James Thompson

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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A Symphony of Light and Atoms: Continuous Lasing and Strong Coupling

Steven Burrows — Mon, 22 Sep 2025 16:41:44 +0000

A Symphony of Light and Atoms: Continuous Lasing and Strong Coupling Steven Burrows Mon, 09/22/2025 - 10:41

Steven Burrows / JILA Science Communications Manager

Experimental setup: continuous lasing of Strontium-88 atoms.

Image credit: Steven Burrows / JILA

Imagine a symphony that never stops. The musicians are atoms, the concert hall that lets their sound build is an optical cavity, and the skilled conductor is a new continuous loading with strong coupling technique—keeping perfect time, seating new players quietly mid-performance, and blending everyone into one steady, pure tone. The 'music' you hear is the single, coherent laser beam emerging from the cavity.

In this experiment, supporting laser beams cool and move the atoms—think of them as the metronome and stage crew, not the performance itself. The performance is the new light the atoms collectively create inside the ring cavity, which then exits as a continuous laser. With this conductor-like control, Professor James K. Thompson, a Fellow of JILA, NIST and the Department of Physics at the University of Colorado, Boulder, and his team, maintain both continuous operation and strong collective coupling between the atoms and the cavity, a key step toward ultra-stable light sources and precision measurement tools.

In a set of papers, published in Nature Physics and Physical Review Letters, the Thompson group demonstrated continuous loading and strong coupling of strontium atoms to a high-finesse optical ring cavity, and continuous recoil-driven lasing with an unexpected pinning of the cavity frequency.

Vera Schäfer, one of the lead researchers, explained, "The original goal of our experiment was to build a continuous superradiant laser, a tool which allows us to make high precision frequency measurements at short timescales. This could help us to explore different regimes to search for dark matter and other new physics."

Zhijing Niu, the graduate student noted, “We managed to realize continuous laser cooling of strontium atoms into an optical ring cavity and to transport them within the cavity. This allowed us to keep a steady stream of extremely cold atoms, which is essential for building a continuous superradiant laser."

Thompson added, "But along the way we found something very curious and unexpected that reflects the fact that nature has a way of self-organizing when you pump energy into a system. We saw laser light coming out of our system when we were just trying to load a very cold gas of atoms between the highly reflective mirrors that form our laser cavity."

Continuous Lasing: Quantum Light Sources

Lasing, or light amplification by stimulated emission of radiation, is a process familiar to many through everyday devices like laser pointers and barcode scanners. Past lasing realized with laser-cooled atoms often involve pulsed operation, where the light source is intermittently active. In contrast, continuous lasing provides a steady stream of coherent light, which is crucial for applications requiring high stability and precision.

The researchers utilized laser-cooled strontium atoms, which were continuously loaded into a high-finesse ring cavity. This setup allowed for the atoms to be trapped and cooled using a series of laser cooling stages, including a three-dimensional red molasses and a vertical slowing beam. The continuous nature of this process ensures that atoms continuously replenished, crucial for sustained lasing.

Niu explained, "We have figured out how to laser cool and load our atoms continuously rather than staggered in time like almost all other experiments in our field do (i.e., cool and load some atoms, briefly do some science, throw them away, repeat).” Thompson added, “However, even before getting a chance to use the very narrow atomic transition, we saw laser light coming out of the cavity, and it would keep going all day long until we went home for the day!"

Recoil-Driven Lasing: The Heartbeat of Quantum Light

To understand recoil-driven lasing, think of a game of pool. When the cue ball strikes another ball, it transfers its momentum, causing the second ball to move. Similarly, in recoil-driven lasing, photons (light particles) transfer their momentum to atoms, causing them to move more quickly. This movement creates a population inversion, a key requirement for lasing.

In traditional lasers, achieving population inversion often involves complex setups and intermittent operation. However, the researchers at JILA have developed a method to maintain this inversion continuously. By using laser-cooled strontium atoms and a high-finesse ring cavity, they have created a system where the atoms are constantly replenished and kept in a low-energy state. This continuous replenishment ensures that the lasing process never stops.

Thompson noted, "We realized that the lasing involved absorbing a photon and then undergoing stimulated emission (the s and e of LASER) to a different momentum state since the atom recoils when it catches a photon of light and then throws it into the cavity.” Niu added, “This appears to be the gain mechanism provided by nature when we put energy into the system via our laser-cooling beams."

The experimental setup: Atoms are cooled and slowed inside a vacuum chamber, until they can be trapped in a lattice inside a cavity (black triangular cavity spacer in the bottom half of the vacuum chamber.) Fluorescence of the atoms can be seen on top in blue from hot atoms interacting with the Zeeman slower laser.

Image credit: Vera Schäfer / JILA

Strong Collective Coupling: Enhancing Atom-Cavity Interactions

In addition to continuous lasing, the study also achieved strong collective coupling of strontium atoms to the optical cavity. This phenomenon occurs when the collective interaction between the atoms and the cavity field is strong enough to significantly alter the properties of the system. The researchers demonstrated this by observing continuous atom-cavity vacuum Rabi splitting, a clear indication of strong coupling. This effect is akin to a dance where the atoms and the cavity photons are in perfect sync, leading to new and exciting quantum behaviors.

Schäfer highlighted, "A lot of the physics we saw only happens because this is a continuous rather than a cyclic experiment. The most interesting lasing regime only appears when starting in a noisier state, and then slowly changing the cavity parameters to a less stable regime that is only upheld by the continuous lasing."

Cavity Frequency Pinning: Stabilizing the Quantum Orchestra

One of the challenges in maintaining continuous lasing and strong coupling is the sensitivity of the system to external disturbances. Any fluctuation in the cavity frequency can disrupt the delicate balance, much like a sudden noise can throw an orchestra off-key. To address this, the researchers discovered a new mechanism that pins or stabilizes the resonance frequency of the cavity. Schäfer notes, “[we] found out that without us even trying, this lasing mechanism stabilized the effective frequency of our cavity.”

Cavity frequency pinning involves stabilizing the frequency of the dressed cavity mode to match the frequency at which there is gain for light inside the cavity. This is achieved through an atomic loss mechanism that adjusts the number of atoms in the cavity based on the lasing intensity. When the cavity frequency drifts, the system automatically compensates by altering the atom number, keeping the cavity frequency and hence the lasing frequency stable.

"This gain mechanism also causes atom-heating which then causes a funny feedback loop that keeps the effective optical cavity frequency to a fixed value, even when we tried our darndest to change the cavity frequency," Thompson explained.

The Future is in Narrow Linewidths

The continuous lasing and strong collective coupling achieved in this study represent an important milestone in laser and quantum science. These advancements not only enhance our understanding of fundamental quantum interactions but also open the door to a wide range of practical applications.
Thompson shared their next steps, "Many different groups in atomic and laser physics are moving towards continuous rather than cyclical operation, whether it be for quantum computing or for ultranarrow linewidth lasers. We plan to really use the narrow linewidth transition in strontium to build incredibly single-color lasers to explore the world."

Soon, this technology could lead to the development of ultra-stable superradiant lasers with millihertz linewidths, which are crucial for high-precision measurements and tests of fundamental physics. Additionally, the techniques developed in this study could be applied to create new quantum sensors and devices that leverage the unique properties of continuous atom-cavity interactions. By orchestrating a tightly synchronized ensemble of atoms and light under conductor‑like control, researchers are not only pushing the boundaries of what’s possible but also laying the groundwork for the next generation of quantum technologies.

This research is supported by the U.S. Department of Energy, Office of Science, National Quantum Information Science Research Centers, Quantum Systems Accelerator, the National Science Foundation JILA Physics Frontier Center and Q-SEnSE QLCI, and the Humboldt Foundation.

In a groundbreaking study researchers at JILA have demonstrated continuous lasing and strong atom-cavity coupling using laser-cooled strontium atoms. This innovative experiment opens new avenues for precision measurement and quantum technologies, promising advancements in quantum sensing and metrology.

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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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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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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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JILA Alumnus Dr. Matthew Norcia is Awarded the IUPAP Early Career Scientist Prize in Atomic, Molecular And Optical Physics 2024

Steven Burrows — Wed, 31 Jul 2024 19:16:41 +0000

JILA Alumnus Dr. Matthew Norcia is Awarded the IUPAP Early Career Scientist Prize in Atomic, Molecular And Optical Physics 2024 Steven Burrows Wed, 07/31/2024 - 13:16

Kenna Hughes-Castleberry / JILA Science Communicator

Dr. Matthew Norcia, a member of JILA’s extensive alumni network, has been awarded the prestigious 2024 International Union of Pure and Applied Physics (IUPAP) Early Career Scientist Prize in Atomic, Molecular, and Optical Physics. The IUPAP Early Career Scientist Prize honors early career physicists for their exceptional contributions within specific subfields, offering recognition through a certificate, medal, and monetary award.

Norcia was awarded this honor specifically “for his seminal contributions to cavity-QED, optical tweezers, and dipolar quantum gases, specifically, the realization of an optical tweezer clock and of two-dimensional supersolids in dipolar quantum gases,” the IUPAP Committee cited.

Norcia did his Ph.D. work at JILA under the mentorship of Professor James K. Thompson, a JILA Fellow with appointments at NIST and the Physics Dept. at the University of Colorado Boulder. Norcia worked on coupling a large ensemble of strontium atoms to an optical cavity via narrow and ultra-narrow linewidth optical transitions. His thesis, “New tools for precision measurement and quantum science with narrow linewidth optical transitions,” highlighted his work in the first demonstration of a superradiant laser on the one mHz linewidth strontium clock transition, achieving the most accurate and precise active atomic clock ever realized.

During his time in the Thompson group, Norcia also discovered novel laser-cooling methods, thought deeply about how atoms can enable gravitational wave detection, realized new laser spectroscopy methods, contributed to state-of-the-art entanglement generation, and observed cavity-mediated spin-exchange interactions for the first time.

“Matt is simply an extraordinary experimental physicist and is more than deserving of this recognition of his high impact and broad advancements of the frontiers of physics. His myriad achievements have opened many new paths within my group and in many groups around the world,” says Thompson. “For instance, his PhD work is now advancing matter wave interferometers, squeezed optical lattice clocks, and even allowing simulations of physics within BCS superconductors.”

Norcia finished his PhD in 2017 and transitioned into the laboratory of JILA Fellow, NIST Physicist, and University of Colorado Boulder Physics Professor Adam Kaufman.

As a postdoctoral researcher in Kaufman's group, Norcia built one of the first experiments for microscopic control over alkaline earth atoms in optical tweezers, culminating in creating one of the world's first tweezer-array optical clocks.

“Matt’s decision to begin his post-doc in an empty lab was risky, to put it lightly,” Kaufman states. “His exceptional experimental capabilities allowed him to bring a new experiment, in a new area for him, up and running in record time. What’s more, he was able to establish new scientific directions with this apparatus, such as using tweezer arrays for clocks. In just two years, he went from an empty room to multiple high-impact scientific publications. His effectiveness and scientific ingenuity is truly remarkable.”

In 2019, Dr. Norcia left JILA and joined Francesca Ferlaino's team at the IQOQI in Innsbruck, where he advanced the study of dipolar supersolids composed of magnetic lanthanide atoms, particularly extending supersolid properties to two dimensions.

Since 2021, he has been at Atom Computing Inc. in Boulder, Colorado, where he continues to innovate in quantum computing with optically trapped alkaline earth atoms.

Dr. Norcia received the award at the International Conference on Atomic Physics (ICAP) in London in mid-Jul 2024, where he also delivered an invited prize talk.

Thompson notes, “At JILA, we are very proud of our many fantastic alumni, who are impacting the frontiers of quantum science and technology at universities, national laboratories, and at companies. So, it is a real delight to see a JILA alumni of Matt’s caliber receive a prestigious recognition such as this.”

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Twisting and Binding Matter Waves with Photons in a Cavity

Steven Burrows — Sun, 28 Apr 2024 17:14:52 +0000

Twisting and Binding Matter Waves with Photons in a Cavity Steven Burrows Sun, 04/28/2024 - 11:14

Kenna Hughes-Castleberry / JILA Science Communicator

Atoms inside of an optical cavity exchange their momentum states by "playing catch" with photons. As the atoms absorb photons from an applied laser, the whole cloud of atoms recoil rather than the individual atoms. Image credit: Steven Burrows / JILA

Precisely measuring the energy states of individual atoms has been a historical challenge for physicists due to atomic recoil. When an atom interacts with a photon, the atom “recoils” in the opposite direction, making it difficult to measure the position and momentum of the atom precisely. This recoil can have big implications for quantum sensing, which detects minute changes in parameters, for example, using changes in gravitational waves to determine the shape of the Earth or even detect dark matter.

In a new paper published in Science, JILA and NIST Fellows Ana Maria Rey and James Thompson, JILA Fellow Murray Holland, and their teams proposed a way to overcome this atomic recoil by demonstrating a new type of atomic interaction called momentum-exchange interaction, where atoms exchanged their momentums by exchanging corresponding photons.

Using a cavity—an enclosed space composed of mirrors—the researchers observed that the atomic recoil was dampened by atoms exchanging energy states within the confined space. This process created a collective absorption of energy and dispersed the recoil among the entire population of particles.

With these results, other researchers can design cavities to dampen recoil and other outside effects in a wide range of experiments, which can help physicists better understand complex systems or discover new aspects of quantum physics. An improved cavity design could also enable more precise simulations of superconductivity, such as in the case of the Bose-Einstein-Condensate-Bardeen-Cooper-Schrift (BEC-BCS) crossover or high-energy physical systems.

For the first time, the momentum-exchange interaction was observed to induce one-axis twisting (OAT) dynamics, an aspect of quantum entanglement, between atomic momentum states. OAT acts like a quantum braid for entangling different molecules, as each quantum state gets twisted and connected to another particle.

Previously, OAT was only seen in atomic internal states, but now, with these new results, it is thought that OAT induced by momentum exchange could help reduce quantum noise from multiple atoms. Being able to entangle momentum states could also lead to improvement in some physical measurements by quantum sensors, such as gravitational waves.

Leveraging a Density Grating

Within this new study, inspired by previous research from Thompson and his team, the researchers examined the effects of quantum superposition, which allows particles like photons or electrons to exist in multiple quantum states simultaneously.

“In this [new] project, the atoms all share the same spinlabel; the only difference is that each atom is in a superposition between two momentum states,” graduate student and first author Chengyi Luo explained.

The researchers found they could better control atomic recoil by forcing the atoms to exchange photons and their associated energies. Similar to a game of dodgeball, one atom may “throw” a “dodgeball” (a photon) and recoil in the opposite direction. That “dodgeball” may be caught by a second atom, which can cause the same amount of recoil for this second atom. This cancels out the two recoils experienced by both atoms and averages them for the entire cavity system.

When two atoms exchange their different photon energies, the resulting wave packet (an atom’s wave distribution) in superposition forms a momentum graph known as a density grating, which looks like a fine-toothed comb.

Luo added. “The formation of the density grating indicates two momentum states [within the atom] are ‘coherent’ with each other such that they could interfere [with each other].” The researchers found that the exchange of photons between atoms caused a binding of the two atoms’ wave packets, so they were no longer separate measurements.

The researchers could induce momentum exchange by exploring the interplay between the density grating and the optical cavity. Because the atoms exchanged energy, any recoil from absorbing a photon was dispersed among the entire community of atoms instead of individual particles.

Dampening the Doppler Shift

Using this new control method, the researchers found that they could also use this recoil-dampening system to help mitigate a separate measurement problem: the Doppler shift.

The Doppler shift, a phenomenon in classical physics, explains why the sound of a siren or train horn changes pitch as it passes a listener or why certain stars appear red or blue in night sky images—it’s the change in the frequency of the wave as the source and observer move toward (or away from) each other. In quantum physics, the Doppler shift describes a particle’s energy change due to relative motion.

For researchers like Luo, the Doppler shift can be a challenge to overcome in getting a precise measurement. “When absorbing photons, the atomic recoil will lead to a Doppler shift of the frequency of the photon, which is a big problem when you talk about precision spectroscopy,” he elaborated. By simulating their new method, the researchers found that it could overcome measurement skewing due to Doppler Shift.

Entangling Momentum Exchange

The researchers also found that the momentum exchange between these atoms could be used as a type of quantum entanglement. As John Wilson, a graduate student in the Holland group, elaborated: “As an atom falls, its motion wiggles the cavity frequency. That, in turn, encourages other atoms to collectively feel that feedback mechanism and nudges them to correlate their motion through the shared wobbles.”

To test this “entanglement” even further, the researchers created a bigger separation between the momentum states of the atoms and then induced the momentum exchange. The researchers found that the atoms continued to behave as if they were connected. “This indicates that the two momentum states are really oscillating concerning each other as if being connected by a spring,” added Luo.

Looking ahead, the researchers plan to probe this new form of quantum entanglement further, hoping to better understand how it can be used to improve various types of quantum devices.

This research was supported by the US Department of Energy, Office of Science, National Quantum Information Science Research Centers, Quantum Systems Accelerator.

In a new paper published in Science, JILA and NIST Fellows Ana Maria Rey and James Thompson, JILA Fellow Murray Holland, and their teams proposed a way to overcome atomic recoil by demonstrating a new type of atomic interaction called momentum-exchange interaction, where atoms exchanged their momentums by exchanging corresponding photons.

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JILA and the University of Colorado Boulder Lead Pioneering Quantum Gravity Research with Heising-Simons Foundation Grant

Steven Burrows — Tue, 27 Feb 2024 18:46:05 +0000

JILA and the University of Colorado Boulder Lead Pioneering Quantum Gravity Research with Heising-Simons Foundation Grant Steven Burrows Tue, 02/27/2024 - 11:46

Kenna Hughes-Castleberry / JILA Science Communicator

Heising-Simons Foundation Awards $3 Million for Informing Gravity Theory

This funding initiative underscores a concerted effort to leverage AMO physics' platforms and techniques to not only probe simple models of quantum gravity but also to aid in the construction of more sophisticated models that better describe our universe.

Among the recipients, a notable grant was awarded to a multi-investigator collaboration spearheaded by the University of Colorado Boulder (��Ҵ�ý�ƽ��) and JILA, a joint institute of ��Ҵ�ý�ƽ�� and the National Institute of Standards and Technology (NIST).

This distinguished team comprises leading experts in their respective fields, including Ana Maria Rey, JILA and NIST Fellow and a ��Ҵ�ý�ƽ�� Professor Adjoint, known for her work in atomic physics; JILA Fellow and ��Ҵ�ý�ƽ�� professor Adam Kaufman and JILA and NIST Fellow and ��Ҵ�ý�ƽ�� professor Dr. James Thompson, both recognized for their pioneering experimental work using tweezer arrays and optical cavities, respectively. Additionally, Dr. Andrew Lucas, Assistant Professor at ��Ҵ�ý�ƽ��, and Dr. Chris Akers, Fellow at the Institute for Advanced Study, experts in holography and quantum gravity, are also included in this team of researchers.

The ��Ҵ�ý�ƽ�� and JILA collaboration is dedicated to addressing two pivotal questions that stand at the forefront of contemporary physics research: firstly, identifying models that are not only practical for experimental implementation but also capable of giving rise to quantum emergent spacetime, and secondly, devising methodologies to verify such emergent spacetime within experimental settings unambiguously. These inquiries are critical for advancing our understanding of the universe at the most fundamental level, potentially unlocking new paradigms in physics.

A Look Beyond Colorado

The Heising-Simons Foundation's initiative extends beyond ��Ҵ�ý�ƽ�� and JILA, with four additional grants awarded to Dr. Brian Swingle at Brandeis University, Dr. Soonwon Choi at the Massachusetts Institute of Technology, Dr. Manuel Endres at the California Institute of Technology, and Dr. Monika Schleier-Smith at Stanford University. Their projects, encompassing research on information dynamics in black holes, the fundamental bounds on quantum dynamics from gravity, and the emergence of geometry from entanglement, represent a broad and ambitious effort to unravel the mysteries of quantum gravity and its implications for our understanding of the cosmos.

This collaborative and multi-faceted approach to exploring the intersection of AMO physics and quantum gravity theories heralds a new era of scientific inquiry, poised to yield insights into some of the most profound and complex questions in physics today.

Through the support of the Heising-Simons Foundation and the collective expertise of the researchers involved, these projects significantly advance our grasp of the universe's foundational principles, marking a pivotal step forward in the ongoing quest to unify the theories of the very large and the very small.

The Heising-Simons Foundation's Science program has announced a generous grant of $3 million over three years, aimed at bolstering theoretical and experimental research efforts to bridge the realms of Atomic, Molecular, and Optical (AMO) physics with quantum gravity theories. Among the recipients, a notable grant was awarded to a multi-investigator collaboration spearheaded by the University of Colorado Boulder (��Ҵ�ý�ƽ��) and JILA, a joint institute of ��Ҵ�ý�ƽ�� and the National Institute of Standards and Technology (NIST).

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B-C-S—Easy as I, II, III: Unveiling Dynamic Superconductivity

Steven Burrows — Wed, 24 Jan 2024 18:37:22 +0000

B-C-S—Easy as I, II, III: Unveiling Dynamic Superconductivity Steven Burrows Wed, 01/24/2024 - 11:37

Kenna Hughes-Castleberry / JILA Science Communicator

Researchers observed the dynamic phases of BCS superconductor interactions in a Cavity QED by measuring the light leakage from the cavity. Image Credit: Steven Burrows / JILA

In physics, scientists have been fascinated by the mysterious behavior of superconductors—materials that can conduct electricity with zero resistance when cooled to extremely low temperatures. Within these superconducting systems, electrons team up in “Cooper pairs” because they're attracted to each other due to vibrations in the material called phonons.

As a thermodynamic phase of matter, superconductors typically exist in an equilibrium state. But recently, researchers at JILA became interested in kicking these materials into excited states and exploring the ensuing dynamics. As reported in a new Nature paper, the theory and experiment teams of JILA and NIST Fellows Ana Maria Rey and James K. Thompson, in collaboration with Prof. Robert Lewis-Swan at the University of Oklahoma, simulated superconductivity under such excited conditions using an atom-cavity system.

Instead of dealing with actual superconducting materials, the scientists harnessed the behavior of strontium atoms, laser-cooled to 10 millionths of a degree above absolute zero and levitated within an optical cavity built out of mirrors. In this simulator, the presence or absence of a Cooper pair was encoded in a two-level system or qubit. In this unique setup, photon-mediated interactions between electrons were realized between the atoms within the cavity.

Thanks to their simulation, the researchers observed three distinct phases of superconducting dynamics, including a rare “Phase III” featuring persistent oscillatory behavior predicted by condensed matter physics theorists but never before observed.

These findings could pave the way for a deeper understanding of superconductivity and its controllability, offering new avenues for engineering unique superconductors. Moreover, it holds promise for enhancing the coherence time for quantum sensing applications, such as improving the sensitivity of optical clocks.

Identifying Superconducting Phases

The JILA team focused on simulating the Barden-Cooper-Schrieffer model, which describes the Cooper pair behavior. As co-first author and JILA graduate student Dylan Young elaborated: “The BCS model has been around since the 1950s and is central to our understanding of how superconductors work. When condensed matter theorists began studying the out-of-equilibrium dynamics of superconductors, they naturally started with this model.”

In the past few decades, condensed matter theorists have predicted three distinct dynamical phases for a superconductor to experience when it evolves. In Phase I, the strength of superconductivity decays rapidly to zero. In contrast, Phase II represents a steady state in which superconductivity is preserved.

However, the previously unobserved Phase III is the most intriguing. “The idea of phase III is that the strength of superconductivity has persistent oscillations with no damping,” explained JILA graduate student and co-first author Anjun Chu. “In the phase III regime, instead of suppressing the oscillations, many-body interactions can lead to a self-generated periodic drive to the system and stabilize the oscillations. Observing this exotic behavior requires precise control of experimental conditions.”

To observe this elusive phase, the team leveraged the collaboration of theory from Rey’s group and experiment from Thompson’s group to create a precisely controlled experimental setup, hoping to fine-tune the experimental parameters to achieve Phase III.

Creating Precise Simulations in a Cavity Setting

While researchers previously tried to observe Phase III in real superconducting systems, measuring this phase has remained elusive due to technical difficulties. “They didn't have the right ‘knobs’ or readout mechanisms,” explained Young. “On the other hand, our implementation in an atom-cavity system gives us access to both tunable controls and useful observables to characterize the dynamics.”

Building on previous work, the researchers trapped a cloud of strontium atoms within an optical cavity. In this “quantum simulator’’, the atoms emulated Cooper pairs and experienced a collective interaction that parallels the attraction experienced by electrons in BCS superconductors. “We think of each atom as representing a Cooper pair,” Young explained. “An atom in the excited state simulates the presence of a Cooper pair, and the ground state represents the absence of one. This mapping is powerful because, as atomic physicists, we know how to manipulate atoms in ways you just can’t with Cooper pairs.”

The researchers applied this knowledge to induce different phases of dynamics in their simulation by a process known as “quenching.” As Young elaborated: “Quenching is when we suddenly change or ‘kick’ our system to see how it responds. In this case, we prepare our atoms in this highly collective superposition state between ground and excited states. Then, we induce a quench by turning on a laser beam that gives all the atoms different energies.”

By changing the nature of this quench, the researchers could see different dynamical phases. They even devised a trick to observe the elusive Phase III, which involved splitting the cloud of atoms in half. “Using two clouds of atoms with separate control over energy shifts is the key idea to achieve Phase III,” Chu remarked.

In superconductors, energy levels of electrons can be split into two sectors, largely occupied or barely occupied, separated by the Fermi level. “Our setup in spin systems does not have a Fermi level intrinsically, so we take account of this using two atomic clouds: one cloud simulates the states below the Fermi level, while another cloud simulates the other [quantum] states,” Chu added.

To measure the dynamics of the superconductor within the cavity, the researchers tracked the light leaking from the optical cavity in real time. Their data found distinct points where the simulated superconductor transitioned between phases, eventually reaching Phase III.

Seeing the first measurements of Phase III surprised many of the team. As Thompson stated: “Actually seeing the wiggles was extremely satisfying.” For her part in the collaboration, Rey was just as excited to see the theory and experiment align. “On the theory side, BCS superfluids/superconductors could, in principle, be observed in actual degenerate fermionic gases, such as the ones Debbie Jin at JILA taught us how to create. However, it has been hard to observe the dynamical phases in these systems. We predicted back in 2021 that all BCS dynamical phases could instead manifest in an atom-cavity experiment. It was so nice to see our theory predictions become a reality and actually observe the dynamical phases in a real experiment!”

Underlying Physics with Broader Applications

While observing Phase III within their system was a significant achievement, the team also found that the measured behaviors could have wider implications beyond superconductivity. As Thompson elaborated, “In terms of the underlying model that you use to describe it, it turns out that this BCS model has all these connections to different types of physics at different energy scales, temperature scales, and timescales, from superconductors to neutron stars to quantum sensors!”

Rey added: “These observations really open a path to simulate unconventional superconductors with fascinating topological properties for realizing robust quantum computers. It will be fantastic to emulate even toy models of these complex systems in our atom-cavity quantum simulator.”

This work was supported in part by the Quantum Systems Accelerator, part of the US Department of Energy, Office of Science, National Quantum Information Science Research Centers.

As a thermodynamic phase of matter, superconductors typically exist in an equilibrium state. But recently, researchers at JILA became interested in kicking these materials into excited states and exploring the ensuing dynamics. As reported in a new Nature paper, the theory and experiment teams of JILA and NIST Fellows Ana Maria Rey and James K. Thompson, in collaboration with Prof. Robert Lewis-Swan at the University of Oklahoma, simulated superconductivity under such excited conditions using an atom-cavity system.

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