The new achievements are expected to open up new prospects for quantum applications

On August 1, Nature Communications published online the research results of Li Lin’s research group of Professor Li Lin of the School of Physics of Huazhong University of Science and Technology. For the first time, the purity and full homogeneity of the Reedborg single photon source were increased to more than 99.9%, and the single photon source was used to achieve the highest fidelity optical quantum logic gate in the world. The research results are expected to open up new prospects for important quantum applications such as optical quantum information processing and distributed optical quantum systems.

Optical quantum logic gate concept diagram Courtesy of Huazhong University of Science and Technology

Single-photon sources are one of the core quantum resources needed for quantum information and precision measurement research. Many important quantum optical applications place extremely high demands on the quality of single photons, for example, to meet applications such as all-optical quantum repeaters and clustered optical quantum computing, the purity of single photons must be more than 99.9%, and the full homogeneity must be greater than 99%. Over the past few decades, different physical systems have been developed to produce single photons, and although the quality of single photons has improved dramatically, achieving single photon sources that meet both high purity and high full homogeneity remains a major challenge.

In recent years, quantum physics research based on Reedberg atoms has developed rapidly, and the extremely strong and controllable interactions between Reedberg atoms have opened up new possibilities for efficient quantum manipulation at the single photon level. Li Lin’s research group has long been committed to the development of quantum information processing and precision measurement technology based on Reedberg atoms, and after several years of continuous efforts, the research group has successfully built a quantum physics experimental platform based on Reedberg atoms.

In this study, the research group used the interaction between reedberg atoms to achieve high-precision excitation and manipulation of superatomic quantum states, and prepared a high-quality single-photon source with a purity of 99.95% and a full homogeneity of 99.94% based on this.

Schematic diagram of preparation of single photon source and quantum logic gate experiment Courtesy of Huazhong University of Science and Technology

The research group also carried out an important application study of single-photon sources – optical quantum logic gates. Quantum logic gates are the core units of important applications such as quantum computing, and their fidelity directly affects the scalability of quantum systems. In existing qubits, photons are the best carrier for transmitting quantum information over long distances, so achieving high-fidelity optical quantum information processing is essential for building large-scale quantum networks, distributed quantum computing. Foreign scholars proposed in 2001 that the use of quantum interference and projection measurements can achieve photon-photon quantum logic gates, but in this scheme, the fidelity of the optical quantum logic gate is limited by the quality of a single photon source. To achieve a quantum logic gate with greater than 99% fidelity based on this scheme, the purity of the single photon must be greater than 99.3%, and the degree of full homogeneity must be greater than 99%. These harsh metrics make optical quantum logic gates with greater than 99% fidelity so far unimplemented.

In this study, the team used a high-quality Reedberg single-photon source to demonstrate near-perfect two-photon quantum interference, applied it to the light quantum logic gate experiment based on the KLM scheme, and successfully increased the fidelity of the truth table to 99.84%. Using this high-fidelity optical quantum logic gate, the research group further demonstrated the establishment of quantum entanglement between two unrelated single photons, and the quantum entanglement measurement was made by quantum chromatography and Bell’s inequality, and the fidelity of the entanglement gate reached 99.69%. Compared with the previous results of similar experiments, this study reduces the error (distortion) of the optical quantum logic gate by more than an order of magnitude.

This research will contribute to the development of all-optical quantum information processing, in which high-quality single-photon sources and high-fidelity metric logic gates can be used to prepare important multiphoton entangled states such as clusters, and thus build optical quantum computing systems with fault tolerance functions. In order to further increase the number of entangled photons, the research group will also explore a new scheme for coupling Rydberg atoms with high-fine resonance chambers to improve the efficiency of light quantum state generation. In addition, the Reedberg atom is also one of the excellent platforms for quantum computing, using its excellent light-matter quantum interaction capabilities, the high-quality light quantum state achieved by the study can connect multiple Reedberg quantum nodes, which is expected to build a quantum computing network with greater scalability. The research group also developed high-precision Reedberg atomic regulation technology in this research, laying the foundation for future quantum precision measurements based on Reedberg atoms. (Source: China Science Daily Chen Bin)

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