China's quantum memory achievement will take one step closer to breaking the 2048-bit RSA password
An important application of quantum computers is to break classical ciphers. Previous studies have shown that the number of qubits required to break the widely used 2048-bit RSA cipher is around 20 million, far exceeding the current state of the art. Recently, researchers from the French National Science Center published a paper in "Physical Review Letters" [1], proving that the use of quantum computers equipped with quantum memory can greatly reduce the number of physical qubits required by quantum computers.
Theoretical analysis shows that it takes 13,000 physical qubits and 177 days to solve the 2048-bit RSA cipher for such a quantum-storage quantum computer. Compared with previous results, this scheme reduces the number of physical bits required for quantum computers by three orders of magnitude, further approaching the current technological level.
This research extends the application of quantum memory to the field of quantum computing, and provides a new technical route for the development of practical quantum computers. However, the French team's scheme has stricter requirements on the number of quantum storage modes and lifetime. The additional cost is a quantum memory of 28 million spatial patterns and 45 temporal patterns, and the storage time is about 2 hours. Quantum storage quantum computers require quantum memory in the microwave band, which is a new technology that needs to be developed urgently.
On January 26, Zhou Zongquan, an associate professor born in the 1990s at the University of Science and Technology of China, published an article in Acta Physica [2] saying that in 2021, Li Chuanfeng and Zhou Zongquan’s research group from Academician Guo Guangcan’s team proposed a noise-free solution in Nature Communications [3]. The photon echo scheme (noiseless photon echo, referred to as NLPE) solves the spontaneous emission noise problem of photon echo, and is expected to further realize microwave quantum storage and apply it in quantum storage quantum computers.
Professor Li Chuanfeng (front) and Associate Professor Zhou Zongquan (back)
The research team performed an NLPE experimental demonstration in the optical band in Eu:YSO crystal [3], successfully observed the photon echo of a single photon with a signal-to-noise ratio exceeding 42.5±7.5, and realized the storage of time-bin qubits. In Eu-doped materials, the storage efficiency and signal-to-noise ratio achieved by NLPE achieved more than fourfold improvement over the only previously available atomic frequency comb scheme [4].
The NLPE scheme fully utilizes the original absorption band of the medium, so it is expected to become an effective storage scheme for quantum U-disk applications. Since the NLPE scheme does not need to cut the atomic absorption band and is as easy to implement as the standard photon echo, it is also expected to be an effective scheme for quantum storage in the microwave band and be applied in quantum storage quantum computers.
Zhou Zongquan explained that previous research on quantum memory has mainly focused on applications in the field of quantum communication, for example, establishing quantum relays based on multi-mode quantum storage to build a long-distance quantum Internet, or realizing mobility based on ultra-long-life quantum storage. Quantum U disk. This achievement of the French team quantitatively demonstrated the value of quantum memory in quantum computers, and successfully promoted the application scope of quantum memory.
However, the work of the French team has not yet considered the efficiency, noise and coherence lifetime of the storage device, which are also significant experimental challenges. Would a quantum computer with matching multimode quantum memory be easier to develop than a larger-scale quantum computer without quantum memory? At present, people do not have a completely definite answer to this question, but the work of the French team provides a feasible technical route for the development of practical quantum computers, which needs to be further explored.
However, a major challenge facing this technical route is that quantum storage in the microwave band has not been realized in any material system so far. The theoretical analysis of Li Chuanfeng and Zhou Zongquan's research group and the experiments in the optical band have proved that NLPE is an effective quantum storage solution, and the actual performance of this solution in the microwave band needs to be further explored experimentally.
references:
[1] Gouzien E and Sangouard N, 2021 Phys. Rev. Lett. 127 140503
[2] "Quantum Memory" Quantum Computers and Noiseless Phton Echoes. Acta Phys. Sin.. doi: 10.7498/aps.71.20212245
[3] Ma YZ, et al., 2021 Nature Communications 12 4378
[4] Jobez P, et al., 2015, Phys. Rev. Lett. 114 230502
