Research Highlights

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

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

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

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

In the quest for ultra-precise timekeeping, scientists have turned to nuclear clocks. Unlike optical atomic clocks—which rely on electronic transitions—nuclear clocks utilize the energy transitions in the atom’s nucleus, which are less affected by outside forces, meaning this type of clock could potentially keep time more accurately than any previously existing technology. 

However, building such a clock has posed major challenges—thorium-229, one of the isotopes used in nuclear clocks, is rare, radioactive, and extremely costly to acquire in the substantial quantities required for this purpose.

Reported recently in a new study published in Nature, a team of researchers, led by JILA and NIST Fellow and University of Colorado Boulder Physics professor Jun Ye, in collaboration with Professor Eric Hudson’s team at UCLA’s Department of Physics and Astronomy, have found a way to make nuclear clocks a thousand times less radioactive and more cost-effective, thanks to a method creating thin films of thorium tetrafluoride (ThF₄). 

The interactions between quantum spins underlie some of the universe’s most interesting phenomena, such as superconductors and magnets. However, physicists have difficulty engineering controllable systems in the lab that replicate these interactions.

Now, in a recently published Nature paper, JILA and NIST Fellow and University of Colorado Boulder Physics Professor Jun Ye and his team, along with collaborators in Mikhail Lukin’s group at Harvard University, used periodic microwave pulses in a process known as Floquet engineering, to tune interactions between ultracold potassium-rubidium molecules in a system appropriate for studying fundamental magnetic systems. Moreover, the researchers observed two-axis twisting dynamics within their system, which can generate entangled states for enhanced quantum sensing in the future. 

An international team of researchers, led by JILA and NIST Fellow and University of Colorado Boulder Physics Professor Jun Ye and his team, has made significant strides in developing a groundbreaking timekeeping device known as a nuclear clock. Their results have been published in the cover article of Nature. 

JILA and NIST Fellow and University of Colorado Boulder Physics professor Jun Ye and his team at JILA, a collaboration between NIST and the University of Colorado Boulder, have developed an atomic clock of unprecedented precision and accuracy. This new clock uses an optical lattice to trap thousands of atoms with visible light waves, allowing for exact measurements. It promises vast improvements in fields such as space navigation, particle searches, and tests of fundamental theories like general relativity.

In a new study published in Science today, JILA and NIST (National Institute of Standards and Technology) Fellow and University of Colorado Boulder physics professor Jun Ye and his research team have taken a significant step in understanding the intricate and collective light-atom interactions within atomic clocks, the most precise clocks in the universe. 

Within atomic and laser physics communities, scientist John “Jan” Hall is a key figure in the history of laser frequency stabilization and precision measurement using lasers. Hall's work revolved around understanding and manipulating stable lasers in ways that were revolutionary for their time. His work laid a technical foundation for measuring a tiny fractional distance change brought by a passing gravitational wave. His work in laser arrays awarded him the Nobel Prize in Physics in 2005. 

Building on this foundation, JILA and NIST Fellow Jun Ye and his team embarked on an ambitious journey to push the boundaries of precision measurement even further. This time, their focus turned to a specialized technique known as the Pound-Drever-Hall (PDH) method (developed by scientists R. V. Pound, Ronald Drever, and Jan Hall himself), which plays a large role in precision optical interferometry and laser frequency stabilization.

While physicists have used the PDH method for decades in ensuring their laser frequency is stably “locked” to an artificial or quantum reference, a limitation arising from the frequency modulation process itself, called residual amplitude modulation (RAM), can still affect the stability and accuracy of the laser’s measurements. 

In a new Optica paper, Ye’s team, working with JILA electronic staff member Ivan Ryger and Hall, describe implementing a new approach for the PDH method, reducing RAM to never-before-seen minimal levels while simultaneously making the system more robust and simpler. 

Historically, JILA (a joint institute established by the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder) has been a world leader in precision timekeeping using optical atomic clocks. These clocks harness the intrinsic properties of atoms to measure time with unparalleled precision and accuracy, representing a significant leap in our quest to quantify the most elusive of dimensions: time.

However, the precision of these clocks has fundamental limits, including the “noise floor,” which is affected by the “quantum projection noise” (QPN). “This comes from the spin-statistics of the individual qubits, the truly quantum nature of the atoms being probed,” elaborated JILA graduate student Maya Miklos. State-of-the-art clock comparisons, like those directed by JILA and NIST Fellow and University of Colorado Boulder Physics professor Jun Ye, are pushing ever closer to this fundamental noise floor limit. However, this limit can be circumvented by generating quantum entanglement in the atomic samples, boosting their stability.

Now, Ye’s team, in collaboration with JILA and NIST Fellow James K. Thompson, has used a specific process known as spin squeezing to generate quantum entanglement, resulting in an enhancement in clock performance operating at the 10-17stability level. Their novel experimental setup, published in Nature Physics, also allowed the researchers to directly compare two independent spin-squeezed ensembles to understand this level of precision in time measurement, a level never before reached with a spin-squeezed optical lattice clock. 

In a recent Science paper, researchers led by JILA and NIST Fellow Jun Ye, along with collaborators JILA and NIST Fellow David Nesbitt, scientists from the University of Nevada, Reno, and Harvard University, observed novel ergodicity-breaking in C60, a highly symmetric molecule composed of 60 carbon atoms arranged on the vertices of a “soccer ball” pattern (with 20 hexagon faces and 12 pentagon faces). Their results revealed ergodicity breaking in the rotations of C60. Remarkably, they found that this ergodicity breaking occurs without symmetry breaking and can even turn on and off as the molecule spins faster and faster. Understanding ergodicity breaking can help scientists design better-optimized materials for energy and heat transfer. 

Many everyday systems exhibit “ergodicity” such as heat spreading across a frying pan and smoke filling a room. In other words, matter or energy spreads evenly over time to all system parts as energy conservation allows. On the other hand, understanding how systems can violate (or “break”) ergodicity, such as magnets or superconductors, helps scientists understand and engineer other exotic states of matter.

Some of the biggest questions about our universe may be solved by scientists using its tiniest particles. Since the 1960s, physicists have been looking at particle interactions to understand an observed imbalance of matter and antimatter in the universe. Much of the work has focused on interactions that violate charge and parity (CP) symmetry. This symmetry refers to a lack of change in our universe if all particles’ charges and orientations were inverted. “This charge and parity symmetry is the symmetry that high-energy physicists say needs to be violated to result in this imbalance between matter and antimatter,” explained JILA research associate Luke Caldwell. To try to find evidence of this violation of CP symmetry, JILA and NIST Fellows Jun Ye and Eric Cornell, and their teams, including Caldwell, collaborated to measure the electron electric dipole moment (eEDM), which is often used as a proxy measure for the CP symmetry violation. The eEDM is an asymmetric distortion of the electron’s charge distribution along the axis of its spin. To try to measure this distortion, the researchers used a complex setup of lasers and a novel ion trap. Their results, published in Science as the cover story and Physical Review A, leveraged a longer experiment time to improve the precision measurement by a factor of 2.4, setting new records. 

JILA researchers have upgraded a breathalyzer based on Nobel Prize-winning frequency-comb technology and combined it with machine learning to detect SARS-CoV-2 infection in 170 volunteer subjects with excellent accuracy. Their achievement represents the first real-world test of the technology’s capability to diagnose disease in exhaled human breath.

Quantum gases of interacting molecules can exhibit unique dynamics. JILA and NIST Physicist Jun Ye has spent years of research to reveal, probe, and control these dynamics with potassium-rubidium molecules. In a new article published in Nature, Ye and his team of researchers describe having combined two threads of previous research—spin and motional dynamics—to reveal rich many-body and collisional physics that are controllable in the laboratory. 

Atomic clocks are essential in building a precise time standard for the world, which is a big focus for researchers at JILA. JILA and NIST Fellow Jun Ye, in particular, has studied atomic clocks for two decades, looking into ways to increase their sensitivity and accuracy. In a new paper published in Science Advances, Ye and his team collaborated with JILA and NIST Fellow Ana Maria Rey and her team to engineer a new design of clock, which demonstrated better theoretical understanding and experimental control of atomic interactions, leading to a breakthrough in the precision achievable in state-of-the-art optical atomic clocks.

More than 400 years later, scientists are in the midst of an equally-important revolution. They’re diving into a previously-hidden realm—far wilder than anything van Leeuwenhoek, known as the “father of microbiology,” could have imagined. Some researchers, like physicists Margaret Murnane and Henry Kapteyn, are exploring this world of even tinier things with microscopes that are many times more precise than the Dutch scientist’s. Others, like Jun Ye, are using lasers to cool clouds of atoms to just a millionth of a degree above absolute zero with the goal of collecting better measurements of natural phenomena. 

Physics has always been a science of rules. In many situations, these rules lead to clear and simple theoretical predictions which, nevertheless, are hard to observe in actual experimental settings where other confounding effects may obscure the desired phenomena. For JILA and NIST Fellows Ana Maria Rey and Jun Ye, one type of phenomena they are especially interested in observing are the interactions between light and atoms, especially those at the heart of the decay of an atom prepared in the excited state. “If you have an atom in the excited state, the atom will eventually decay to the ground state while emitting a photon,” explained Rey. “This process is called spontaneous emission.” The spontaneous emission rate can be manipulated by scientists, making it longer or shorter, depending on the experimental conditions. Many years ago it was predicted that one way to suppress or slow down spontaneous emission was by applying a special type of statistics known as Fermi statistics which prevents two identical fermions from being in the same quantum state, known as the Pauli Exclusion Principle

This principle is similar to a game of Duck, Duck, Goose, where two individuals fight over an open spot in a circle in order to avoid being “it.” Like children in this game, the atoms must find an empty quantum state to decay into. If they cannot find an empty state, interesting things begin to happen. “If an excited atom wants to decay, but the ground state is already filled, then the decay is “Pauli blocked” and the atom will stay in the excited state longer, or even forever,” Rey said. Nevertheless, the experimental observation of this effect happened to be challenging.  It was not until last year  that the Ye group observed Pauli blocking of radiation for the first time indirectly by measuring the light scattered by the atoms—but a direct observation of Pauli blocking by measuring  the lifetime of atoms in the steady state was lacking. More recently, Ye’s and Rey’s groups collaborated in a joint study, and were able to find an appropriate experimental setting where they were able to observe Pauli blocking of spontaneous emission by direct measurements of the excited state population. The results have been published in the journal Physical Review Letters. 

Worldwide, many researchers are interested in controlling atomic and molecular interactions. This includes JILA and NIST fellows Jun Ye and Ana Maria Rey, both of whom have spent years researching interacting potassium-rubidium (KRb) molecules, which were originally created in a collaboration between Ye and the late Deborah Jin. In the newest collaboration between the experimental (Ye) and theory (Rey) groups, the researchers have developed a new way to control two-dimensional gaseous layers of molecules, publishing their exciting new results in the journal Science.

JILA physicists have measured Albert Einstein’s theory of general relativity, or more specifically, the effect called time dilation, at the smallest scale ever, showing that two tiny atomic clocks, separated by just a millimeter or the width of a sharp pencil tip, tick at different rates.

The experiments, described in the Feb. 17 issue of Nature, suggest how to make atomic clocks 50 times more precise than today’s best designs and offer a route to perhaps revealing how relativity and gravity interact with quantum mechanics, a major quandary in physics.
 

Last week, U.S. Rep. Joe Neguse got a first-hand look at the future of ultrafast lasers, record-setting clocks, and quantum computers on the CU Boulder campus. Neguse visited the university Thursday to tour facilities at JILA, a research partnership between CU Boulder and the National Institute of Standards and Technology (NIST).

How atoms interact with light reflects some of the most basic principles in physics. On a quantum level, how atoms and light interact has been a topic of interest in the worldwide scientific community for many years. Light scattering is a process where incoming light excites an atom to a higher-lying energy state from which it subsequently decays back to its ground state by reemitting a quantum of light. In the quantum realm, there are many factors that affect light scattering. In a new paper published in Science, JILA and NIST Fellow Jun Ye and his laboratory members report on how light scattering is affected by the quantum nature of the atoms, more specifically, thequantum statistical rule such as the Pauli Exclusion Principle.

The second quantum revolution is underway, a period marked by significant advances in quantum technology, and huge discoveries within quantum science. From tech giants like Google and IBM, who build their own quantum computers, to quantum network startups like Aliro Quantum, companies are eager to profit from this revolution. However, doing so takes a new type of workforce, one trained in quantum physics and quantum technology. The skillset required for this occupation is unique, and few universities expose students to real-world quantum technology.