Researchers realize time reversal through indeterminacy of inputs and outputs

The research team constructed a coherent superposition of quantum evolution with two opposite directions in a photonic system and confirmed its advantage in characterizing the uncertainty of inputs and outputs. The study was published in Physical inspection letters.

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

More precisely, the fundamental equations of motion of both classical and quantum mechanics are reversible, and a change in the direction of the time coordinate system of the dynamical process (perhaps along with the direction of some other parameters) still represents a valid evolutionary process.

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

To solve this problem, a team led by Academician Guo Guangcan, Professor Li Chuanfeng and Professor 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 of the University of Hong Kong constructed a class of quantum evolution processes in a photonic setup by extending time reversal to the inversion of the input and output of a quantum device.

By swapping the input and output ports of the quantum device, the resulting evolution satisfied the time-reversal properties of the initial evolution, yielding a time-reversal simulator for quantum evolution.

On this basis, the team further quantized the temporal direction of evolution, achieving a coherent superposition of quantum evolution and its inverse evolution. They also characterized the structures using quantum witness techniques.

Compared to the scenario of a certain evolutionary time direction, time direction quantization showed significant advantages in quantum channel identification.

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

The study reveals the potential of input-output uncertainty as a valuable resource for advancing quantum information and photonic quantum technologies.