The efficiency of quantum entanglement purification has increased by more than 6,000 times, far exceeding the international level

At present, quantum technology represented by quantum information science is constantly advancing, forming a new scientific frontier, stimulating scientific and technological innovation, and nurturing disruptive technologies that have a huge impact on human society. Countries around the world are paying close attention to it, and major technology companies and research institutes are also making plans.

As a scientific and technological power, my country attaches great importance to the development of quantum information technology. It has made breakthroughs in a series of important scientific problems and key core technologies in the field of quantum information technology, and produced a number of results with significant international influence.

News from USTC shows that recently, the research group of Li Chuanfeng and Liu Biheng from the team of Academician Guo Guangcan of USTC, in collaboration with Sheng Yubo and others from Nanjing University of Posts and Telecommunications, used a high-quality super-entangled source to achieve for the first time the long-distance quantum entanglement purification of 11 kilometers. The purification efficiency has increased by more than 6,000 times over the previous best international level, making important progress in the research of quantum communications and quantum networks.

It is reported that in quantum communication transmission, how to achieve long-distance quantum communication has always been a hot topic in international research. There are currently two main solutions to this problem:

One is to use satellites to extend the distance of quantum communication in outer space, which is almost a vacuum and has minimal quantum signal loss. For example, my country successfully launched the world's first quantum science experimental satellite "Micius" in 2016, successfully verifying the feasibility of this solution.

The second is to use quantum repeaters in fiber optic networks to divide a long-distance fiber optic channel into multiple shorter channels, so that the quantum signal no longer decays exponentially with increasing distance, thereby extending the distance of quantum communication.

Quantum relay is an important way to achieve long-distance quantum communication in noisy channels. It has important scientific research and application value. The international competition for quantum repeater research is very fierce. Traditional quantum repeaters need to be based on three essential technologies: entanglement exchange, entanglement purification, and quantum storage.

Among them, quantum entanglement purification is a key operation in quantum relay. It can be used to extract an entangled state with a higher degree of entanglement from two entangled states with a lower degree of entanglement. Previous entanglement purification protocols were implemented using two pairs of photons with a low degree of entanglement, but the research group and its collaborators proposed an entanglement purification scheme that only requires one pair of super-entangled photons.

So, how to purify high-quality quantum entangled states?

The research team prepared super-entangled photon pairs with entangled polarization and path, distributed the entanglement in an 11-kilometer-long multi-core optical fiber, and then performed quantum entanglement purification operations. The experimental results show that when the initial fidelity of the distributed polarization entanglement and path entanglement is about 0.665, the fidelity of the purified entangled state can be improved to 0.774, and when the initial fidelity is about 0.771, the fidelity after purification can be improved to 0.887.

In addition, the research team used entanglement purification for quantum key distribution for the first time, increasing the effective key rate from 0 to 0.371.

This achievement made by USTC can be said to be a key step in the transition of entanglement purification from laboratory platform to long distance. At the same time, it has greatly improved the efficiency of entanglement purification and provided a strong technical guarantee for the realization of high-efficiency quantum relay in the future.