Research Highlights

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. 

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.

While it may not look like it, the interstellar space between stars is far from empty. Atoms, ions, molecules, and more reside in this ethereal environment known as the Interstellar Medium (ISM). The ISM has fascinated scientists for decades, as at least 200 unique molecules form in its cold, low-pressure environment. It’s a subject that ties together the fields of chemistry, physics, and astronomy, as scientists from each field work to determine what types of chemical reactions happen there. 

Now, in the recently published cover article of the Journal of Physical Chemistry A, JILA Fellow and University of Colorado Boulder Physics Professor Heather Lewandowski and former JILA graduate student Olivia Krohn highlight their work to mimic ISM conditions by using Coulomb crystals, a cold pseudo-crystalline structure, to watch ions and neutral molecules interact with each other. 

While atomic clocks are already the most precise timekeeping devices in the universe, physicists are working hard to improve their accuracy even further. One way is by leveraging spin-squeezed states in clock atoms. Spin-squeezed states are entangled states in which particles in the system conspire to cancel their intrinsic quantum noise. These states, therefore, offer great opportunities for quantum-enhanced metrology since they allow for more precise measurements. Yet, spin-squeezed states in the desired optical transitions with little outside noise have been hard to prepare and maintain. 

One particular way to generate a spin-squeezed state, or squeezing, is by placing the clock atoms into an optical cavity, a set of mirrors where light can bounce back and forth many times. In the cavity, atoms can synchronize their photon emissions and emit a burst of light far brighter than from any one atom alone, a phenomenon referred to as superradiance. Depending on how superradiance is used, it can lead to entanglement, or alternatively, it can instead disrupt the desired quantum state. 

In a prior study, done in a collaboration between JILA and NIST Fellows, Ana Maria Rey and James Thompson, the researchers discovered that multilevel atoms (with more than two internal energy states) offer unique opportunities to harness superradiant emission by instead inducing the atoms to cancel each other’s emissions and remain dark. 

Now, reported in a pair of new papers published in Physical Review Letters and Physical Review A, Rey and her team discovered a method for how to not only create dark states in a cavity, but more importantly, make these states spin squeezed. Their findings could open remarkable opportunities for generating entangled clocks, which could push the frontier of quantum metrology in a fascinating way. 

Astronomers have long sought to understand the early universe, and thanks to the James Webb Space Telescope (JWST), a critical piece of the puzzle has emerged. The telescope's infrared detecting “eyes” have spotted an array of small, red dots, identified as some of the earliest galaxies formed in the universe. 

This surprising discovery is not just a visual marvel, it's a clue that could unlock the secrets of how galaxies and their enigmatic black holes began their cosmic journey.
“The astonishing discovery from James Webb is that not only does the universe have these very compact and infrared bright objects, but they're probably regions where huge black holes already exist,” explains JILA Fellow and University of Colorado Boulder astrophysics professor Mitch Begelman. “That was thought to be impossible.” 

Begelman and a team of other astronomers, including Joe Silk, a professor of astronomy at Johns Hopkins University, published their findings in The Astrophysical Journal Letters, suggesting that new theories of galactic creation are needed to explain the existence of these huge black holes. 

When it comes to drug development, membrane proteins play a crucial role, with about 50% of drugs targeting these molecules. Understanding the function of these membrane proteins, which connect to the membranes of cells, is important for designing the next line of powerful drugs. To do this, scientists study model proteins, such as bacteriorhodopsin (bR), which, when triggered by light, pump protons across the membrane of cells. 

While bR has been studied for half a century, physicists have recently developed techniques to observe its folding mechanisms and energetics in the native environment of the cell’s lipid bilayer membrane. In a new study published by Proceedings of the National Academy of Sciences (PNAS), JILA and NIST Fellow Thomas Perkins and his team advanced these methods by combining atomic force microscopy (AFM), a conventional nanoscience measurement tool, with precisely timed light triggers to study the functionality of the protein function in real-time. 

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. 

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. 

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 new study published in Scientific Reports, JILA Fellow and University of Colorado Boulder physics professor Andreas Becker and his team theorized a new method to produce extreme ultraviolet (EUV) and x-ray light with elliptical polarization, a special shape in which the direction of light waves’ oscillation is changing. This method could provide experimentalists with a simple technique to generate such light, which is beneficial for physicists to further understand the interactions between electrons in materials on the quantum level, paving the way for designing better electronic devices such as circuit boards, solar panels, and more.

Deep within every piece of magnetic material, electrons dance to the invisible tune of quantum mechanics. Their spins, akin to tiny atomic tops, dictate the magnetic behavior of the material they inhabit. This microscopic ballet is the cornerstone of magnetic phenomena, and it's these spins that a team of JILA researchers—headed by JILA Fellows and University of Colorado Boulder physics professors Margaret Murnane and Henry Kapteyn—has learned to control with remarkable precision, potentially redefining the future of electronics and data storage. 

As reported in a new Science Advances paper, the JILA team and collaborators from universities in Sweden, Greece, and Germany probed the spin dynamics within a special material known as a Heusler compound: a mixture of metals that behaves like a single magnetic material. For this study, the researchers utilized a compound of cobalt, manganese, and gallium, which behaved as a conductor for electrons whose spins were aligned upwards and as an insulator for electrons whose spins were aligned downwards.

Using a form of light called extreme ultraviolet high-harmonic generation (EUV HHG) as a probe, the researchers could track the re-orientations of the spins inside the compound after exciting it with a femtosecond laser, which caused the sample to change its magnetic properties. The key to accurately interpreting the spin re-orientations was the ability to tune the color of the EUV HHG probe light.
“In the past, people haven't done this color tuning of HHG,” explained co-first author and JILA graduate student Sinéad Ryan. “Usually, scientists only measured the signal at a few different colors, maybe one or two per magnetic element at most.” In a historic first, the JILA team tuned their EUV HHG light probe across the magnetic resonances of each element within the compound to track the spin changes with a precision down to femtoseconds (a quadrillionth of a second).

“On top of that, we also changed the laser excitation fluence, so we were changing how much power we used to manipulate the spins,” Ryan elaborated, highlighting that that step was also an experimental first for this type of research. By changing the power, the researchers could influence the spin changes within the compound.

When measuring minor changes for quantities like forces, magnetic fields, masses of small particles, or even gravitational waves, physicists use micro-mechanical resonators, which act like tuning forks, resonating at specific frequencies. Traditionally, it was assumed that the temperature across these devices is uniform. 

However, new research from JILA Fellow and University of Colorado Boulder physics professor Cindy Regal and her team, Dr. Ravid Shaniv and graduate student Chris Reetz has found that in specific scenarios, such as advanced studies looking at the interactions between light and mechanical objects, where the temperature might differ in various resonator parts, which lead to unexpected behaviors. Their observations, published in Physical Review Research, can potentially revolutionize the design of micro-mechanical resonators for quantum technology and precision sensing.

Quantum sensors help physicists understand the world better by measuring time passage, gravity fluctuations, and other effects at the tiniest scales. For example,  one quantum sensor, the LIGO gravitational wave detector, uses quantum entanglement (or the interdependence of quantum states between particles) within a laser beam to detect distance changes in gravitational waves up to one thousand times smaller than the width of a proton! 

LIGO isn’t the only quantum sensor harnessing the power of quantum entanglement. This is because entangled particles are generally more sensitive to specific parameters, giving more accurate measurements. 

While researchers can generate entanglement between particles, the entanglement may only be useful sometimes for sensing something of interest. To measure the “usefulness” of quantum entanglement for quantum sensing, physicists calculate a mathematical value, known as the Quantum Fisher Information (QFI), for their system. However, physicists have found that the more quantum states in the system, the harder it becomes to determine which QFI to calculate for each state. 

To overcome this challenge, JILA Fellow Murray Holland and his research team proposed an algorithm that uses the Quantum Fisher Information Matrix (QFIM), a set of mathematical values that can determine the usefulness of entangled states in a complicated system. 

Their results, published in Physical Review Letters as an Editor’s Suggestion, could offer significant benefits in developing the next generation of quantum sensors by acting as a type of “shortcut” to find the best measurements without needing a complicated model.

In quantum information science, many particles can act as “bits,” from individual atoms to photons. At JILA, researchers utilize these bits as “qubits,” storing and processing quantum 1s or 0s through a unique system. 

While many JILA Fellows focus on qubits found in nature, such as atoms and ions, JILA Associate Fellow and University of Colorado Boulder Assistant Professor of Physics Shuo Sun is taking a different approach by using “artificial atoms,” or semiconducting nanocrystals with unique electronic properties. By exploiting the atomic dynamics inside fabricated diamond crystals, physicists like Sun can produce a new type of qubit, known as a “solid-state qubit,” or an artificial atom.

To study nanoscale patterns in tiny electronic or photonic components, a new method based on lensless imaging allows for near-perfect high-resolution microscopy. This is especially important at wavelengths shorter than ultraviolet, which can image with higher spatial resolution than visible light but where image-forming optics are imperfect. 

The most powerful form of lensless imaging is called ptychography, which works by scanning a laser-like beam across a sample, collecting the scattered light, and then using a computer algorithm to reconstruct an image of the sample. 

While ptychography can visualize many nanostructures, this special microscope has trouble analyzing samples with very regular, repeating patterns. This is because the scattered light does not change as a periodic sample is scanned, so the computer algorithm gets confused and cannot reconstruct a good image.

Taking on this challenge, recently graduated Ph.D. researchers Bin Wang and Nathan Brooks, working with JILA Fellows Margaret Murnane and Henry Kapteyn, developed a novel method that uses short-wavelength light with a special vortex or donut shape to scan these repeating surfaces, resulting in more varied diffraction patterns. This allowed the researchers to capture high-fidelity image reconstructions using this new approach, which they recently published in Optica. This result will also be highlighted in the Optica Magazine Optics and Photonics News in the annual highlights of Optics in 2023. 

Opening new possibilities for quantum sensors, atomic clocks and tests of fundamental physics, JILA researchers have developed new ways of “entangling” or interlinking the properties of large numbers of particles. In the process they have devised ways to measure large groups of atoms more accurately even in disruptive, noisy environments. 

The new techniques are described in a pair of papers published in Nature. JILA is a joint institute of the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder. 

To engineer materials with unique properties, like superconductivity, scientists dive into the quantum interactions between electrons and vibrational particles called phonons. When electrons and phonons strongly interact, they behave as “quasi” particles, not single isolated particles. These interactions occur on extremely short timescales: electrons interact with each other in femtoseconds (10^-15 seconds) or even faster, while phonons respond more slowly, within hundreds of femtoseconds, since the heavier atoms move more slowly than electrons. 

To investigate these interactions, scientists often change a material's temperature, pressure, or chemical composition and measure its electrical properties to learn about the interactions. However, materials that host different interactions can exhibit very similar properties, making it challenging to pinpoint the exact nature of these interactions.

To overcome this issue, JILA graduate student Yingchao Zhang, working with JILA Fellows Henry Kapteyn and Margaret Murnane and University of Colorado Boulder physics professor Rahul Nandkishore, utilized a powerful new method to precisely identify phonon interactions within quantum materials, the results of which were published in Nano Letters. Using ultraprecise, timed laser pulses, and extreme ultraviolet pulses, they measured the response times and saw precisely how the electrons and phonons interacted.  This method paves the way for better control and manipulation of quantum materials.

In a new paper in The Astrophysical Journal, JILA Fellow Jason Dexter, graduate student Kirk Long, and other collaborators compared two main theoretical models for emission data for a specific quasar, 3C 273. Using these theoretical models, astrophysicists like Dexter can better understand how these quasars form and change over time.

Quasars, or active galactic nuclei (AGN), are believed to be powered by supermassive black holes at their centers. Among the brightest objects in the universe, quasars emit a brilliant array of light across the electromagnetic spectrum. This emission carries vital information about the nature of the black hole and surrounding regions, providing clues that astrophysicists can exploit to better understand the black hole's dynamics. 

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.