Research Highlights

​A team of physicists at the University of Colorado Boulder and the National Institute of Standards and Technology (NIST) has developed a groundbreaking laser-based device capable of analyzing gas samples to identify a vast array of molecules at extremely low concentrations, down to parts per trillion. Their findings were recently published in Nature. 

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.

Black holes have been fascinating subjects of study, not just because they are cosmic vacuum cleaners, but also as engines of immense power capable of extracting and redistributing energy on a staggering scale. These dark giants are often surrounded by swirling disks of gas and dust, known as accretion disks. When these disks are strongly magnetized, they can act like galactic power plants, extracting energy from the black hole’s spin in a process known as the Blandford-Znajek (BZ) effect.

While scientists have theorized that the BZ effect is the primary mechanism in the energy extraction process, many unknowns remain, like what determines how much energy is funneled into powerful jets—powerful streams of particles and energy ejected along the black hole's poles—or dissipated as heat.

To answer these questions, JILA postdoctoral researcher Prasun Dhang, and JILA Fellows and University of Colorado Boulder Astrophysical and Planetary Sciences professors Mitch Begelman and Jason Dexter, turned to advanced computer simulations. By modeling black holes surrounded by thin, highly magnetized accretion disks, they sought to uncover the underlying physics that drives these enigmatic systems. Their findings, published in The Astrophysical Journal, offer crucial insights into the complex physics around black holes and could redefine how we understand their role in shaping galaxies.

When atoms collide, their exact structure—for example, the number of electrons they have or even the quantum spin of their nuclei—has a lot to say about how they bounce off each other. This is especially true for atoms cooled to near-zero Kelvin, where quantum mechanical effects give rise to unexpected phenomena.  Collisions of these cold atoms can sometimes be caused by incoming laser light, resulting in the colliding atom-pair forming a short-lived molecular state before disassociating and releasing an enormous amount of energy. These so-called light-assisted collisions, which can happen very quickly, impact a broad range of quantum science applications, yet many details of the underlying mechanisms are not well understood. 

In a new study published in Physical Review Letters, JILA Fellow and University of Colorado Boulder physics professor Cindy Regal, along with former JILA Associate Fellow Jose D’Incao (currently an assistant professor of physics at the University of Massachusetts, Boston) and their teams developed new experimental and theoretical techniques for studying the rates at which light-assisted collisions occur in the presence of small atomic energy splittings.  Their results rely upon optical tweezers—focused lasers capable of trapping individual atoms—that the team used to isolate and study the products of individual pairs of atoms.

Researchers at the University of Colorado Boulder have developed a novel method to measure magnetic field orientations using atoms as minuscule compasses. The research, a collaboration between JILA Fellow and CU Boulder physics professor Cindy Regal and Svenja Knappe, a research professor in the Paul M. Rady Department of Mechanical Engineering, was recently published as the cover article in the journal Optica.

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.  

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. 

Ultrawide-bandgap semiconductors—such as diamond—are promising for next-generation electronics due to a larger energy gap between the valence and conduction bands, allowing them to handle higher voltages, operate at higher frequencies, and provide greater efficiency compared to traditional materials like silicon. However, their unique properties make it challenging to probe and understand how charge and heat move on nanometer-to-micron scales. Visible light has a very limited ability to probe nanoscale properties, and moreover, it is not absorbed by diamond, so it cannot be used to launch currents or rapid heating.

Now, researchers at JILA, led by JILA Fellows and University of Colorado physics professors Margaret Murnane and Henry Kapteyn, along with graduate students Emma Nelson, Theodore Culman, Brendan McBennett, and former JILA postdoctoral researchers Albert Beardo and Joshua Knobloch, have developed a novel microscope that makes examining these materials possible on an unprecedented scale. The team’s work, recently published in Physical Review Applied, introduces a tabletop deep-ultraviolet (DUV) laser that can excite and probe nanoscale transport behaviors in materials such as diamond. This microscope uses high-energy DUV laser light to create a nanoscale interference pattern on a material’s surface, heating it in a controlled, periodic pattern. Observing how this pattern fades over time provides insights into the electronic, thermal, and mechanical properties at spatial resolutions as fine as 287 nanometers, well below the wavelength of visible light. 

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). 

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

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

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

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

Physics lab courses are vital to science education, providing hands-on experience and technical skills that lectures can’t offer. Yet, it’s challenging for those in Physics Education Research (PER) to compare course to course, especially since these courses vary wildly worldwide. 

To better understand these differences, JILA Fellow and University of Colorado Boulder physics professor Heather Lewandowski and a group of international collaborators are working towards creating a global taxonomy, a classification system that could create a more equitable way to compare these courses. Their findings were recently published in Physical Review Physics Education Research.

JILA Fellow and NIST (National Institute of Standards and Technology) Physicist and University of Colorado Boulder Physics professor Adam Kaufman and his team have ventured into the minuscule realms of atoms and electrons. Their research involves creating an advanced optical atomic clock using a lattice of strontium atoms, enhanced by quantum entanglement—a phenomenon that binds the fate of particles together. This ambitious project could revolutionize timekeeping, potentially surpassing the "standard quantum limit" of precision. 

In collaboration with JILA and NIST Fellow Jun Ye, the team highlighted their findings in Nature, demonstrating how their clock, operating under certain conditions, could exceed conventional accuracy benchmarks. Their work advances timekeeping and opens doors to new quantum technologies, such as precise environmental sensors.

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.

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. 

Many quantum devices, from quantum sensors to quantum computers, use ions or charged atoms trapped with electric and magnetic fields as a hardware platform to process information. 

However, current trapped-ion systems face important challenges. Most experiments are limited to one-dimensional chains or two-dimensional planes of ions, which constrain the scalability and functionality of quantum devices. Scientists have long dreamed of stacking these ions into three-dimensional structures, but this has been very difficult because it’s hard to keep the ions stable and well-controlled when arranged in more complex ways.

To address these challenges, an international collaboration of physicists from India, Austria, and the USA—including JILA and NIST Fellow Ana Maria Rey, along with NIST scientists Allison Carter and John Bollinger—proposed that tweaking the electric fields that trap the ions can create stable, multilayered structures, opening up exciting new possibilities for future quantum technologies. The researchers published their findings in Physical Review X.
 

In the quiet halls of the Duane Physics building at the University of Colorado Boulder, two JILA researchers, postdoctoral research associate Catie LeDesma and graduate student Kendall Mehling, combine machine learning with atom interferometry to create the next generation of quantum sensors. Because these quantum sensors can be applied to various fields, from satellite navigation to measuring Earth’s composition, any advancement has major implications for numerous industries. 

As reported in a recent article preprint, the researchers successfully demonstrated how to build a quantum sensor using atoms moving through crystals made entirely of laser light. They applied accelerated forces to atoms along multiple directions and, using this sensor, measured the results, which closely matched values predicted by quantum theory. LeDesma and Mehling also showed that their device could accurately detect accelerations from just one run of their experiment, a feat that is very difficult to accomplish with traditional cold atom interferometry. 

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.

One of the biggest challenges in quantum technology and quantum sensing is “noise”–seemingly random environmental disturbances that can disrupt the delicate quantum states of qubits, the fundamental units of quantum information. Looking deeper at this issue, JILA Associate Fellow and University of Colorado Boulder Physics assistant professor Shuo Sun recently collaborated with Andrés Montoya-Castillo, assistant professor of chemistry (also at CU Boulder), and his team to develop a new method for better understanding and controlling this noise, potentially paving the way for significant advancements in quantum computing, sensing, and control. Their new method, which uses a mathematical technique called a Fourier transform, was published recently in the journal npj Quantum Information. 

In daily life, when two objects are “indistinguishable,” it’s due to an imperfect state of knowledge. As a street magician scrambles the cups and balls, you could, in principle, keep track of which ball is which as they are passed between the cups. However, at the smallest scales in nature, even the magician cannot tell one ball from another. True indistinguishability of this type can fundamentally alter how the balls behave. For example, in a classic experiment by Hong, Ou, and Mandel, two identical photons (balls) striking opposite sides of a half-reflective mirror are always found to exit from the same side of the mirror (in the same cup). This results from a special kind of interference, not any interaction between the photons. With more photons, and more mirrors, this interference becomes enormously complicated.

Measuring the pattern of photons that emerges from a given maze of mirrors is known as “boson sampling.” Boson sampling is believed to be infeasible to simulate on a classical computer for more than a few tens of photons. As a result, there has been a significant effort to perform such experiments with actual photons and demonstrate that a quantum device is performing a specific computational task that cannot be performed classically. This effort has culminated in recent claims of quantum advantage using photons.

Now, in a recently published Nature paper, JILA Fellow and NIST Physicist and University of Colorado Boulder Physics Professor Adam Kaufman and his team, along with collaborators at NIST (the National Institute of Standards and Technology), have demonstrated a novel method of boson sampling using ultracold atoms (specifically, bosonic atoms) in a two-dimensional optical lattice of intersecting laser beams. 

Dead stars known as white dwarfs, have a mass like the Sun while being similar in size to Earth. They are common in our galaxy, as 97% of stars are white dwarfs. As stars reach the end of their lives, their cores collapse into the dense ball of a white dwarf, making our galaxy seem like an ethereal graveyard. 

Despite their prevalence, the chemical makeup of these stellar remnants has been a conundrum for astronomers for years. The presence of heavy metal elements—like silicon, magnesium, and calcium—on the surface of many of these compact objects is a perplexing discovery that defies our expectations of stellar behavior. 

“We know that if these heavy metals are present on the surface of the white dwarf, the white dwarf is dense enough that these heavy metals should very quickly sink toward the core,” explains JILA graduate student Tatsuya Akiba. “So, you shouldn't see any metals on the surface of a white dwarf unless the white dwarf is actively eating something.” 

While white dwarfs can consume various nearby objects, such as comets or asteroids (known as planetesimals), the intricacies of this process have yet to be fully explored. However, this behavior could hold the key to unraveling the mystery of a white dwarf's metal composition, potentially leading to exciting revelations about white dwarf dynamics. 

In results reported in a new paper in The Astrophysical Journal Letters, Akiba, along with JILA Fellow and University of Colorado Boulder Astrophysical and Planetary Sciences professor Ann-Marie Madigan and undergraduate student Selah McIntyre, believe they have found a reason why these stellar zombies eat their nearby planetesimals. Using computer simulations, the researchers simulated the white dwarf receiving a “natal kick” during its formation (which has been observed) caused by asymmetric mass loss, altering its motion and the dynamics of any surrounding material.