Research Highlights

When it comes to creating ever more intriguing quantum systems, a constant need is finding new ways to observe them in a wide range of physical scenarios.  JILA Fellow Cindy Regal and JILA and NIST Fellow Ana Maria Rey have teamed up with Oriol Romero-Isart, a professor at the University of Innsbruck and IQOQI (Institute for Quantum Optics and Quantum Information) to show that a trapped particle in the form of an atom readily reveals its full quantum state with quite simple ingredients, opening up opportunities for studies of the quantum state of ever larger particles.

There are many methods to determine what the limits are for certain processes. Many of these methods look to reach the upper and lower bounds to identify them for making accurate measurements and calculations. In the growing field of quantum sensing, these limits have yet to be found.  That may change, thanks to research done by JILA Fellow Graeme Smith and his research team, with JILA and NIST Fellow James Thompson In a new study published in Physical Review Applied, the JILA and NIST researchers collaborated with scientists at the quantum company Quantinuum (previously Honeywell Quantum Solutions) to try and identify the upper limits of quantum sensing.

At ultra-cold temperatures, quantum mechanics dictate how particles bump into each other. The collisions depend both on the quantum statistics of the colliding partners (their location within the medium) and on their collisional energy and angular momentum.  The angular momentum of the particles creates an energy barrier, a field of energy that prevents two molecules from interacting, and which can also affect particle dynamics in the quantum realm. The two main types of interactions at the quantum level are s-waves and p-waves. S-wave types of collisions happen naturally between fermions when they exist together in two different internal states and happen with zero angular momenta, which creates a low energy barrier. That means that atoms can collide “head-on.” S-wave collisions have been very well studied and characterized.  However, quantum statistics prevents identical fermions (those having the same internal state) to collide via s-wave interactions, instead forcing them to interact via the so-called “p-wave” channel. 

However, quantum statistics prevents identical fermions (having the same internal state) to collide via s-wave interactions, instead forcing them to interact via the so-called “p-wave” channel.  In contrast with s-wave interactions, p-wave interactions are penalized by the aforementioned energy barrier.In order to collide, particles need to carry a non-zero angular momentum in order to overcome that barrier—they need to spin around each other, like a pair of dancers. The net angular momentum of the partners can give rise to rich quantum behaviors and phases of matter that have been intensively sought in real materials and cold atoms, but which have not yet been found. Besides the energy barrier, the dynamics of three-body recombination, which involves interactions when three atoms are present rather than two, can make it complicated to study p-wave interactions in an isolated space. To overcome these problems, and to measure coherent p-wave interactions between two particles for the first time, JILA and NIST Fellow Ana Maria Rey and her group, together with JILA theorist Jose D’Incao, collaborated with the University of Toronto experimentalist team led by Joseph Thywissen. They devised a method to isolate pairs of atoms in an optical lattice, a web of laser light that helps isolate and control particle interactions, then gave the particles the necessary angular momentum, or twist, for the atoms to collide via p-wave using specific laser beam frequencies. This resulted in the first observation of p-wave interactions in an experiment. The researchers have published their findings in the journal Nature.

Sitting 150 million kilometers away from the Earth, the Sun produces puzzling phenomena, like solar flares, that physicists are working to understand. One of these puzzles involves the Sun's tachocline, a belt of heat transition. “A tachocline is when the radiative interior of a star rotates like a solid ball, but the convection zone [an unstable outer heat layer in a star] rotates differently,” explained former JILA graduate student Loren Matilsky. “For geometric reference in the Sun, the outer 30% by radius is the convection zone, and the inner 70% by radius is the radiative interior.” Before leaving JILA to become a postdoctoral researcher at the University of California Santa Cruz, Matilsky collaborated with JILA Fellow Juri Toomre and his group at JILA to study the Sun's tachocline using computer simulations. In a new paper published in The Astrophysical Journal Letters, Matilsky and Toomre developed a new type of simulation, one where the tachocline is self-consistent and not artificially enforced, meaning that it arises on its own. According to Matilsky: “As far as we know, it's the first time this type of self-consistent tachocline behavior has been published for a fully nonlinear fluid dynamical global simulation.”

Atomic clocks are crucial for everyday living as they help our telecommunications, electrical power grids, GPS systems, transportation, and other processes around the world keep precise time. Some of these clocks use lasers and special resonator cavities to measure time intervals. They are some of the most accurate clocks in the world and the most fragile. The cesium atomic clocks play a consequential role, as a specific atomic transition induced in the atomic cesium is used to define the unit of time: the SI second.  The National Institute of Standards and Technology (NIST) laboratories in Boulder, Colorado have housed atomic clocks—including the cesium atomic clock NIST-F1 which serves as the United States' primary time and frequency standard—for decades, as researchers continue to improve the clocks' accuracies through cutting-edge research. For the NIST-F1 cesium clock specifically, this process has included rebuilding parts of the clock. 

JILA and NIST Fellow James K. Thompson’s team of researchers have for the first time successfully combined two of the “spookiest” features of quantum mechanics to make a better quantum sensor:  entanglement between atoms and delocalization of atoms.  Einstein originally referred to entanglement as creating spooky action at a distance—the strange effect of quantum mechanics in which what happens to one atom somehow influences another atom somewhere else. Entanglement is at the heart of hoped-for quantum computers, quantum simulators and quantum sensors.  A second rather spooky aspect of quantum mechanics is delocalization, the fact that a single atom can be in more than one place at the same time.  As described in their paper recently published in Nature, the Thompson group has combined the spookiness of both entanglement and delocalization to realize a matter-wave interferometer that can sense accelerations with a precision that surpasses the standard quantum limit (a limit on the accuracy of an experimental measurement at a quantum level) for the first time.  By doubling down on the spookiness, future quantum sensors will be able to provide more precise navigation, explore for needed natural resources, more precisely determine fundamental constants such as the fine structure and gravitational constants, look more precisely for dark matter, or maybe even one day detect gravitational waves.

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.

Of all the atoms that quantum physicists study, alkaline atoms hold a special place due to their unique structure. Found in the second column of the periodic table, these atoms have two outer electrons, allowing the atoms to interact with one another in intriguing ways. “They have received a lot of attention in recent years among the physics community because of two reasons,” explained JILA and NIST Fellow Ana Maria Rey. “One is that they have a unique atomic structure, which makes them ideal for atomic clocks. This is because they have a long-lived electronic excited state that can live longer than 100 seconds. The second is that their electronic and nuclear spin degrees of freedom are highly decoupled and therefore the nuclear spins do not participate in the atomic collisions.”

Like planets orbiting the sun while rotating, an atom's electrons orbit the nucleus while spinning. The nucleus itself also spins, and this spin can be linked, or “coupled” to the electrons' spins. If the nuclear spin is coupled, it (indirectly) participates in collisions with other atoms. If it is not coupled (decoupled), the nuclear spin is uninvolved in these collisions. For decoupled nuclei, their properties give rise to a unique symmetry called SU(n) symmetry, where the strength of the interactions between the atoms is uninfluenced by what nuclear spins are involved in the collisions. “Here n corresponds to the number of nuclear spin states,” Rey added. “In an alkaline earth atom like strontium, it can be up to 10.” In a new paper published in PRX Quantum, Rey and her team of researchers proposed a new method for seeing the quantum effects enabled by SU(n) symmetry in current experimental conditions, something that has been historically challenging for physicists.

Quantum science promises a range of technological breakthroughs, such as quantum computers that can help discover new pharmaceuticals or quantum sensors for navigation. These capabilities rest on two unusual properties of quantum systems, superposition and entanglement. Just as a computer register stores information in the zeros or ones of classical bits, quantum bits, or qubits, store quantum information—but in the quantum world, superposition allows the qubit to be both a zero and a one at the same time. Furthermore, multiple qubits can be bizarrely correlated through a process called entanglement. When two qubits are entangled with each other, each qubit individually looks to be in a random state, but measuring one qubit reveals perfect information about its entangled partner. These properties of superposition and entanglement make qubits quite special, as they can work more efficiently than a classical computer’s bits.

However, a common challenge in actually using these quantum systems arises due to their limited memory time, or “coherence” time, which is often measured in milliseconds. Many researchers at JILA study and use superposition and entanglement of quantum systems, including JILA fellow Adam Kaufman. Previously, Kaufman and his research team focused on improving the coherence time of the strontium atoms’ superposition between the ground state and the “clock” state, so named because these two states form the basis for state-of-the-art atomic clocks. As reported in two new papers, researchers from this lab have extended these studies to much larger system sizes, with an atom in a superposition of hundreds of locations, and separately, demonstrating optical clock entanglement with seconds-scale coherence time.

Lasers have not only fascinated scientists for decades, but they have also become an integral part of many electronic devices. To create scientific-grade lasers, physicists try to control the temporal, spatial, phase, and polarization properties of the laser beam’s pulse to be able to manipulate it. One of these properties is called the orbital angular momentum (OAM), and its phase, or shape, swirls as the doughnut-shaped laser beam travels through space. There are two types of OAM, spatial (S-OAM) and spatial-temporal (ST-OAM). S-OAM describes the angular momentum of the laser beam that is parallel to the light source's propagation direction. In contrast, ST-OAM has angular momentum that moves in a motion perpendicular to the light source’s  propagation direction, which creates a time component to the momentum  [1, 2].  Because of these differences, ST-OAM is more difficult to study due to this time component. According to senior scientist Dr. Chen-Ting Liao: “The problem is that ST-OAM is very difficult to see or measure. And if we can't see or measure this easily, there's no way we can fully understand and optimize it, let alone use it for potential future applications.” To try to overcome this difficulty, a collaboration led by Dr. Liao and other researchers, including JILA Fellows Margaret Murnane and Henry Kapteyn, worked out a method to image and better analyze ST-OAM beams. Their work was subsequently published in ACS Photonics and featured on the cover [3].

JILA Fellow Cindy Regal and her team, along with researchers at the National Institute of Standards and Technology (NIST), have for the first time demonstrated that they can trap single atoms using a novel miniaturized version of “optical tweezers” — a system that grabs atoms using a laser beam as chopsticks.

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. 

The process of developing a quantum computer has seen significant progress in the past 20 years. Quantum computers are designed to solve complex problems using the intricacies of quantum mechanics. These computers can also communicate with each other by using entangled photons (photons that have connected quantum states). As a result of this entanglement, quantum communication can provide a more secure form of communication, and has been seen as a promising method for the future of a more private and faster internet.

Qubits are a basic building block for quantum computers, but they’re also notoriously fragile—tricky to observe without erasing their information in the process. Now, new research from CU Boulder and the National Institute of Standards and Technology (NIST) may be a leap forward for handling qubits with a light touch.  

In the study, a team of physicists demonstrated that it could read out the signals from a type of qubit called a superconducting qubit using laser light—and without destroying the qubit at the same time.

Artist's depiction of an electro-optic transducer, an ultra-thin wafer that can read out the information from a superconducting qubit.

Artist's depiction of an electro-optic transducer, an ultra-thin device that can capture and transform the signals coming from a superconducting qubit. (Credit: Steven Burrows/JILA)

The group’s results could be a major step toward building a quantum internet, the researchers say. Such a network would link up dozens or even hundreds of quantum chips, allowing engineers to solve problems that are beyond the reach of even the fastest supercomputers around today. They could also, theoretically, use a similar set of tools to send unbreakable codes over long distances. 

The study, published June 15 in the journal Nature, was led by JILA, a joint research institute between CU Boulder and NIST.

Of the many different objects in the galaxy, M-dwarf stars, also known as red dwarf stars, are of particular interest to astrophysicists. These small objects are the most common type of star in the universe and have unique properties. “If you lay out all of the different types of stars [in a plot of stellar color versus brightness] we can see, based on what color they are and how bright they are, [that] most stars fall on a line we call the ‘main sequence’,” explained graduate student Connor Bice. “That's where they are born, and they stay in that same spot for most of their lives. Down at the tail end of that [line] are red dwarfs. They're the least massive, the coldest, and the smallest type of main-sequence stars.” Bice is a researcher in JILA Fellow and astrophysicist Juri Toomre's group, and both he and Toomre have been looking at some of a red dwarf's unique properties, mainly their magnetic fields and convective flows. In a new paper published in the Astrophysical Journal, Bice and Toomre have found a link between the star’s convective cycles, or the heat cycles in a star’s atmosphere, and its magnetic fields, using fluid dynamics simulations.

An international team of astrophysicists, including scientists from CU Boulder, may have pinpointed the cause of that shift. The magnetic field lines threading through the black hole appear to have flipped upside down, causing a rapid but short-lived change in the object’s properties. It was as if compasses on Earth suddenly started pointing south instead of north. 

The findings, published May 5 in The Astrophysical Journal, could change how scientists look at supermassive black holes, said study coauthor Nicolas Scepi. 

JILA has a long history in quantum research, advancing the state of the art in the field as its Fellows study various quantum effects. One of these Fellowsis Adam Kaufman. Kaufman and his laboratory team work on quantum systems that are based on neutral atoms, investigating their capacities for quantum information storage and manipulation. The researchers utilize optical tweezers—arrays of highly focused laser beams which hold and move atoms—to study these systems. Optical tweezers allow researchers exquisite, single-particle experimental control. In a new paper published in Physical Review X, Kaufman and his team demonstrate that a specific isotope, ytterbium-171 (171Yb), has the capacity to store quantum information in decoherence-resistant (i.e., stable) nuclear qubits, allows for the ability to quickly manipulate the qubits, and finally, permits the production of such qubits in large, uniformly filled arrays. 

Functional materials—like molecular electronics, biomaterials, light-emitting diodes, or new photovoltaic materials—gain their electronic or photonic properties from complex and multifaceted interactions occurring at the elementary scales of their atomic or molecular constituents. In addition, the ability to control the functions of these materials through external stimuli , e.g., in the form of strong optical excitations, enables new properties in the materials, making them appealing for new technological applications. However, a major obstacle to overcome is the combination of the very fast time (billionths of a second) scales and the very small spatial (nanometer) scales which define the many-body interactions of the elementary excitations in the material which define its function. The extremely high time and spatial resolutions needed have been extremely difficult to achieve simultaneously. Many physicists have, therefore, struggled to visualize the interactions within these materials. In a paper recently published in Nature Communications, JILA Fellow Markus Raschke and his team report on a new ultrafast imaging technique that could solve this issue.

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. 

Colorado has a reputation for being a quantum ecosystem hotspot and a recent panel discussion further bolstered this image. It was hosted by JILA, a world-leading physics institute created by a partnership between the University of Colorado Boulder and NIST; and the CUbit Quantum Initiative, a CU Boulder research center. The panel, titled "Women in Quantum: What Does It Take," brought in individuals from both quantum research and the quantum industry. With panelists from some of the biggest names in the quantum industry, including ColdQuanta, Maybell Quantum, Quantinuum, and Vescent, the discussions about the industry itself were relevant and engaging.