Scientists Create Quantum Steps Reshaping the Quantum Arrow of Time

Scientists at Los Alamos National Laboratory have developed quantum control procedures that, according to a new study published in Physical Review X, generate processes that appear more compatible with time flowing backward than forward.

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Protocols, methods for controlling quantum systems, can alter a system’s “arrow of time,” the concept that time moves in a single forward direction. These findings open new possibilities, including extracting energy from quantum systems and improving quantum state preparation.

The principles of quantum mechanics govern any quantum system, including collections of qubits. The team’s control procedures can suppress the emergence of the arrow of time within a quantum system, or even reverse its direction, making time appear to run backward at the quantum level. As part of the study, the researchers applied their control algorithms to build a measurement engine capable of extracting energy from quantum measurements performed on the system.

Unlike phenomena we observe around us, at the microscopic level, most fundamental laws of physics see forward and backward movement in time as physically possible. In other words, those laws of physics are symmetrical under time reversal; the equations work just as well if you reverse time. For quantum systems, which operate at that microscopic level, the tools we’ve constructed can manipulate the perceived arrow of time, leading to surprising, novel ways to control quantum systems.

Luis Pedro García-Pintos, Physicist, Los Alamos National Laboratory

Time-Reversed Trajectories

Unlike in classical physics, where measurements have little impact on the phenomena being observed, quantum measurements stochastically influence the state of a system, giving rise to an arrow of time. In this study, the researchers used measurements combined with feedback to generate time-reversed stochastic trajectories, causing quantum systems to behave as though they are evolving backward in time.

The researchers created a control Hamiltonian (a series of fields and pulses) to simulate the impact of measurements. Using that Hamiltonian in a feedback mechanism, they could then cancel, magnify, or overcompensate for measurement imperfections, resulting in new trajectories that corresponded to stretched, blurred, or even inverted time arrows.

In the 19^th-century thought experiment known as "Maxwell's demon," altering the direction of hot and cold particles reduces entropy in a system, seemingly contradicting the second rule of thermodynamics, which states that entropy should rise or remain constant as the natural order. (Later physics has demonstrated that the second rule is not broken when all sources of thermodynamic costs are included.)

The Laboratory team’s quantum “demon” uses knowledge of a quantum system's state and measurement results to drive similar anomalous processes, reversing the natural order (i.e. the arrow of time) in a quantum system.

Quantum Feedback Control for Superconducting Qubits

The tools developed by the team can modulate the flow of energy into and out of a quantum system. This capability is particularly useful for powering a continuous measurement engine, which can extract energy directly from the act of monitoring the system. Quantum measurements can therefore be treated as a thermodynamic resource, from which energy can be drawn, either to power other processes or to store in a quantum battery.

The next phase of this research will focus on experimentally demonstrating Hamiltonian measurement processes for quantum feedback control, in particular in superconducting qubits, a platform well-suited for rapid feedback, high detection efficiency, and even implementations of quantum versions of Maxwell’s demon. In parallel, the team plans to apply these methodologies to develop more advanced quantum state preparation protocols.

The US Department of Energy’s Office of Science, Advanced Scientific Computing Research program, the Beyond Moore's Law project of the Advanced Simulation and Computing Program at Los Alamos, and the National Science Foundation all supported his study.