QSA quantum devices and techniques further our understanding of physics
The Quantum Systems Accelerator (QSA), a National Quantum Information Science Research Center funded by the U.S. Department of Energy Office of Science, takes a science-first approach to exploring new frontiers in physics. By co-designing cutting-edge quantum devices across three major technologies—neutral atoms, trapped ions, and superconducting circuits—scientists at QSA are exploring big science questions about our world that are not feasible to tackle with classical methods. QSA includes researchers in quantum information science, sharing experimental techniques, novel scalable technologies, and theoretical tools of quantum information across both application spaces. Crucial to QSA's cutting-edge experiments that advance our fundamental understanding of physics is a collaboration between institutions and a strong vision to explore profound questions with current quantum technologies.
Increased Quantum Coherence
A notable example of QSA's researchers pushing the boundaries in science by leveraging the properties of quantum physics was published in Nature in 2022. The experimental team led by JILA, a joint institute at the University of Colorado Boulder and the National Institute of Standards and Technology (NIST), demonstrated once more the validity of the theory of general relativity by improving the accuracy of precise measurements. Albert Einstein's theory of general relativity refers to how gravity and massive objects curve space-time, affecting the passage of time – a phenomenon described as time dilation. The research probed time dilation on a millimeter-scale atomic ensemble with unprecedented precision.
Using roughly 100,000 ultracold strontium atoms in an optical lattice at ultralow temperatures close to absolute zero and spaced across one millimeter vertically, QSA scientists demonstrated with unprecedented precision that the clocks ticked at different rates due to the effects of gravity when atoms are separated by a few hundred microns. The team employed these atoms in the optical lattice to reduce disturbances from atomic motion, allowing for more precise measurements over ten days. The groundbreaking accuracy in detecting the minimal difference in time due to gravity, known as "redshift," was 50 times better than in any previous clock, establishing a new standard in precision and quantum coherence.
"QSA has been instrumental in providing a stimulating intellectual environment for us to pursue the absolute forefront of quantum science, where students can inspire each other to create new ideas and develop new technologies," explained the Principal Investigator, Jun Ye of JILA.
The team also focused on improving control over the clock's quantum states throughout the experiment because extended atomic coherence times are critical for clock accuracy and precision. To achieve this, they adjusted the depth of the optical traps to maximize coherence times and the stability of measurements, proving that this critical requirement for clocks is also valuable for other programmable quantum devices.
"This work sets the pace for advancing quantum coherence to the macroscopic scale of tens of seconds, with more recent work now demonstrating 2 minutes 2-minute-long coherence time for quantum superposition at optical frequencies. The QSA collaboration has also stimulated the use of such coherent quantum system for entangled quantum sensors, with breakthrough results over the past two years," said Ye.
Sensing with Waves of Matter Beyond the Standard Quantum Limit
Another QSA team, led by Professor James K. Thompson at JILA, NIST, and the Department of Physics, University of Colorado, Boulder, pioneered a unique approach to enhance the precision of a quantum sensor. In a standard matter-wave interferometer, researchers harness the fact that atoms can act as both a particle and a wave in quantum mechanics to measure tiny changes in gravity or motion. However, the random behavior of atoms in quantum mechanics limits the device's precision, known as the standard quantum limit or SQL. The QSA team demonstrated for the first time how a special phenomenon in quantum mechanics can make an extremely sensitive matter-wave interferometer even more precise.
Publishing their experiment in Nature in 2022, the team went beyond the SQL by using quantum entanglement, where the quantum states of roughly 700 ultracold rubidium atoms are linked in such a way that their properties become correlated, regardless of distance. The collective measurements of these states reduce the randomness in their interferometer results, achieving up to 1.7 decibels of improvement.
"Single atoms in quantum mechanics are fundamentally very fuzzy and uncertain little objects. However, we were able to get the atoms to work together such that the quantum fuzziness of one atom is partially cancelled by the quantum fuzziness of another atom," said Thompson.
Similar to other QSA researchers using quantum entanglement for quantum computing, reducing noise in quantum sensors requires creating entanglement between atoms, thus relying on the same techniques and fundamental physics.
The team directed roughly 700 rubidium atoms inside a high-finesse optical cavity along its axis to achieve this. A high-finesse cavity consists of two highly reflective, nearly flawless mirrors that trap light and keep it bouncing back and forth through the atoms many times. The sustained confinement of light inside the cavity strengthens its interaction with the atoms, enabling advanced quantum effects and enhancing the interferometer's precision beyond SQL.
"The team, co-led by then Ph.D. students Graham Greve and Chengyi Luo, used two very different approaches that both worked great! In the first approach, they used light to essentially measure the quantum noise of the atoms and cancel it. In the second approach, they used the light as a kind of shared quantum network that allowed the atoms to carry on a quantum conversation with each other so that they could conspire to work together to be quieter," explained Thompson.
By linking the quantum states of hundreds of atoms, the team's experiment has broadened novel possibilities for ultra-precise measurements in physics.
"QSA provided critical support to realize the first entanglement-enhanced matterwave interferometer with broad technological and scientific impacts, including the next generation of precision inertial navigation sensors, natural resource discovery, tests of fundamental physics, searches for dark matter, and even future gravitational wave detectors," concluded Thompson.
New Capabilities for Exploring Quantum Materials
(QSA) is is making significant progress in quantum technology, including developing quantum devices to measure important properties of other quantum systems and testing for defects. Testing quantum devices is often slow and expensive due to lengthy preparation, often requiring extensive calibration and characterization before any operation. Early screening of devices can save time, provided the screening tools are sensitive enough.
An original approach to this kind of sensitive, non-invasive screening involves the use of quantum sensors based on defects in diamond. Diamond can behave like a tiny magnet sensitive to magnetic fields when two neighboring carbon atoms are replaced by a nitrogen atom and a vacancy, creating a nitrogen-vacancy (NV) center. Researchers at Sandia National Laboratories (Sandia Labs), led by Andrew Mounce, Pauli Kehayias, and Luca Bass, developed a non-invasive method to measure microwave frequency magnetic fields using an NV-based quantum sensor. Publishing their approach in Journal of Applied Physics, they demonstrated a new way to test quantum devices and screen for errors without damaging them, while also verifying that the devices behaved as expected compared to computer simulations.
"These developments are really cool: we're using a quantum system (nitrogen-vacancies in diamond) to evaluate quantum devices… Double quantum!" described Andrew Mounce, research physicist at Sandia Labs.
This research builds on earlier work in trapped-ion quantum computers by the same Sandia National Laboratories team. In that work, they investigated localized electrical shorts in ion traps, specifically the trench capacitors that help manage RF voltages on control electrodes, also published in the Journal of Applied Physics. The team extended this capability by demonstrating that their devices can also measure magnetic fields at higher frequencies. These higher-frequency measurements are relevant to various quantum computing platforms, including trapped-ion and superconducting systems.
A deeper understanding of how magnetic fields and quantum phenomena affect the electronic structure of materials is a vital step toward improving fabrication methods for quantum technologies. A team at Lawrence Berkeley National Laboratory (Berkeley Lab), QSA's lead laboratory, introduced a magnetoARPES technique in the Journal of Electron Spectroscopy and Related Phenomena—a modified version of the widely used Angle-Resolved Photoemission Spectroscopy (ARPES) technique, which uses electromagnetic radiation to eject electrons from a material's surface.
Historically, applying a magnetic field during ARPES has been challenging because it distorts the trajectory of electrons. The novel MagnetoARPES technique tackles this challenge by confining the magnetic field into a thin layer roughly 100 micrometers from the sample surface, which consists of graphene deposited on thin silicon carbide (SiC) substrates.
The team leveraged the world-class synchrotron light source capabilities at Berkeley Lab's Advanced Light Source, a DOE user facility, to produce a strong, focused X-ray beam to strike the graphene sample. This intense X-ray radiation with very thin samples released photoelectrons that traveled briefly through the confined magnetic field with minor deflection or distortion, reaching a special device called a hemispherical electron analyzer. The analyzer provided high-resolution measurements of electron energy and emission angles and was rotated to compensate for any shifts induced by the magnetic field. To further test this technique, the team compared their experimental data to computer simulations, demonstrating that magnetoARPES can be used in future experiments to explore quantum materials and fundamental questions in physics.
"Although the magnetic field strengths we can apply are modest, they're still around 1000 times higher than we could previously study using electron spectroscopy. These fields allow us to observe how quantum materials coherently process and transfer information through subtle magnetic interactions, offering new insights for quantum information science through our magnetoARPES technique," said Eli Rotenberg, senior staff scientist at Berkeley Lab's Advanced Light Source and principal investigator.
These groundbreaking experiments at multiple partner institutions across the QSA ecosystem highlight QSA's unique ability as a DOE National QIS Research Center to bring together forward-looking pioneers and co-design tools and methods for a deeper understanding of fundamental physics. From proving time dilation on a millimeter scale to enhancing sensor precision beyond classical limits, QSA researchers use quantum tools and methods to explore exciting new questions in science.
More information:
doi.org/10.1038/s41586-021-04349-7
doi.org/10.1038/s41586-022-05197-9
doi.org/10.1063/5.0253730
doi.org/10.1063/5.0234269
doi.org/10.1016/j.elspec.2023.147357
Provided by Lawrence Berkeley National Laboratory
