First time: China achieves major breakthrough in quantum relay

Recently, Duan Luming's research group at the Institute of Cross-Disciplinary Information Sciences at Tsinghua University has made important progress in the field of quantum information. For the first time, they achieved an efficient entangled connection between two relay modules in a quantum relay protocol in an experiment, successfully demonstrating the large-scale improvement in the connection efficiency of quantum relay modules.

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This is seen as a key step towards practical quantum repeaters.

It is understood that the exponential attenuation of photons when propagating in optical fibers is the main problem faced in the realization of long-distance quantum communication and large-scale quantum networks, and the quantum relay protocol is the best solution to solve the optical fiber propagation loss.

In 2001, Duan Luming and his collaborators proposed the famous DLCZ (Duan-Lukin-Cirac-Zoller) quantum relay scheme, which used the combination of atomic quantum memory and single-photon channels to overcome the exponential decay problem of optical quantum signals in optical fibers. It has since continued to be a research hotspot in this field.

After nearly 20 years of hard work by research teams from all over the world, the experimental implementation of the DLCZ quantum relay protocol has made great progress in many aspects.

However, a key step in the quantum relay protocol, namely how to efficiently connect small-scale relay modules with adjacent relay modules through the storage of quantum memories to form a larger relay module, thereby expanding the distribution of quantum entanglement in space, has not yet been achieved due to difficulties in experimental technology.

Schematic diagram of the experimental system

During the research, the researchers first trapped ultra-low temperature rubidium atomic gas in a one-dimensional optical lattice, prepared the atoms in a clock state that is insensitive to magnetic field changes through optical pumping, and precisely controlled the magnetic field applied to the atoms. They successfully increased the coherence time of cold atom quantum relays to tens of milliseconds and ensured that the read quantum state had high fidelity.

Secondly, combined with a high-speed control system with real-time feedback, the asynchronous preparation of quantum entanglement inside two adjacent quantum relay modules is achieved by storing the relay module that generates quantum entanglement first until the adjacent relay module also generates quantum entanglement; finally, efficient entanglement connection of quantum relay modules is achieved through entanglement exchange between the two modules.

By making entangled connections in this way, the connection efficiency is linearly proportional to the time required to prepare the entanglement within a single module , which changes the complexity of the connection efficiency at scale compared to the quadratic time required to synchronously prepare quantum entanglement within two relay modules without using quantum storage in previous studies;

At the same time, when the probability of entanglement preparation within a single quantum relay module is 0.1%, the efficiency of the entanglement connection between two quantum relay modules can be increased by 353 times .

When the number of quantum relay modules expands from two to N in the future, this efficiency improvement corresponds to the exponential improvement in the efficiency of quantum entanglement distribution in direct quantum communications by quantum repeaters.

This experimental study achieved on-demand entanglement connection between different quantum relay modules for the first time by using quantum storage, and the connection efficiency was improved on a large scale, demonstrating the core acceleration capability of quantum repeaters for long-distance quantum communications.

Experimental process and quantum relay module entanglement connection efficiency improvement