Scientists have developed a new way to control quantum systems whose behavior appears more consistent as time moves backwards rather than forwards. This study Physical Review XHere we present a quantum control protocol that reconstructs the system’s “time arrow,” or the concept that time naturally moves in only one direction. This approach could ultimately support new ways to extract energy from quantum systems and prepare quantum states.
Quantum systems, such as groups of qubits, follow the rules of quantum mechanics rather than classical physics. Using newly developed control protocols, researchers can suppress the normal appearance of the arrow of time or reverse its apparent direction, making it appear as if quantum processes are unfolding in the opposite direction. As a demonstration of this technology, the team also created a measurement engine that can harvest energy from the act of making quantum measurements.
“Unlike the phenomena observed around us, at the microscopic level, most of the fundamental laws of physics consider forward and backward movement in time to be physically possible,” said Luis Pedro García-Pintos, a physicist at Los Alamos National Laboratory. “In other words, these physical laws are symmetric under time reversal, and the equations work the same way when time is reversed. For quantum systems operating at that microscopic level, the tools we have built can manipulate the arrow of perceived time, leading to surprising and novel ways to control quantum systems.”
Engineering time inversion quantum behavior
In everyday classical physics, measurements have little effect on the object being observed. Quantum systems behave completely differently. When measured, the state changes randomly, naturally creating an arrow of time.
To overcome this effect, the researchers combined measurements and feedback to generate time-reversed stochastic trajectories. This allowed quantum systems to follow paths that appeared to be consistent with time flowing backwards.
The team achieved this by designing a controlled Hamiltonian, a carefully planned sequence of magnetic fields and pulses that reproduces the effects of quantum measurements. Incorporating the Hamiltonian into a feedback system can cancel, enhance, or even overcompensate for disturbances caused by measurements. As a result, the system can generate trajectories corresponding to stretched, blurred, or reversed time arrows.
Quantum version of Maxwell’s demon
The study is also based on a famous 19th century thought experiment known as “Maxwell’s Demon.” In that scenario, a hypothetical observer would selectively classify hot and cold particles, apparently decreasing entropy and challenging the second law of thermodynamics, which states that entropy either naturally increases or remains constant. (Subsequent physics has shown that the second law is not violated if all sources of thermodynamic cost are considered.)
The Los Alamos team’s quantum “demons” use information about the state and measurements of a quantum system to cause similarly anomalous behavior, effectively reversing the system’s natural flow of time.
Extracting energy from quantum measurements
New control methods also allow researchers to influence how energy enters and exits quantum systems. This feature has the potential to power a continuous measurement engine that extracts useful energy directly from the monitoring process.
In this framework, quantum measurements become thermodynamic resources that can be harnessed to perform tasks such as driving other quantum processes or storing energy in quantum batteries.
Looking ahead, the researchers plan to experimentally demonstrate a Hamiltonian-based measurement process for quantum feedback control using superconducting qubits. These systems support rapid feedback, high efficiency detection, and are already being used to implement a quantum version of Maxwell’s Demon. Future research will also apply new techniques to develop improved quantum state preparation protocols.
Funding: This research was supported by the U.S. Department of Energy, Office of Science, Advanced Scientific Computing Research Program, Los Alamos’ Beyond Moore’s Law Project of the Advanced Simulation and Computing Program, and the National Science Foundation.
