From The DOE’s Los Alamos National Laboratory : “Finding coherence in quantum chaos” 

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From The DOE’s Los Alamos National Laboratory

May 26, 2022
Charles Poling
(505)257-8006
cpoling@lanl.gov

Theoretical breakthrough creates path to manipulating quantum chaos for laboratory experiments; quantum computing and black-hole research.

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By balancing energy gain and loss symmetrically in an open quantum system, Los Alamos National Laboratory physicists have challenged a long-held theory about quantum chaos and decoherence.

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.

“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.”

Quantum chaos differs from classical-physics chaos theory. The latter seeks to understand deterministic, or non-random, patterns and systems that are highly sensitive to initial conditions. The so-called butterfly effect is the most familiar example, whereby the flap of a butterfly’s wings in Texas could, through a bewilderingly complicated but not random chain of cause and effect, lead to a tornado in Kansas.

On the other hand, 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.

It turns out that’s not entirely true. Saxena, University of Luxembourg physicists Aurelia Chenu and Adolfo del Campo, and collaborators found that the dynamical signatures of quantum chaos are actually enhanced, not suppressed, in some instances.

“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.

“Balanced energy gain and loss provides a physical mechanism to realize in the laboratory the kind of energy-spectral filtering that has become ubiquitous in theoretical and numerical studies of complex many-body quantum systems,” del Campo said. “Specifically, balanced energy gain and loss in energy dephasing leads to the optimal spectral filter. Thus, one could leverage balanced energy gain and loss as an experimental tool not only to probe quantum chaos but to study many-body quantum systems in general.”

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 team’s paper builds on previous theoretical work by Carl Bender (of Washington University at St. Louis and former Ulam scholar at Los Alamos) and Stefan Boettcher (formerly of Los Alamos and now at Emory University). They found that, contrary to the accepted paradigm from the early twentieth century, some quantum systems yielded real energies under certain symmetries even though their Hamiltonian was not Hermitian, which means it satisfies certain mathematical relations. In general, such systems are known as non-Hermitian Hamiltonians. A Hamiltonian defines the energy of the system.

“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.

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The DOE’s Los Alamos National Laboratory mission is to solve national security challenges through scientific excellence.

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DOE’s Los Alamos National Laboratory, a multidisciplinary research institution engaged in strategic science on behalf of national security, is managed by Triad, a public service oriented, national security science organization equally owned by its three founding members: The University of California Texas A&M University, Battelle Memorial Institute (Battelle) for the Department of Energy’s National Nuclear Security Administration. Los Alamos enhances national security by ensuring the safety and reliability of the U.S. nuclear stockpile, developing technologies to reduce threats from weapons of mass destruction, and solving problems related to energy, environment, infrastructure, health, and global security concerns.

The United States Department of Energy (DOE) is a cabinet-level department of the United States Government concerned with the United States’ policies regarding energy and safety in handling nuclear material. Its responsibilities include the nation’s nuclear weapons program; nuclear reactor production for the United States Navy; energy conservation; energy-related research; radioactive waste disposal; and domestic energy production. It also directs research in genomics. the Human Genome Project originated in a DOE initiative. DOE sponsors more research in the physical sciences than any other U.S. federal agency, the majority of which is conducted through its system of National Laboratories. The agency is led by the United States Secretary of Energy, and its headquarters are located in Southwest Washington, D.C., on Independence Avenue in the James V. Forrestal Building, named for James Forrestal, as well as in Germantown, Maryland.

Formation and consolidation

In 1942, during World War II, the United States started the Manhattan Project, a project to develop the atomic bomb, under the eye of the U.S. Army Corps of Engineers. After the war in 1946, the Atomic Energy Commission (AEC) was created to control the future of the project. The Atomic Energy Act of 1946 also created the framework for the first National Laboratories. Among other nuclear projects, the AEC produced fabricated uranium fuel cores at locations such as Fernald Feed Materials Production Center in Cincinnati, Ohio. In 1974, the AEC gave way to the Nuclear Regulatory Commission, which was tasked with regulating the nuclear power industry and the Energy Research and Development Administration, which was tasked to manage the nuclear weapon; naval reactor; and energy development programs.

The 1973 oil crisis called attention to the need to consolidate energy policy. On August 4, 1977, President Jimmy Carter signed into law The Department of Energy Organization Act of 1977 (Pub.L. 95–91, 91 Stat. 565, enacted August 4, 1977), which created the Department of Energy. The new agency, which began operations on October 1, 1977, consolidated the Federal Energy Administration; the Energy Research and Development Administration; the Federal Power Commission; and programs of various other agencies. Former Secretary of Defense James Schlesinger, who served under Presidents Nixon and Ford during the Vietnam War, was appointed as the first secretary.

President Carter created the Department of Energy with the goal of promoting energy conservation and developing alternative sources of energy. He wanted to not be dependent on foreign oil and reduce the use of fossil fuels. With international energy’s future uncertain for America, Carter acted quickly to have the department come into action the first year of his presidency. This was an extremely important issue of the time as the oil crisis was causing shortages and inflation. With the Three-Mile Island disaster, Carter was able to intervene with the help of the department. Carter made switches within the Nuclear Regulatory Commission in this case to fix the management and procedures. This was possible as nuclear energy and weapons are responsibility of the Department of Energy.

Recent

On March 28, 2017, a supervisor in the Office of International Climate and Clean Energy asked staff to avoid the phrases “climate change,” “emissions reduction,” or “Paris Agreement” in written memos, briefings or other written communication. A DOE spokesperson denied that phrases had been banned.

In a May 2019 press release concerning natural gas exports from a Texas facility, the DOE used the term ‘freedom gas’ to refer to natural gas. The phrase originated from a speech made by Secretary Rick Perry in Brussels earlier that month. Washington Governor Jay Inslee decried the term “a joke”.

Facilities

The Department of Energy operates a system of national laboratories and technical facilities for research and development, as follows:

Ames Laboratory
Argonne National Laboratory
Brookhaven National Laboratory
Fermi National Accelerator Laboratory
Idaho National Laboratory
Lawrence Berkeley National Laboratory
Lawrence Livermore National Laboratory
Los Alamos National Laboratory
National Energy Technology Laboratory
National Renewable Energy Laboratory
Oak Ridge National Laboratory
Pacific Northwest National Laboratory
Princeton Plasma Physics Laboratory
Sandia National Laboratories
Savannah River National Laboratory
SLAC National Accelerator Laboratory
Thomas Jefferson National Accelerator Facility

Other major DOE facilities include
Albany Research Center
Bannister Federal Complex
Bettis Atomic Power Laboratory – focuses on the design and development of nuclear power for the U.S. Navy
Kansas City Plant
Knolls Atomic Power Laboratory – operates for Naval Reactors Program Research under the DOE (not a National Laboratory)
National Petroleum Technology Office
Nevada Test Site
New Brunswick Laboratory
Office of River Protection
Pantex
Radiological and Environmental Laboratory
Y-12 National Security Complex
Yucca Mountain nuclear waste repository
Other:

Pahute Mesa Airstrip – Nye County, Nevada, in supporting Nevada National Security Site