Insider Brief
- A study in Physical Review X reports quantum control protocols that can suppress or invert a system’s “arrow of time,” enabling processes that appear to run backward and offering new approaches to energy extraction and quantum state preparation.
- The researchers use measurement-driven feedback and engineered control Hamiltonians to cancel or overcompensate for measurement disturbances, producing stochastic trajectories consistent with reversed or altered time flow.
- The work demonstrates a measurement-based “quantum engine” that extracts energy from quantum observations and outlines future experimental validation in platforms such as superconducting qubits.
- Photo by Donald Wu on Unsplash
In new research published in Physical Review X, scientists have designed quantum control protocols that generate processes more consistent with time flowing backward than forward. The protocols — techniques to control quantum systems — modify a quantum system’s “arrow of time,” the concept of time as moving in one forward direction. The work opens up possibilities for energy extraction from quantum systems and for quantum state preparation.
A quantum system, such as a collection of qubits, is governed by the laws of quantum mechanics. The team’s control protocols can prevent the emergence of the arrow of time in a quantum system or even invert its direction — that is, cause quantum time to appear to flow in reverse. As an application of their research, the team leveraged their control protocols to design a measurement engine that extracts 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,” said Los Alamos National Laboratory physicist Luis Pedro García-Pintos. “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.”
Time-reversed trajectories
Unlike classical physics, where measurements have little influence on the phenomenon being observed, in quantum physics, measurements stochastically change the system’s state, inducing an arrow of time. The research team used measurements and feedback to engineer time-reversed stochastic trajectories, making quantum systems behave in a way perceived as going backward in time.
The team designed a control Hamiltonian — a sequence of fields and pulses — that could emulate the effects of measurements. Using that Hamiltonian in a feedback process, the team could cancel, amplify or overcompensate for measurement disturbances, generating new trajectories consistent with stretched, blurred or even inverted arrows of time.
In the 19th-century thought experiment known as “Maxwell’s demon,” manipulating the direction of hot and cold particles decreases entropy in a system, seemingly violating the second law of thermodynamics, which posits that entropy should increase or stay constant as the natural order. (Later physics has shown that the second law is not violated when all sources of thermodynamic costs are accounted for.) The Laboratory team’s quantum “demon” exploits knowledge of a quantum system’s state and measurement outcomes to drive similarly anomalous processes, reversing the natural order — the arrow of time — in a quantum system.
Quantum feedback control for superconducting qubits
The tools developed by the team can modify the flow of energy in and out of the system. Such a capability is useful to power a continuous measurement engine that can extract energy from the monitoring process. The quantum measurements, therefore, are exploited as a thermodynamic resource from which energy can be drawn; for instance, to drive another process or store in a quantum battery.
Next steps in the research will include experimentally demonstrating the use of Hamiltonian measurement processes for quantum feedback control; for example, in superconducting qubits, a platform that allows for rapid feedback and high detection efficiencies and in which quantum versions of Maxwell’s demon have been implemented. In follow-up work, the new techniques are used to design quantum state preparation protocols.
Paper: “Reshaping the Quantum Arrow of Time” Physical Review X. DOI: 10.1103/l18s-9vmh
Funding: This work is supported by the U.S. Department of Energy, 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.
