Researchers reverse the apparent arrow of time in quantum systems
Los Alamos-led work uses quantum measurements and feedback to make systems appear to run backward and to draw energy from monitoring.
By Priya Raghavan · Science Reporter
3 min read
Researchers have devised quantum control protocols that can make a quantum system behave as if its arrow of time has been weakened or reversed, Los Alamos National Laboratory said. The work matters for quantum technology because the same methods could help extract energy from measurements and prepare quantum states for computing devices.
The study, published in Physical Review X, was written by Luis Pedro García-Pintos, Yi-Kai Liu and Alexey V. Gorshkov. Los Alamos said the techniques apply to systems governed by quantum mechanics, including groups of qubits.
The arrow of time describes the familiar direction in which events seem to proceed. According to Los Alamos, many microscopic laws of physics are symmetric under time reversal, meaning their equations can describe processes running forward or backward.
Quantum measurements complicate that picture. Los Alamos said measuring a quantum system can randomly change its state, creating a directionality that resembles time moving forward.
How the protocol works
The researchers addressed that measurement effect by pairing measurements with feedback control, according to Los Alamos. The result was a way to produce stochastic quantum trajectories that match what would be expected if the system were evolving backward in time.
The team used what the study calls a control Hamiltonian: a designed sequence of fields and pulses that can reproduce the effects of quantum measurements. When used in a feedback loop, Los Alamos said, that control can offset, amplify or overcorrect the disturbance caused by measuring the system.
That gives researchers several options for changing how time’s direction shows up in a quantum process. Los Alamos said the method can suppress the usual arrow of time, stretch or blur it, or make it appear inverted.
García-Pintos, a physicist at Los Alamos, said the tools are meant to control the perceived arrow of time in microscopic quantum systems, where the underlying physics does not treat forward and backward time evolution in the same way people experience everyday events. He said that control could open new ways to manage quantum systems.
A quantum measurement engine
The study also connects the work to Maxwell’s demon, the 19th-century thought experiment in which an observer sorts particles by energy and appears to reduce entropy. Los Alamos noted that later physics accounts for the thermodynamic costs involved, preserving the second law of thermodynamics.
In the new work, Los Alamos said a quantum version of that idea uses information about the system and the results of measurements to create behavior that can effectively reverse the system’s natural time direction. The researchers also demonstrated a measurement engine that can draw useful energy from the act of monitoring a quantum system.
Under that framework, measurements act as a thermodynamic resource, Los Alamos said. The energy extracted from measurement could be used to drive another quantum process or be stored in a quantum battery, according to the laboratory.
The researchers plan to test Hamiltonian-based measurement processes experimentally with superconducting qubits, Los Alamos said. The laboratory said those systems are suited to fast feedback and efficient detection, and have already been used in quantum versions of Maxwell’s demon.
Los Alamos said future work will also apply the control methods to improve quantum state preparation. The research was supported by the U.S. Department of Energy Office of Science, Los Alamos programs including Beyond Moore’s Law and Advanced Simulation and Computing, and the National Science Foundation.
This story draws on original reporting from ScienceDaily.