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CHIPS Articles: Researchers See Signs of Interactive Form of Quantum Matter

Researchers See Signs of Interactive Form of Quantum Matter
By CHIPS Magazine - November 9, 2018
JILA researchers have, for the first time, isolated groups of a few atoms and precisely measured their multi-particle interactions within an atomic clock. The discovery will help scientists control interacting quantum matter, which is expected to boost the performance of atomic clocks, many other types of sensors, and quantum information systems.

The research is published online in Nature. JILA is jointly operated by the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder.

NIST scientists have forecasted “many body” physics and its advantages for years, but the new JILA findings provides the first quantitative evidence of exactly what happens when packing together a few fermions—atoms that cannot be in the same quantum state and location at the same time, according to a release.

“We are trying to understand the emergence of complexity when multiple particles—atoms here—interact with each other,” NIST and JILA Fellow Jun Ye said. “Even though we may understand the rules perfectly on how two atoms interact, when multiple atoms get together there are always surprises. We want to understand the surprises quantitatively.”

Today’s best tools for measuring quantities, such as time and frequency, are based on control of individual quantum particles. This is the case even when ensembles of thousands of atoms are used in an atomic clock. These measurements are approaching the so-called standard quantum limit—a “wall” preventing further improvements using independent particles, according to the release.

Connecting many-particle interactions could push that wall back or even break through it, because an engineered quantum state could suppress atom collisions and protect quantum states against interference, or noise, according to JILA research. In addition, atoms in such systems could be arranged to cancel each other’s quantum noise such that sensors would get better as more atoms were added, promising significant increases in precision and data-carrying capacity.

In the new research, the JILA team used their three-dimensioned strontium lattice clock, which offers precise atom control. They created arrays of between one and five atoms per lattice cell, and then used a laser to set the clock “ticking,” or switching at a specific frequency between two energy levels in the atoms. JILA’s new imaging technique was used to measure the atoms’ quantum states.

The researchers observed unexpected results when three or more atoms were together in a cell. The results were nonlinear, or unpredicted based on past experience, a hallmark of multi-particle interactions. The researchers combined their measurements with theoretical predictions by NIST colleagues Ana Maria Rey and Paul Julienne to conclude that multi-particle interactions occurred.

Specifically, the clock’s frequency shifted in unexpected ways when three or more atoms were in a lattice site. The shift is different from what one would expect from summing up various pairs of atoms. For example, five atoms per cell caused a shift of 20 percent compared to what would normally be expected, according to the JILA research.

“Once you get three atoms per cell, the rules change,” Ye said. This is because the atoms’ nuclear spins and electronic configurations play together to determine the overall quantum state, and the atoms can all interact simultaneously instead of in a pair-wise fashion, he said.

Multi-particle effects also appeared in crowded lattice cells in the form of an unusual, rapid decay process. Two atoms per triad formed a molecule and one atom remained loose, but all had enough energy to escape the trap. By contrast, a single atom is likely to remain in a cell for a much longer time, Ye explained.

“What this means is, we can make sure there is only one atom per cell in our atomic clock,” Ye said. Understanding these processes will allow us to figure out a better path for making improved clocks, as particles inevitably will interact if we pack enough of them nearby to improve signal strength.”

The JILA team also found that packing three or more atoms into a cell could result in long-lived, highly entangled states, meaning the atoms’ quantum properties were linked in a stable way. This simple method of entangling multiple atoms may be a useful resource for quantum information processing.

This research is supported by NIST, the Defense Advanced Research Projects Agency, the Army Research Office, the Air Force Office of Scientific Research, National Science Foundation and National Aeronautics and Space Administration.

Paper: A. Goban, R.B. Hutson, G.E. Marti, S.L. Campbell, M.A. Perlin, P.S. Julienne, J.P. D’Incao, A.M. Rey and J. Ye. 2018. Emergence of multi-body interactions in few-atom sites of a fermionic lattice clock. Nature. Published online 31 October 2018. DOI: 10.1038/s41586-018-0661-6

About JILA
When JILA was formed in 1962 as a joint institute of CU Boulder and NIST, the acronym stood for "Joint Institute for Laboratory Astrophysics." Then JILA's research quickly expanded to include fields like atomic, molecular and optical physics, as well as biophysics, quantum information, precision measurement, and more! So although the name "JILA" has stuck, it no longer spell outs the acronym that it had outgrown.

JILA scientists explore some of today's most challenging and fundamental scientific questions about quantum physics, the design of precision optical and X-ray lasers, the fundamental principles underlying the interaction of light and matter, and processes that have governed the evolution of the Universe for nearly 14 billion years.

Research topics range from the small, frigid world governed by the laws of quantum mechanics through the physics of biological and chemical systems to the processes that shape the stars and galaxies. JILA science encompasses eight broad categories: astrophysics, atomic & molecular physics, biophysics, chemical physics, laser physics, nanoscience, precision measurement, and quantum information science & technology.

False-color images showing variations in atom numbers (1 to 5 atoms, left to right) and density in different lattice cells of JILA’s strontium lattice atomic clock. JILA researchers observed shifts in the clock’s frequency that arise from the emergence of multi-particle interactions when three or more atoms occupy a single cell. Credit: Aki Goban, Ye group/JILA
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