John Bohn

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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Colorado Representative Yadira Caraveo visits JILA and the University of Colorado Boulder for Quantum Discussions

Steven Burrows — Wed, 03 Jul 2024 19:33:56 +0000

Colorado Representative Yadira Caraveo visits JILA and the University of Colorado Boulder for Quantum Discussions Steven Burrows Wed, 07/03/2024 - 13:33

Kenna Hughes-Castleberry / JILA Science Communicator

JILA and NIST Fellow Jun Ye shows his experimental set up to Colorado Representative Yarida Caraveo during her visit to JILA. Credit: University of Colorado Boulder/Casey Cass

On July 3, 2024, Colorado Congresswoman Yadira Caraveo delved into the quantum realm during her first official visit to JILA, a joint institute established by the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder.

As a House Committee on Science, Space, and Technology member, Caraveo's visit came just a day after the Mountain West was granted a $127 million boost for quantum technology and workforce development. During her tour, including a visit to Jun Ye’s renowned lab, Caraveo expressed her commitment to ensuring robust funding for agencies vital to quantum research. Accompanied by university leaders and distinguished researchers, Caraveo's visit highlighted the critical role of federal support in maintaining the U.S.'s leadership in quantum innovation.

Read about Congresswoman Caraveo's visit at this link.

On July 3, 2024, Colorado Congresswoman Yadira Caraveo delved into the quantum realm during her first official visit to ]JILA, a joint institute established by the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder.

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JILA Fellow and University of Colorado Boulder Physics professor John Bohn and JILA and NIST Fellow and University of Colorado Boulder Physics Professor Eric Cornell are awarded 2024 Physics Department Teaching Awards

Steven Burrows — Wed, 01 May 2024 17:24:52 +0000

JILA Fellow and University of Colorado Boulder Physics professor John Bohn and JILA and NIST Fellow and University of Colorado Boulder Physics Professor Eric Cornell are awarded 2024 Physics Department Teaching Awards Steven Burrows Wed, 05/01/2024 - 11:24

Kenna Hughes-Castleberry / JILA Science Communicator

Eric Cornell and John Bohn.

JILA and the University of Colorado Boulder's Department of Physics proudly announce two 2024 Physics Department Teaching Award recipients: JILA Fellow, NIST Fellow, and Professor Eric Cornell, and JILA Fellow and Professor John Bohn. These awards recognize their exceptional dedication to teaching and their profound impact on students at different levels of their academic journey.

The awards committee cited Professor Eric Cornell as being honored for his engaging approach to introductory physics, which brings the wonder of experimental atomic, molecular, and optical (AMO) physics to the classroom. His innovative teaching methods and dedication to foundational concepts helped students connect deeply with the material.

Cornell, who also won the Nobel Prize in Physics in 2001, was instrumental in developing courses that educate and inspire first-year physics, astrophysics, and engineering physics students. His collaboration with Professor Paul Beale in designing the General Physics for Majors course was particularly influential, creating a community where budding physicists can thrive.

Similarly, Professor John Bohn was recognized for his outstanding contributions at the physics major level. Bohn's ability to translate complex concepts into accessible learning experiences, along with some humorous jokes, has made him a pivotal figure in the academic development of advanced students.

Both professors were also lauded for their outreach efforts and commitment to teaching beyond the traditional classroom setting. This dedication ensures their influence extends beyond their immediate academic circles, inspiring a broader audience and fostering a greater appreciation of physics.

JILA and the University of Colorado Boulder's Department of Physics proudly announce two 2024 Physics Department Teaching Award recipients: JILA Fellow and NIST Fellow and Professor Eric Cornell and JILA Fellow and Professor John Bohn. These awards recognize their exceptional dedication to teaching and their profound impact on students at different levels of their academic journey.

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Colorado Congressman Joe Neguse Announces Significant Funding Headed to NIST & CU’s JILA Lab Renovation

Steven Burrows — Thu, 07 Sep 2023 21:34:16 +0000

Colorado Congressman Joe Neguse Announces Significant Funding Headed to NIST & CU’s JILA Lab Renovation Steven Burrows Thu, 09/07/2023 - 15:34

Kenna Hughes-Castleberry / JILA Science Communicator

JILA's A-wing tower

Since its inception in 1962, JILA has been a vital part of the University of Colorado’s physics research department, leading the way in the science of precision measurement while also teaching the next generation of physicists. Congressman Neguse recently released an update on funds for JILA provided by the Community Project Funding (PCF) status, saying: “Colorado has become world-renowned for its research ecosystem, and I could not be more excited that we’ve secured funding to help support the development of these groundbreaking labs in Colorado’s Second District. Thanks to this funding, JILA researchers and scientists will be able to complete much needed renovations to their lab—equipping this facility with the tools needed to remain a leader in their field.”

Last year, Colorado Congressman Joe Neguse helped to secure over $2,000,000 in funding for JILA as part of CPF. “I am very pleased the omnibus funding bill includes resources for JILA, a joint research institute between ��Ҵ�ý�ƽ�� and the National Institute of Standards and Technology," said University of Colorado Boulder Chancellor Philip DiStefano in 2022. “JILA is a leading institute in quantum technology research and training and has a long history of high-impact discoveries. I appreciate Congressman Neguse's strong support for our JILA scientists as well as the university's decades-long partnership with NIST.”

In response to Congressman Neguse’s update, JILA Associate Chair Dr. John Bohn stated: “JILA is grateful for this federal funding championed by Congressman Neguse. This support provides critical infrastructure for the JILA facility, notably HVAC improvements essential to maintaining stable, high-quality laboratory space. This space will enable JILA scientists to continue to explore cutting-edge experiments in the physical sciences, including quantum science and technology. JILA's dual research and workforce development mission benefits Colorado and the Nation.” The resources provided by CPF will also focus on ensuring JILA’s facilities can precisely control and stabilize temperatures within the laboratories, as many quantum experiments require specific temperatures for success.

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A Tale of Two Dipoles

Steven Burrows — Fri, 21 Apr 2023 18:02:08 +0000

A Tale of Two Dipoles Steven Burrows Fri, 04/21/2023 - 12:02

Kenna Hughes-Castleberry / JILA Science Communicator

Dipolar BEC Gas. Image credit: Steven Burrows / JILA

Dipolar gases have become an increasingly important topic in the field of quantum physics in recent years. These gases consist of atoms or molecules that possess a non-zero electric dipole moment, which gives rise to long-range dipole-dipole interactions between particles. These interactions can lead to a variety of interesting and exotic quantum phenomena that are not observed in conventional gases.

One important area of research involving dipolar gases is the study of quantum phases of matter. In particular, dipolar gases have been used to explore the behavior of so-called “quantum droplets”, which are self-adhering quantum states of atoms or molecules that arise due to the interplay between the molecule’s dipole-dipole interactions and quantum fluctuations. These droplets are stable and have been observed experimentally in a variation of two different systems. In a new paper published in Physical Review A, JILA Fellow John Bohn and graduate student Eli Halperin looked into the different patterns these droplets made, specifically within a Bose-Einstein Condensate (BEC) dipolar gas. “We’re looking at these arrays of droplets, which have different symmetries,” Halperin explained. “But the most common one, or the one that’s been reproduced in the lab, is this six-fold, hexagonal array of droplets.” From their work, Halperin and Bohn found that the dipolar BEC gas droplets could be disturbed enough to form intermediate patterns and symmetries, thereby creating a method for fine-tuning these interactions.

A Square Lattice and A Frustrated Gas

To look at how these symmetries formed within the BEC droplets, Halperin and Bohn used a method appropriately called “geometric frustration,” or simply, “frustration.” As Bohn elaborated: “This idea of frustration is very important. The atoms in these experiments are left to themselves and may want to do six-fold symmetry. But instead, you might try to force four-fold symmetry on them. And now the atoms are under stress, so they’re going to make a compromise. The frustration here is that compromise.” To apply frustration to the gaseous system, Halperin and Bohn posited using a square optical lattice, a type of web using different lasers, as a weak constraining force on the BEC droplets. “If the square lattice is really strong, you don’t just get the BEC sitting in the lattice,” Halperin added, alluding to the strength of the lattice depending on the laser frequency. However, a weak square lattice creates a weak force on the BEC system, which keeps the BEC in the lattice, and the gaseous droplets begin to struggle with each other to balance out their energy levels in order to reach the lowest energy states.

A Tale of Two Patterns

For Bohn and Halperin, this frustration caused some interesting effects. “We found this intermediate regime where it doesn’t always have six-fold symmetry or four-fold symmetry, it can have neither type of symmetry,” Halperin stated. This created different pattern regimes within the droplets, which the researchers mapped out, showing the different ground state of each regime. “It can have different regions where one section arranges in one pattern and a different section arranges in a different pattern,” Halperin added. “This half and half system ends up being an overall lower energy than when one pattern dominates the whole gas.” Because the dipolar BEC droplets acted as a superfluid system, (where the system has no viscosity and can flow without losing kinetic energy) instead of having more distinct particles such as in other gases, this half and half pattern of droplet frustration suggested something new for further exploration of the dipolar BEC gas. “This frustration gives another kind of knob to turn when you’re looking at patterns that change in the BEC,” Halperin stated.

Besides being a fine-tuning knob, this process of frustration has bigger implications for the field of quantum physics. As Bohn added: “You see frustration all over physics. The interesting stuff happens when there’s two competing things going on and the system has to find its way in-between.” Dipolar interactions can lead to the formation of complex patterns and structures in the gas, such as long-range order or the formation of exotic phases such as super solids. These systems are of great interest to researchers studying condensed matter physics and are being explored in a variety of different experimental systems. Both Bohn and Halperin are hopeful that other researchers could use their theory to further study this unique system.

Directing Sound at Dipoles

Another way to perturb a dipolar gas is to push different types of waves through it. In the research done by Bohn and graduate student Reuben Wang, also reported in Physical Review A, the waves being utilized were sound waves. Instead of utilizing the dipolar BEC gas that Halperin used, Wang and Bohn instead studied the interactions of sound waves on a dipolar fermionic system. Fermionic gases are unique in that fermions are difficult to condense, due to the Pauli Exclusion principle which asserts that two particles cannot share identical quantum states. However, by cooling these fermionic gases to ultracold temperatures, researchers can coax the gas to condense into a BEC-type formation and study the gas as one cohesive unit. Above certain temperatures, the fermions are studied as separate units. As Wang explained: “We look at the dipolar gas molecules as distinct things that whiz around and are thermally distributed, but also collide with each other and interact in quantum ways, where the quantum ways are scattering between particles.” Besides the scattering interactions, Wang and Bohn also studied the dipolar interactions between polar gases, also known as the mean-field. By taking the gases out of equilibrium, using sound waves, Wang and Bohn hoped to understand how these gases responded to the perturbation.

To disturb the system, the researchers decided to use sound waves, which are a type of compression wave. “Sound is a rather simple probe for this system,” Wang added. “We say: ‘Well, if I weakly poke it, there’s these linear excitations that go on top of the gas.’ So, we want to understand how this evolves with molecular collisions in the gas.” Looking at what would happen when sound waves penetrated the polar gas like ripples in a pond, Wang and Bohn were excited to see that the ripples were unequal. Instead of being symmetric in all directions, they found that the sound deformed the gas based on the directions of the dipole interactions. “The sound moves relatively faster to the direction in which the dipoles were aligned, and only slowly propagates in another direction, creating more oval-shaped ripples,” said Wang. “We’ve seen a somewhat similar thing in other literature on condensates.”

Studying Gaseous Viscosity

Understanding how sound waves move though the dipolar fermionic gas suggested other implications for studying these gaseous systems. As Wang explained, part of understanding dipolar gas dynamics was to look at their viscosity, which he described as a “…form of friction of the gas. “It arises microscopically, from the bumping of all the different molecules and atoms in the system.” Viscosity can tell physicists more about the fluctuations within the system, giving more insight into interactions happening at the quantum level. Like liquids with different viscosities, or runniness, gases with different viscosities behave differently. However, finding viscosity was not a straightforward method. “The method for figuring out the viscosity of a gas is well established and really complicated,” Wang added. “But it’s so much more complicated when the gas is dipolar. Usually, people will just put a number into viscosity, because you need to have some friction coefficient. But in our case, the viscosity becomes this object, which is also characterized by different directions in space.” By using sound waves to disturb the gas, Wang and Bohn found a new method that could yield greater accuracy to calculations of the viscosity of a dipolar gas.

Thanks to the different disturbances they were able to create in dipolar gaseous systems, Bohn, Halperin, and Wang were able to identify new dynamics within these special gases. Because dipolar gases are utilized in many different systems, from creating highly accurate atomic clocks to designing new types of sensors that are capable of detecting tiny variations in electric fields, understanding more about how these gases work can help advance many different subfields within quantum physics. As researchers continue to explore the properties of dipolar gases, we can expect to see many exciting new discoveries in the years to come.

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Don’t React, Interact: Looking Into Inert Molecular Gases

Steven Burrows — Tue, 12 Oct 2021 18:24:50 +0000

Don’t React, Interact: Looking Into Inert Molecular Gases Steven Burrows Tue, 10/12/2021 - 12:24

Kenna Hughes-Castleberry / JILA Science Communicator

The dipolar interactions within a molecular gas. Image credit: Steven Burrows / JILA

One of the major strengths of JILA are the frequent and ongoing collaborations between experimentalists and theorists, which have led to incredible discoveries in physics. One of these partnerships is between JILA Fellow John Bohn and JILA and NIST Fellow Jun Ye. Bohn's team of theorists has partnered with Ye's experimentalist laboratory for nearly twenty years, from the very beginning of Ye’s cold molecule research when he became a JILA Fellow. Recently in their collaborations, the researchers have been studying a three-dimensional molecular gas made of ⁴⁰K⁸⁷Rb molecules. In a paper published in Nature Physics, the combined team illustrated new quantum mechanical tricks in making this gas unreactive, thus enjoying a long life (for a gas), while at the same time letting the molecules in the gas interact and socialize (thermalize) with each other.

Creating a Gas

The work of creating this ultracold three-dimensional gas at JILA began more than 10 years ago with Fellows Jun Ye and Deborah Jin. Once they had created the gas, it kept reacting. In fact, many research groups worldwide have now achieved similar types of molecular gases in the quantum regime, and everybody has had to deal with the loss of molecules due to chemical reaction. Bohn and his team worked with Ye and his team to come up with a theory for stopping this reaction. The postulated theory involved a process called resonant shielding. “The shielding says, basically, if you take these molecules, and you put them in their quantum mechanical ground state, they attract each other, like all atoms or molecules attract each other by so called Van der Waals forces,” Bohn explained. “That's a quantum mechanical thing that goes way, way back. And that's what we're trying to avoid. Because if the molecules attract each other, they run into each other and react chemically, changing themselves.” In order to avoid chemical reactions, the molecular gas goes though the resonant shielding process where: “If you put the molecule in an excited state where it's already rotating, then you put the electric field just so, then the fluctuations in this dipole orientation make the molecules repel each other,” Bohn added. Because of the repulsion, the molecules in the gas don't get close enough with each other to react, thereby allowing for better control of the gas, such as turning on the dipolar interactions that remain effective at longer distances. From the data in the recently published paper, Bohn explained what the team found when the data was plotted: “The rate at which chemical reactions are happening, and it's high, high, high, and the experiment gets to a certain field–then bam! It drops by two orders of magnitude.” The fine-tuning of this gas allowed the researchers to examine other aspects of the gas without worrying about causing chemical reactions. Bohn's team theorized the resonant shielding as a recipe for creating this inert gas. Ye’s team then took the theory and began experimenting in the laboratory.

Cooling It Down

The opportunity of studying the unreactive gas was too good for the researchers to pass up. According to first author Jun-Ru Li: “Achieving such a collision resonance allows us to do the evaporative cooling of the molecules, which produces molecular gases with very low entropy. This allows us to use this platform to study a range of many-body phenomena related to dipolar quantum gases.” The many-body problem has intrigued physicists for years, as it refers to a category of issues relating to microscopic systems with interacting particles. Having an ultra-cold three-dimensional molecular gas that didn't react is valuable for the researchers to engineer and tune interparticle interactions that can give rise to exotic quantum many-body dynamics.

The researchers looked into the evaporative cooling of the gas. The process of evaporative cooling is what cools your body when you sweat. As the sweat evaporates, its molecules use heat energy to convert from a liquid to a vapor and escape from the surface of your skin, leading to a decrease in temperature as the heat is transferred into the evaporating sweat. In order to measure the evaporative cooling of their inert gas, the team looked at the gas’s thermalization signature which contributed to evaporative cooling. “It's the number of collisions between particles that is required for them to thermalize with each other,” graduate student Reuben Wang said. “We measure this signature to determine if the gas is interacting while not reacting. And it very nicely tallies with our theoretical prediction.” The researchers were excited to see the theory and experiment corroborate each other. “Those data are explained beautifully by Reuben's theories,” Bohn stated. “This is the first ultracold molecular gas in three dimensions where they saw evaporative cooling.” Bohn and the rest of the team looked forward to testing other characteristics of this gas.

A Valuable Collaboration

In looking back on the research, Bohn was grateful to have the partnership with Ye's laboratory. “That is one of the strengths of JILA as a whole, is a strong experimental-theory coupling. Not only is it a lot of fun, but it makes the science go faster.” Bohn has been at JILA long enough to understand the dynamics of this relationship. “Any number of times, an experimentalist will see something. And they burst into the theorist's office and say: ‘What is this?’” As a collaborating experimentalist, Li is thankful for the effort done by the theorists. “We achieve something in the lab, and then the theorists begin working on that. And then the theory predicts something even more interesting. And we use experiment to explore the theory. So, it's kind of a back-and-forth process of helping each other.” As the collaboration continues, the research becomes more rewarding, as both types of physicists share in their hard work to advance knowledge and develop new technology.

One of the major strengths of JILA are the frequent and ongoing collaborations between experimentalists and theorists, which have led to incredible discoveries in physics. One of these partnerships is between JILA Fellow John Bohn and JILA and NIST Fellow Jun Ye. Bohn's team of theorists has partnered with Ye's experimentalist laboratory for nearly twenty years, from the very beginning of Ye’s cold molecule research when he became a JILA Fellow. Recently in their collaborations, the researchers have been studying a three-dimensional molecular gas made of 40K87Rb molecules. In a paper published in Nature Physics, the combined team illustrated new quantum mechanical tricks in making this gas unreactive, thus enjoying a long life (for a gas), while at the same time letting the molecules in the gas interact and socialize (thermalize) with each other.

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From Liquid to Gas: A Way to study BEC

Steven Burrows — Wed, 29 Sep 2021 18:34:11 +0000

From Liquid to Gas: A Way to study BEC Steven Burrows Wed, 09/29/2021 - 12:34

Kenna Hughes-Castleberry / JILA Science Communicator

Model of the BEC interactions. Image credit: Steven Burrows / JILA

The Bose-Einstein Condensate (BEC) has been studied for decades, ever since its prediction by scientists Satyandra Nath Bose and Albert Einstein nearly 100 years ago. The BEC is a gas of atoms cooled to almost absolute zero. At low enough temperatures, quantum mechanics allows the locations of the atoms in the BEC to be uncertain to the extent that they can’t be located individually in the gas. The BEC has a special history with JILA, as it was at JILA that the first gaseous condensate was produced in 1995 by JILA Fellows Eric Cornell (NIST) and Carl Wieman (University of Colorado Boulder). The original BEC was a gas of rubidium atoms, but using other atoms, such as dysprosium, a BEC can also be produced that will have dipolar interactions. Unlike a normal BEC, the atoms in a dipolar BEC have two opposing poles, similar to the two ends of a magnet. This dipolar BEC has been shown to create self-binding droplets, which cohere even in the absence of an electric potential to hold them together. The non-polar BEC does not create these droplets. The droplets are of interest to physicists who have researched ways to describe the droplet’s energetic excitations.

Since 2005, research on dipolar BEC has continued, using different theories to describe the droplet’s interactions. In a paper recently published in Physical Review A, first author, and graduate student, Eli Halperin and JILA fellow John Bohn theorize a way to study the BEC using a hyperspherical approach. While the name may sound intimidating, the hyperspherical approach is simply a systematic way to look at a many-body problem. The many body problem refers to a large category of problems regarding microscopic systems with interacting particles. Bohn and Halperin applied this approach to a dipolar BEC specifically. When speaking of applying this approach, Halperin explained: "Twenty years ago, John wrote this paper applying the hyperspherical approach to a spherically symmetric system... I think the main advantage we get here is this nice linear way of looking at a many-body problem, without too much loss of accuracy." This approach could be used to help researchers describe the energies and wavefunctions of the BEC droplets using relatively straightforward quantum mechanics rather than more sophisticated quantum field theories.

As a theoretical physicist, Bohn was particularly interested in finding ways to describe interactions of the BEC. According to Bohn: "The problem is, whatever one atom is doing affects what all the other atoms are doing. So how do you keep track of them all? The way people normally solve this is to take a representative atom and follow it as it moves around in the background of other atoms that are, basically, exactly like this atom. Our approach with hyperspherical coordinates is to say, ‘There are a lot of atoms there, let's treat them all at once, as opposed to individually.’ So, we have a coordinate, the hyperradius, which is sort of the size of the whole droplet. It accounts for all atoms at the same time, leaves behind this independent particle picture, and tries to look at things in a collective way.” Bohn was not the first to use this approach. "We didn't invent this. Hyperspherical coordinates have been around since the 1930s. They're used all the time in chemistry and nuclear physics," explained Bohn. "And there's a vast literature out there that we basically stole to put to use for this purpose. And even then, it's hard to do." The hyperspherical approach, while effective in making a many-body problem easier to study for the BEC, is difficult to set up, but, as mentioned above, leads to interpretations of the results.

From Liquid to Gas:

While the hyperspherical approach has revealed a new way to look at BEC droplets, both Halperin and Bohn look forward to using it to further study the interactions of the BEC, particularly with its excitations. In clarifying research around the BEC, Bohn stated: "normally in Bose Einstein condensation, you have a trap where you hold the atoms in place with magnetic fields or optical fields. An experiment done in 2016 showed that if you tuned things just right, the dipoles in the gas align head to tail, which makes them attract each other. It turns out, there's a kind of quantum mechanical fluctuation effect that keeps them from crashing in on themselves. Under the right circumstances, you could turn off the trap, and the thing holds together in the middle of the experiment as a self-bound droplet, like a liquid droplet would. There's a transition that happens where this liquid droplet can transform into a gas. And what we're applying this method to is the transition between liquid to gas." Looking at the transition, both Halperin and Bohn hope to find more about the excitations of the BEC as it changes shape when oscillating between a liquid like droplet and gas.

Halperin explained that it was important to look at the energy states for both the droplet and the the BEC as they had similar energies but different excitations. "We think it might be possible to find a regime where you could get the gas to naturally oscillate between the droplet state and the gas state," Halperin said. "We're interested to see if you can first use the hyper-spherical approach and see this oscillation between the liquid and gas states. And then to test this also in a quantum field theory." Being able to describe the oscillation from liquid to gas using this approach would allow a better understanding of the BEC, specifically at a quantum level. According to Bohn: "You think of liquid and gas as an either-or situation. But in quantum mechanics, why can't it be both?" As the work continues, it will be interesting to see how the quantum interactions within the BEC build on the field of quantum physics.

The Bose-Einstein Condensate (BEC) has been studied for decades, ever since its prediction by scientists Satyandra Nath Bose and Albert Einstein nearly 100 years ago. The BEC is a gas of atoms cooled to almost absolute zero. At low enough temperatures, quantum mechanics allows the locations of the atoms in the BEC to be uncertain to the extent that they can’t be located individually in the gas. The BEC has a special history with JILA, as it was at JILA that the first gaseous condensate was produced in 1995 by JILA Fellows Eric Cornell (NIST) and Carl Wieman (University of Colorado Boulder). Since 2005, research on dipolar BEC has continued, using different theories to describe the droplet’s interactions. In a paper recently published in Physical Review A, first author, and graduate student, Eli Halperin and JILA fellow John Bohn theorize a way to study the BEC using a hyperspherical approach. While the name may sound intimidating, the hyperspherical approach is simply a systematic way to look at a many-body problem. The many body problem refers to a large category of problems regarding microscopic systems with interacting particles. Bohn and Halperin applied this approach to a dipolar BEC specifically.

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Quantum Adventures with Cold Molecules

Steven Burrows — Thu, 07 Sep 2017 17:31:35 +0000

Quantum Adventures with Cold Molecules Steven Burrows Thu, 09/07/2017 - 11:31

Julie Phillips / Science Communicator

Artist’s conception of ultracold potassium-rubidium (KRb) molecules pinned in individual optical lattice sites. JILA studies of KRb molecules have opened the door to controlling the internal quantum states of these molecules and understanding how they interact with each other. This work may one day lead to the design and fabrication of advanced quantum materials. Image credit: Steven Burrows / JILA

Researchers at JILA and around the world are starting a grand adventure of precisely controlling the internal and external quantum states of ultracold molecules after years of intense experimental and theoretical study. Such control of small molecules, which are the most complex quantum systems that can currently be completely understood from the principles of quantum mechanics, will allow researchers to probe the quantum interactions of individual molecules with other molecules, investigate what happens to molecules during collisions, and study how molecules behave in chemical reactions. Armed with such fundamental insights into the workings of molecules, researchers anticipate developing tools not only to control reaction chemistry, but also to design and manufacture advanced quantum materials.

With these goals in mind, Fellows John Bohn, Ana Maria Rey, and Jun Ye collaborated on a review article discussing how progress over the last dozen or so years in cold-molecule research by a large scientific community has laid the groundwork for the exquisite control of molecules and their interaction processes. The article, entitled “Cold Molecules: Progress in Quantum Engineering of Chemistry and Quantum Matter,” appeared online in Science on September 8, 2017.

“We can control the initial state of molecules and how they approach each other, monitor intermediate states, and analyze the end products,” explained Ye. “Being able to control these three steps in a state by state fashion gives you resolution limited only by quantum mechanics for the study of a molecular reaction process.” Ye said that it’s also possible to use ultracold (quantum) molecules to simulate quantum magnetism and study fundamental reaction processes in the quantum regime.

“We have now reached the stage where we have the capability to start controlling molecules,” Rey added. “Now we want to understand how they react and how they interact. And, this understanding is orienting us on a path to learning more about chemical reactions from start to finish.”

Bohn summed it up this way: “The gist of all this is that now we’re starting to get a handle on anything you may want to know about a chemical reaction.”

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