USTC realizes generation and application of quantum coherence based on particle indistinguishability

The team of Academician Guo Guangcan of USTC has made important progress in the basic research of quantum physics. The team Li Chuanfeng, Xu Jinshi, Sun Kai and other international collaborators such as Professor Rosario Lo Franco of the University of Palermo in Italy achieved the generation of quantum coherence by regulating the spatial indistinguishability of photons, and demonstrated its performance in quantum metrology tasks. practical application. The results were published in the internationally renowned journal Proceedings of the National Academy of Sciences on May 20.

Quantum coherence is the most basic essential property of quantum mechanics, which makes quantum systems appear incomprehensible phenomena such as "Schrödinger's cat" that are difficult to understand from a classical perspective. For a single-particle quantum system, quantum coherence is reflected in the superposition state of the computational fundamental vectors; for a multi-particle quantum system, if these particles are identical particles, even if no particle is in a coherent superposition state, the entire quantum system is in a state of superposition. There can be coherence. This coherence is due to the spatial indistinguishability of wave functions between identical particles.

However, for the quantum coherent resources based on particle indistinguishability, its experimental research faces two problems: one is that the indistinguishability between identical particles needs to be controlled to generate quantum resources with different degrees of coherence; the other is the need to demonstrate this The practical application of coherence to specific quantum information tasks, thereby demonstrating that it is a physically available quantum resource and not just the result of a specific mathematical form describing identical particles.

Schematic diagram of the quantum coherence of an identical particle system and its application in the phase channel identification task.

In recent years, Li Chuanfeng, Xu Jinshi, Sun Kai and others have continued to carry out experimental research on the indistinguishability of isotonic particles in optical systems. 6410 (2020)].

In this work, the research group developed an on-demand control technique for the spatial distribution of the wave function of identical particles in a polarization-path mixed-encoded two-photon system, thereby realizing a controllable photon coherence synthesizer, which generates a The fidelity of the most coherent state reaches 98.8%. The research group further designed a phase discrimination task to demonstrate the practical application of the generated quantum coherent resources in quantum metrology. The experimental results show that quantum coherence based on the indistinguishability of identical particles can improve the success probability of phase discrimination, and can coexist with coherence based on coherent superposition of individual particles, so it will find applications in specific quantum information tasks.

Experimental results of the quantitative relationship between coherence and indistinguishability. The inset is the density matrix of the most coherent states.

This work demonstrates that the indistinguishability of isotonic particles can be used as a quantum resource for physical applications, and provides a method to experimentally control the indistinguishability, thereby laying the foundation for its further application in quantum information tasks.

The reviewers spoke highly of the work: "...what is really relevant is to prove a direct quantitative connection between coherence and spatial indistinguishability and the use of this coherence for metrological applications. Therefore, I believe that the results are really outstanding…" (Proving that there is a quantitative link between coherence and indistinguishability and applying it to quantum metrology tasks is an important achievement. So I believe the experimental results are indeed outstanding.)

The co-first authors of the paper are Dr. Sun Kai, a special associate researcher of the Key Laboratory of Quantum Information, Chinese Academy of Sciences, and Liu Zhenghao, a doctoral student. This research was supported by the Ministry of Science and Technology, the National Foundation of China, the Chinese Academy of Sciences, and Anhui Province.

Paper link: https://www.pnas.org/doi/epdf/10.1073/pnas.2119765119