting it in turn. Such an exchange of energy—known as photon-mediated interactions—is key to making particles interact, even when they cannot physically touch each other.

Such types of quantum interactions can compete with gravitational effects on individual atoms inside the cavity. Typically, particles positioned at different “heights” within a gravitational field experience slight differences in how they “tick” due to gravitational redshift. Without interactions between particles, the slight difference in oscillation frequencies will cause them to fall out of sync over time.

However, when photon-mediated interactions were introduced, something remarkable happened: the particles began to synchronize, effectively “locking” their ticking together despite the differences in oscillation frequencies induced by gravity.

“It’s fascinating,” Chu says. “You can think of each particle as its own little clock. But when they interact, they start to tick in unison, even though gravity is trying to pull their timing apart.”

This synchronization showcased a fascinating interplay between gravitational effects and quantum interactions, where the latter can override the natural desynchronization caused by gravitational redshift.

This synchronization wasn’t just an oddity—it also led to the creation of quantum entanglement, a phenomenon where particles become interconnected, with the state of one instantly affecting the other. Remarkably, the researchers found that the speed of synchronization could also serve as an indirect measure of entanglement, offering an insight into quantifying the interplay between two effects. “Synchronization is the first phenomenon we can see that reveals this competition between gravitational redshift and quantum interactions,” adds JILA postdoctoral researcher Dr. Kyungtae Kim. “It’s a window into how these two forces balance each other.”

Advancing Physics Research

While this study revealed the initial interactions between these two fields of physics, the protocols developed could help refine experimental techniques, making them even more precise—with applications ranging from quantum computing to fundamental physics experiments.

“Detecting this GR-facilitated entanglement would be a groundbreaking achievement, and our theoretical calculations suggest that it is within reach of current or near-term experiments,” says Rey.

Future experiments could explore how particles behave under different conditions or how interactions can amplify gravitational effects, bringing us closer to unifying the two great pillars of modern physics.

This research was supported by the Sloan Foundation, the Simons Foundation and the Heising-Simons Foundation along with the JILA PFC.

Researchers led by JILA and NIST Fellows and University of Colorado Boulder physics professors Jun Ye and Ana Maria Rey—in collaboration with scientists at the Leibnitz University in Hanover, the Austrian Academy of Sciences, and the University of Innsbruck—proposed practical protocols to explore the effects of relativity, such as the gravitational redshift, on quantum entanglement and interactions in an optical atomic clock. Their work revealed that the interplay between gravitational effects and quantum interactions can lead to unexpected phenomena, such as atomic synchronization and quantum entanglement among particles.

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Making a Leap by Using “Another State to Entangle”

Steven Burrows — Fri, 24 Jan 2025 20:01:43 +0000

Making a Leap by Using “Another State to Entangle” Steven Burrows Fri, 01/24/2025 - 13:01

Kenna Hughes-Castleberry / JILA Science Communicator

Interactions between atoms and light rule the behavior of our physical world, but, at the same time, can be extremely complex. Understanding and harnessing them is one of the major challenges for the development of quantum technologies.

To understand light-mediated interactions between atoms, it is common to isolate only two atomic levels, a ground level and an excited level, and view the atoms as tiny antennas with two poles that talk to each other. So, when an atom in a crystal lattice array is prepared in the excited state, it relaxes back to the ground state after some time by emitting a photon. The emitted photon does not necessarily escape out of the array, but instead, it can get absorbed by another ground-state atom, which then gets excited. Such an exchange of excitations also referred to as dipole-dipole interaction, is key for making atoms interact, even when they cannot bump into each other.

“While the underlying idea is very simple, as many photons are exchanged between many atoms, the state of the system can become correlated, or highly entangled, quickly,” explains JILA and NIST Fellow and University of Colorado Boulder physics professor Ana Maria Rey. “I cannot think of a single atom as an independent object. Instead, I need to keep track of how its state depends on the state of many other atoms in the array. This is intractable with current computational methods. In the absence of an external drive, the generated entanglement typically disappears since all atoms relax to the ground state.”

Atoms can, however, have more than two atomic levels. Interactions in the system can change drastically if more than two internal levels are allowed to participate in the dynamics. In a two-level system (weak excitation) with only one photon and, at most, one excited atom in the array, one just needs to track the single excited atom. While this is numerically tractable, it is not so helpful for quantum technologies since the atoms could be thought of more as classical antennas.

In contrast, by allowing just one additional ground level per atom, even with a single excitation, the number of configurations accessible to the system grows exponentially, drastically increasing the complexity. Understanding atom-light interaction in multi-level settings is an extremely difficult problem, and up to now it has eluded both theorists and experimentalists.

Schematic of the multi-level atomic array structure used in this study. Image credit: Steven Burrows / JILA

Rey explains, “However, it can be extremely useful, not only because it can generate highly entangled states which can be preserved in the absence of a drive since atoms in the ground levels do not decay.’’

Now, in a recent study published in Physical Review Letters, Rey and JILA and NIST Fellow James K. Thompson, along with graduate student Sanaa Agarwal and researcher Asier Piñeiro Orioli from the University of Strasbourg, studied atom-light interactions in the case of effective four-level atoms, two ground (or metastable) and two excited levels arranged in specific one-dimensional and two-dimensional crystal lattices.

“We know that including the full multilevel structure of atoms can give us richer physics and new phenomena, which are promising for entangled state generation,” says Agarwal, the paper’s first author. With quantum technologies like computing and secure communications requiring entanglement, understanding how to create stable, interconnected atomic systems has become a priority.

From Two to Four Internal States

For this study, the researchers focused on isolating four energy levels in strontium atoms, arranged in either one-dimensional (1D) or two-dimensional (2D) configurations where the atoms are loaded in a special configuration in which they are closer to each other than the wavelength of the laser light used to excite them.

The study concentrated on a set of internal levels with a much smaller energy separation than typical optical transitions. Instead of using truly ground-state levels, they proposed using metastable levels where atoms can live for a very long time.

This very interesting set of levels has not been explored much before since it requires a special laser with a very long wavelength, but Thompson plans to have this laser in his lab.

“We plan to build the necessary capabilities in our lab to first knock the atom into an excited state that lives for a really long time,” Thompson says. “This will let us use a 2.9-micron wavelength transition between this so-called metastable-state excited-state 3P2 in strontium and another excited-state 3D3 state. This wavelength is about eight times longer than the usual separation between nearby atoms trapped in an optical lattice in our lab. By having a transition wavelength much longer than the trapping light wavelength, we will be able to realize strong and programmable interactions via this photon exchange that happens when the atoms are jammed in close to each other.”

Agarwal adds, “The atoms need to be very close, as interactions weaken with distance, eventually becoming lost due to other sources of decoherence [noise]. Keeping atoms close allows interactions to dominate, preserving the growth of entanglement.”

The team focused on the weak and far-from-resonance regime where atoms are allowed to virtually “trade” photons, i.e., moving them between ground states without permanently occupying an excited state.

“By exchanging photons, atoms are effectively only moving between different configurations in the ground state levels, which simplifies our calculations by reducing the number of states accessible to the system,” Agarwal adds. “It’s easier to eliminate the excited states and focus on the metastable state dynamics, where we observe growing correlations, which furthermore can be preserved when the laser is turned off.

Creating A Spin Model for Entanglement

In the regime where the excited levels are only “virtually” populated, and only atoms can occupy the metastable state levels, the four-level problem can be reduced back to a two-level system at the cost of dealing with much more complex interactions, which involves not only pair-wise interactions but multi-atom interaction.

Rey explains, “We focused in the far from a resonant regime where to leading order, only two atoms interact at a given time. In this case, the Hamiltonian describing the metastable state dynamics maps back to a well-characterized spin model.”

The team used this well-known model to study what are called “spin waves”—coordinated low-energy excitations of atomic spins—across the lattice arrangement. Moreover, by controlling the polarization and propagation direction of the photons of the laser exciting the atoms, the researchers could determine which “spin-wave pattern” became dominantly entangled. The entanglement observed was spin-squeezing, a specific form of entanglement that has increased sensitivity to external noise and thus useful for metrology.

“The spin squeezing in our system can be experimentally measured and serves as a witness of quantum entanglement. Our setup also has possible applications in simulating many-body physics,” Agarwal says.

This finding is especially significant, as it implies that quantum systems could maintain entanglement over long periods, without needing constant intervention to prevent decoherence.

Limitations in Simulations

While the team’s model offered promising insights, it also faced limitations in accurately simulating the system over time. One of the key limitations arose from the dipole-dipole interactions, which, unlike simpler interactions, involve long-range forces that couple atoms both near and far in the lattice. Furthermore, these couplings are anisotropic and depend on the relative orientation of the atomic dipoles, making the system more complex. Each atom interacts differently with its neighbors spaced along different directions in the lattice, leading to varying interaction strengths and signs across the array.

Other popular simulation techniques designed for short-range interactions fail when applied to long-range interactions, as they aren’t equipped to handle the many correlations that develop over time. Although some other methods are more appropriate for long-range atomic interactions, they are constrained to small atom numbers due to their computational complexity, limiting the researchers’ ability to observe the long-time progression of correlations in a large system.

Diving Further into Internal States

The team’s findings could open new avenues in quantum information science and quantum computing, offering a potential path for the development of highly entangled and scalable quantum systems.

“We’re inching closer to systems that could sustain entanglement reliably, which is a crucial step for future quantum applications,” says Agarwal.

Looking forward, the research team plans to explore how more extensive multilevel systems might enhance entanglement potential.

“In atoms like strontium, with as many as 10 ground and excited levels each, the complexity grows significantly, and we want to see how this impacts entanglement,” Agarwal says. “Furthermore, while we have focused here on interactions between atoms in free space, one excited extension is to understand how these interactions can interplay with additional photon-mediated interactions [that are] built when atoms are instead placed inside an optical cavity or in nanophotonic devices,” she adds

“The competition between the infinite range interactions mediated by the cavity photons and the dipole-dipole interactions described here can open fantastic opportunities to harness light-mediated quantum gates, entanglement distribution, and programmable quantum many-body physics,” says Thompson.

This work was supported by the Vannevar Bush Faculty Fellowship (VBFF), the National Science Foundation (NSF), the Joint Institute for Laboratory Astrophysics-Physics Frontier Center (JILA-PFC), the Quantum Systems Accelerator (QSA), and the National Institute of Standards and Technology (NIST)

In a recent study published in Physical Review Letters, Rey and JILA and NIST Fellow James K. Thompson, along with graduate student Sanaa Agarwal and researcher Asier Piñeiro Orioli from the University of Strasbourg, studied atom-light interactions in the case of effective four-level atoms, two ground (or metastable) and two excited levels arranged in specific one-dimensional and two-dimensional crystal lattices.

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No Cavity, No Party: Free-Space Atoms Give Superradiant Transition a Pass

Steven Burrows — Tue, 17 Dec 2024 19:51:10 +0000

No Cavity, No Party: Free-Space Atoms Give Superradiant Transition a Pass Steven Burrows Tue, 12/17/2024 - 12:51

Kenna Hughes-Castleberry / JILA Science Communicator

A pencil-shaped ultracold gas of frozen two-level atoms interacting via photon-mediated interactions, with elastic and inelastic components. A continuous laser drive excites the atoms on-resonance. Atoms also spontaneously emit photons into free-space. Credit: Steven Burrows / JILA

Isolated atoms in free space radiate energy at their own individual pace. However, atoms in an optical cavity interact with the photons bouncing back and forth from the cavity mirrors, and by doing so, they coordinate their photon emission and radiate collectively, all in sync. This enhanced light emission before all the atoms reach the ground state is known as superradiance. Interestingly, if an external laser is used to excite the atoms inside the cavity moderately, the absorption of light by the atoms and the collective emission can balance each other, letting the atoms relax to a steady state with finite excitations.

However, above a certain laser energy level, the nature of the steady state drastically changes since atoms inside the cavity cannot collectively emit light fast enough to balance the incoming light. As a result, the atoms keep emitting and absorbing photons without reaching a stable, steady state. While this change in steady-state behaviors was theoretically predicted decades ago, it hasn’t yet been observed experimentally.

To better understand these findings, JILA and NIST Fellow Ana Maria Rey and her theory team collaborated with an international team of experimentalists. The theorists found that atoms in free space can only partially synchronize their emission, suggesting that the free-space experiment did not observe the superradiant phase transition. These results are published in PRX Quantum.

“While our current simulations were able to reproduce the experimental data, and explained why full synchronization cannot take place under current experimental conditions, a remaining open question is whether the phase transition could happen under different conditions, and at higher densities, where our theoretical methods fail and instead a genuine quantum description is required,” explains Rey.

From Experiment to Theory

In physics, solving complex problems often requires the combined efforts of both theorists and experimentalists. Theorists develop mathematical models and simulations to predict how systems should behave. Conversely, experimentalists conduct experiments to test and challenge these predictions. This collaboration helps bridge the gap between abstract ideas and observable phenomena.

“One of the big questions people are trying to answer is if it's possible to create entangled states in different atomic systems,” explains Sanaa Agarwal, a graduate student in Rey’s group and the paper’s first author. “In a cavity system, this is enabled by these collective all-to-all interactions [atoms interacting one-to-one], but in free space, that still needs to be clarified.”

A cavity system can be fine-tuned to drive atoms into specific quantum states. In contrast, free-space systems are less controlled.

“In free space, there are many effects to look at, like interaction-induced frequency,” says Agarwal. “You also have emission into all possible directions, not just predominantly into the cavity system. So, these effects are expected to change the physics in the system, and that's why we started looking into it, and indeed, we found it’s quite different.”

Simulating the Free-Space System

The specific free-space experimental conditions raised the question of whether the observed behaviors were truly superradiant or coincidental.

To answer these questions, the researchers carried out a series of theoretical simulations using a model that accounted for each atom as a dipole, absorbing and emitting photons from the laser and the light emitted by the other atoms.

“This was an interesting challenge, as the number of accessible states increases linearly in the cavity, but in free space, it can increase exponentially with system size,” Rey elaborates. “In many cases, the interactions can be weak enough that simplified treatments are possible, but it was initially not clear if that was going to be the case in this experiment.”

Argawal adds, “We considered a microscopic model, in which every atom acts like a dipole, and used it to study the emergent properties of the entire atomic cloud. The laser beam is a plane wave, imprinting a specific phase pattern on the atoms, which is crucial in determining how the atoms interact.”

The researchers simulated different conditions, including varying laser power and atom density, to see how these factors influenced the system's behavior.

“Our simulations showed that a “mean-field approximation”, which reduces the complexity greatly by treating the atoms as classical magnets, was enough to reproduce the physics,” Rey notes.

This model was validated with more complex approaches to ensure consistent results.

When Theory Agrees with Experiment

“When we were comparing the theory with the data, we were unsure if it would agree,” Agarwal says. “Some of the data was fairly easy to compare because there was less ambiguity in the experimental apparatus. So when our findings agreed with those results, it gave us a vote of confidence that what we're doing makes sense.”

From their simulations, the researchers concluded that while the free-space experiment agreed with the cavity model, within a narrow range of laser intensities and atom densities, the two systems generally behaved very differently. As the laser power increased beyond a certain threshold, the collective effects that gave rise to superradiance in the cavity disappeared into free space, and the atoms acted more like independent emitters rather than a coordinated group.

Further Enhancing Fundamental Physics

These findings open new research avenues in quantum physics and validate the great value of experiment-theory collaborations to gain a better understanding of the underlying physics.

“While our simulations were able to reproduce the experimental observations in the regime where the system is dilute, and the mean-field approximation is valid, it will be very exciting to study new regimes where our current theory models become obsolete and better treatments are required,” Rey adds. “Our group will be looking for ways to improve our calculations and to prepare us for the new exciting measurements coming ahead.”

This work was supported by the JILA Physics Frontier Center (PFC), the U.S. Department of Energy’s Office of Science, the National Quantum Information Science Research Centers’ Quantum Systems Accelerator, and the National Institute of Standards and Technology (NIST).

Written by Kenna Hughes-Castleberry, JILA Science Communicator

Recent research at the Laboratoire Charles Fabry and the Institut d’Optique in Paris studied a collection of atoms in free space forming an elongated, pencil-shaped cloud and reported the potential observation of this desired phase transition. Yet, the results of this study puzzled other experimentalists since atoms in free space don’t easily synchronize.

To better understand these findings, JILA and NIST Fellow Ana Maria Rey and her theory team collaborated with an international team of experimentalists. The theorists found that atoms in free space can only partially synchronize their emission, suggesting that the free-space experiment did not observe the superradiant phase transition. These results are published in PRX Quantum.

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To Measure or Not to Measure, but Dynamically Evolve—That is the Question

Steven Burrows — Mon, 07 Oct 2024 19:28:31 +0000

To Measure or Not to Measure, but Dynamically Evolve—That is the Question Steven Burrows Mon, 10/07/2024 - 13:28

Kenna Hughes-Castleberry / JILA Science Communicator

When the detection efficiency of the quantum nondemolition (QND) measurement is above 0.19, QND outperforms unitary evolution for the preparation of spin squeezing in a QED cavity. (Image Credit: Steven Burrows / JILA)

In the world of quantum technology, measuring with extreme accuracy is key. Despite impressive developments, state-of-the-art matter-wave interferometers and clocks still operate with collections of independent atoms, and the fundamental laws of quantum mechanics limit their precision.

One way to get around this fundamental quantum fuzziness is to entangle the atoms or make them talk so that one cannot independently describe their quantum states. In this case, it is possible to create a situation where the quantum noise of one atom in a sensor can be partially canceled by the quantum noise of another atom such that the total noise is quieter than one would expect for independent atoms. This type of entangled state is called a “squeezed state,” which can be visualized as if one had made a clock hand narrower to tell the time more precisely, a precision that comes at the expense of making the fuzziness along the clock hand worse. However, preparing spin-squeezed states is no easy feat.

Up to now, there have been two leading ways to generate squeezed states, using atoms that interact with light. One way, unitary evolution, is by transforming an initially uncorrelated (not entangled) state into a spin-squeezed state via dynamical evolution via a specific type of unitary interaction. One can imagine the initially uncorrelated state as a round piece of dough where your hand slowly squeezes the dough in one direction while making the other direction wider.

The other way is to perform quantum nondemolition measurements (QND) that allow one to pre-measure the quantum noise and subtract it from the final measurement outcome. The QND approach has currently realized the largest amounts of observed squeezing between the two methods, but it is not clear which protocol is actually optimal, given fundamental experimental constraints, or even if it would be better to use both protocols at the same time.

This is why JILA and NIST Fellows and University of Colorado Boulder Physics professors Ana Maria Rey and James K. Thompson and their teams wanted to guide the community on which protocol is best to use under fundamental and realistic experimental conditions. Their results, published in Physical Review Research, revealed that when measurement efficiency is greater than 19%, the QND measurement protocol outperformed unitary dynamical evolution. This finding can have big implications for quantum metrology.

“We were able to build a sort of map, where, if you have this experiment with these specific parameters, here’s what you should do,” explains first author former JILA graduate student Diego Barberena, now a postdoc at the University of Cambridge. “I think that it's beneficial to have all the results together in a single place because they are scattered all over the literature and written in ways that may be easier to parse for some physicists but not others, given the technical language of each different experiment. So, we’re happy to give people a paper where all protocols are in one place.”

A Tale of Two Methods

Squeezing is related to the Heisenberg Uncertainty Principle, which limits how accurately a researcher can measure two related properties simultaneously—such as the momentum and position of a particle—where the more researchers know about one parameter than the other. Squeezing overcomes this limitation by making one of these variables more uncertain, or less known, allowing a more accurate measurement of one of the variables.

Within quantum research, unitary evolution and QND are two of the most commonly used methods to create spin-squeezing in atoms. Both revolve around an ensemble of atoms placed in an optical cavity. By measuring the light leaking out of the cavity in QND experiments, the researchers can determine if the atoms are in a spin-squeezed state or not.

In the unitary evolution method, atoms interact by exchanging photons inside the optical cavity while swapping their internal levels or spins, a process that allow them to evolve in a controlled, predictable way which shapes their noise distribution in a specific desirable way: from a circle to an ellipse. This process is governed by a well-defined set of rules that describe the system's evolution, and no external measurements are involved, meaning the researchers didn’t observe or measure the light leakage during the experiment.

“You send the laser into the system, and then you let the atoms do their own thing,” says Barberena. “We call it unitary evolution because the atoms are evolving via their interactions. They are already in a setup where the evolution alone would help enhance measurement precision.”

In contrast, QND uses a different method to measure quantum dynamics.

Thompson elaborates, “QND measurements are very special. They involve measuring the light that leaks from the optical cavity to gather information about how many atoms are in which quantum state without knowing which atoms are in which quantum state.”

This measurement projects or collapses the original quantum state into a state with less quantum noise, with the measurement outcome telling the experimentalist which quantum state they have available to use.
“We have used both methods to generate squeezing here at JILA and NIST. Given that both approaches seem to work, it would be great to understand with certainty which one is the best one to use,” adds Rey.

Trading Off Efficiencies

To compare the two methods, researchers developed a detailed simulation that modeled how atoms interact with a shared light field inside an optical cavity. In this simulation, they accounted for real-world factors such as quantum noise, imperfect optical cavities, decoherence, and a crucial parameter known as quantum efficiency. Quantum efficiency refers to the fraction of all the information that is accessible to the experimentalist.

“Quantum efficiency is basically a quantity describing how well you can measure a system,” adds former JILA graduate student Anjun Chu, second author and now a postdoctoral fellow at the University of Chicago. “It revolves around the percentage of light leakage that can be measured out of the cavity. The efficiency is one, if you can perfectly detect all photons coming out of the cavity. In this case, it would be better to turn off unitary evolution and focus only on QND. If the efficiency is zero, you’re measuring no light, so you get no information from a measurement. In this case, it would be better to suppress the light leakage and let the system evolve near unitary. Between zero and one, there is a combination of measurement and unitary evolution. We are trying to determine which of the two methods or their combination would win.”

The simulation tested different levels of quantum efficiency to see how both methods performed under various conditions. The results showed that when quantum efficiency was above 19%, the QND method outperformed unitary evolution in generating spin-squeezed states. This was because the high efficiency allowed the QND process to gather enough information from the system to reduce uncertainty and improve precision. Below that threshold, unitary evolution was more effective.

Thompson notes, “In previous experimental work, we achieved net quantum efficiencies above 30%, and it is just a matter of technology development to achieve >90%. However, in quantum sensing, one often juggles many competing requirements that might make unitary evolution more advantageous. Now we know when to switch between one approach versus the other.”

The researchers also found that the combination of QND and unitary evolution did not provide the distinct advantage they expected, suggesting that one or the other method should be favored instead.

Improvements in Precision Measurement

This result is significant for quantum metrology, where the goal is to achieve more accurate measurements than the standard quantum limit (SQL)—the fundamental limit of precision achievable with uncorrelated particles—which can be achieved using spin-squeezed states.

“I think this paper is a helpful guideline for future experiments,” says Chu. “A researcher is going to want to know what the best way is to generate spin-squeezing. We give a very straightforward answer: look at the quantum efficiency of the experiment.”

This research was supported by the National Science Foundation (NSF), the NSF-Funded center Q-SEnSE and NIST.

JILA and NIST Fellows and University of Colorado Boulder Physics professors Ana Maria Rey and James K. Thompson and their teams wanted to guide the community on which protocol is best to use under fundamental and realistic experimental conditions. Their results, published in Physical Review Research, revealed that when measurement efficiency is greater than 19%, the QND measurement protocol outperformed unitary dynamical evolution. This finding can have big implications for quantum metrology.

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