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Time-Reversal Simulator Outperforms Classical Methods

A research team led by Guangcan Guo from the University of Science and Technology of China, along with Professors from the University of Hong Kong, constructed a coherent superposition of quantum evolution with two opposite directions in a photonic system and confirmed its advantage in characterizing input-output indefiniteness. Their study was published in the journal Physical Review LettersPhysical Review Letters.

Time-Reversal Simulator Outperforms Classical Methods
Experimental setup of the superposition of the quantum evolution and its inverse evolution. Image Credit: Professor Chuanfeng Li's team

People believe that time moves inexorably from the past to the future. However, the direction of time is not explicitly distinguished by the laws of physics that control how objects move in the microscopic world.

To be more precise, the fundamental equations of motion of both quantum and classical physics are reversible, and a dynamical process that has its time coordinate system changed (maybe along with some other parameters) still qualifies as an evolution process. This is known as time-reversal symmetry. 

Time reversal has generated a lot of interest in quantum information science because of its applicability in inverting unknown quantum evolutions, simulations of closed timelike curves, and multi-time quantum states. However, time reversal is challenging to achieve experimentally.

By extending the time reversal to the input-output inversion of a quantum device, the researchers created a class of quantum evolution processes in a photonic setup to address this issue.

A time-reversal simulator for quantum evolution was obtained by swapping the input and output ports of a quantum device, which led to an evolution that satiated the time-reversal qualities of the original evolution.

The team achieved the coherent superposition of the quantum evolution and its inverse evolution by further quantizing the evolution time direction based on this basis. They also used quantum witness techniques to characterize the structures.

The quantization of the time direction demonstrated notable benefits in quantum channel identification compared to the case of a definite evolution time direction. In this investigation, the maximum success probability of a certain time direction method was only 89 % with the same resource consumption; in contrast, researchers employed the device to identify between two sets of quantum channels with a 99.6 % success probability.

The work demonstrated that input-output indefiniteness can be useful for developing photonic quantum technologies and quantum information.

Journal Reference:

Guo, Y., et al. (2024) Experimental Demonstration of Input-Output Indefiniteness in a Single Quantum Device.Physical Review Letters.

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