Yu-Ao Chen

Quantum networks, which integrate multiple quantum computers and the channels connecting them, are crucial for distributed quantum information processing but remain inherently susceptible to channel noise. The channel purification protocol emerges as a promising technique for directly suppressing noise in quantum channels without complex encoding and decoding operations, making it particularly suitable for remote quantum information transmission in optical systems. In this work, leveraging the spatial and polarization degrees of freedom of photons, we propose a novel experimental configuration that efficiently implements the channel purification protocol, utilizing two Fredkin gates to coherently interfere independent noise channels. Based on this configuration, we experimentally demonstrate that the protocol can suppress the noise with various noise levels and forms. Furthermore, we apply our protocol in a practical application of entanglement distribution, showing that the channel purification can effectively protect the distributed entanglement from channel noise.

The inherent irreversibility of quantum dynamics for open systems poses a significant barrier to the inversion of unknown quantum processes. To tackle this challenge, we propose the framework of virtual combs that exploit the unknown process iteratively with additional classical post-processing to simulate the process inverse. Our research establishes a path to achieving the exact inverse of unknown channels with certain conditions, accompanied by a no-go theorem that underscores the intrinsic limitations imposed by quantum mechanics on such tasks. Notably, we demonstrate that an n-slot virtual comb can exactly reverse a depolarizing channel with one unknown noise parameter out of n+1 potential candidates, and a 1-slot virtual comb can exactly reverse an arbitrary pair of quantum channels. We further explore the approximate inverse of an unknown channel within a given channel set. For any unknown depolarizing channels within a specified noise region, we unveil a worst-case error decay of O(n^−1) of reversing the channel via virtual combs. Moreover, we show that virtual combs with constant slots can be applied to universally reverse unitary operations and investigate the trade-off between the slot number and the sampling overhead.