Researchers find coherence in quantum chaos; could open new paths into quantum information & quantum computing

By Sandra Helsel posted 27 May 2022

(HPCWire) A theoretical breakthrough in understanding quantum chaos could open new paths into researching quantum information and quantum computing, many-body physics, black holes, and the still-elusive quantum to classical transition.
Quantum chaos describes chaotic classical dynamical systems in terms of quantum theory. Quantum chaos is responsible for the scrambling of information occurring in complex systems such as blackholes. It reveals itself in the energy spectra of the system, in the form of correlations between its characteristic modes and frequencies.
It has been believed that as a quantum system loses coherence, or its “quantumness,” by coupling to the environment outside the system—the so-called quantum to classical transition—the signatures of quantum chaos are suppressed. That means they can’t be exploited as quantum information or as a state that can be manipulated.
“By applying balanced energy gain and loss to an open quantum system, we found a way to overcome a previously held limitation that assumed interactions with the surrounding environment would decrease quantum chaos,” said Avadh Saxena, a theoretical physicist at Los Alamos National Laboratory and member of the team that published the paper on quantum chaos in Physical Review Letters. “This discovery points to new directions in studying quantum simulations and quantum information theory.”
“Our work challenges the expectation that decoherence generally suppresses quantum chaos,” Saxena said.
The energy values in the spectra of the quantum system were previously thought to be complex numbers—that is, numbers with an imaginary number component—and thus not useful in an experimental setting. But by adding energy gain and loss at symmetrical points in the system, the research team found real values for the energy spectra, provided that the strength of gain or loss is below a critical value.
By changing the decoherence, Saxena and del Campo explained, the filter allows better control of energy distribution in the system. That can be useful in quantum information, for example. “Decoherence limits quantum computing, so it follows that because increasing quantum chaos reduces decoherence, you can keep computing longer,” Saxena said.
“The prevailing understanding was that decoherence suppresses quantum chaos for Hermitian systems, with real energy values,” Saxena said. “So we thought, what if we take a non-Hermitian system?”
The research paper studied the example of pumping energy into a wave guide at a particular point—that’s the gain—then pumping energy out again—the loss—symmetrically. The wave guide is an open system, able to exchange energy with the environment. Instead of causing decoherence, they found, the process and interactions increase coherence and quantum chaos.

Sandra K. Helsel, Ph.D. has been researching and reporting on frontier technologies since 1990.  She has her Ph.D. from the University of Arizona.


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