Researchers achieve time reversal through input-output indeterminacy

A research team has constructed a coherent superposition of quantum evolution with two opposite directions in a photonic system and confirmed its advantage in characterizing input-output indeterminacy. The study was published in Physical assessment letters.

The idea that time flows inexorably from the past to the future is deeply ingrained in the human mind. However, the physical laws that govern the movement of objects in the microscopic world do not intentionally distinguish the direction of time.

To be more specific, the basic equations of motion of both classical and quantum mechanics are reversible. Changing the direction of the time coordinate system of a dynamical process (possibly together with the direction of some other parameters) still constitutes a valid evolutionary process.

This is known as time reversal symmetry. In quantum information science, time reversal has attracted much interest because of its applications to multi-time quantum states, simulations of closed timelike curves, and inversion of unknown quantum evolutions. However, time reversal is difficult to realize experimentally.

To address this problem, the team, led by Academician Guo Guangcan, Prof. Li Chuanfeng and Prof. Liu Biheng from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS), in collaboration with Prof. Giulio Chiribella from the University of Hong Kong, constructed a class of quantum evolution processes in a photonic setup by extending time reversal to the input-output inversion of a quantum device.

When the input and output ports of a quantum device were swapped, the resulting evolution satisfied the time-reversal properties of the original evolution, creating a time-reversal simulator for quantum evolution.

On this basis, the team further quantified the evolutionary time direction, achieving the coherent superposition of quantum evolution and its inverse evolution. They also characterized the structures using quantum witnessing techniques.

Compared with the scenario of a certain evolution time direction, the quantification of the time direction showed significant advantages in the identification of quantum channels.

In this study, researchers used the device to distinguish between two sets of quantum channels with a 99.6% success rate, while the maximum success rate of a strategy with a given time direction was only 89% at the same resource consumption.

The research shows that input-output indeterminacy can be a valuable resource for developments in quantum information and photonic quantum technologies.