Multi-functional solid state quantum memory for multi-degree-of-freedom multiplexing realized by Zhongke University

[ China Instrument Network Instrument Development ] Recently, Guo Guangcan, a member of the Chinese Academy of Sciences and a professor at the University of Science and Technology of China, made new progress in the field of quantum storage. The team Li Chuanfeng and Zhou Zongquan successfully developed a solid-state quantum memory with multi-degree-of-freedom parallel multiplexing. For the first time in the world, it realizes the reuse of quantum storage across three degrees of freedom, and demonstrates the arbitrary photon pulse operation functions of time and frequency degrees of freedom. The results were published in the international journal Nature-Communication on August 24.

Figure 1 Schematic diagram of multi-degree-of-freedom multiplexing quantum storage

Due to insurmountable fiber channel losses, the current ground-based quantum communication distance is limited to the order of 100 kilometers. The quantum memory-based quantum relay scheme can effectively overcome the channel loss and expand the working distance of quantum communication. Therefore, quantum memory is the core device of future long-range quantum communication and quantum network. The key indicator of the practical application of quantum networks is the communication rate, and the multi-mode multiplexing quantum memory can greatly improve the communication rate of the quantum network. For a classic memory, such as a hard disk or a USB flash drive, one of its storage units can only store one bit at a time. For quantum memory, because of its quantum coherence, a memory cell can store a large number of qubits at a time, which is the concept of multiplexing. In principle, the degrees of freedom of the quantum memory can be multiplexed.

In recent years, Li Chuanfeng's research group has been working on the experimental study of multiplexed quantum storage based on rare earth doped crystals. In 2015, for the first time, the spatial freedom of photons was used to realize the reuse of quantum storage. The number of storage dimensions reached 51 dimensions, and the highest level of solid-state quantum storage dimensions has been maintained so far [Physical Review Letters 115, 070502 (2015)]. One dimension as a model, then there are 51 modes of spatial freedom. In the same year, using the time freedom of photons, 100 modes of deterministic single-photon quantum storage were realized, and the highest number of modes of multiplexed solid-state quantum storage has been maintained so far [Nature Communications 6, 8652 (2015)].

In order to further enhance the multiplexing capacity of quantum memories, the research group innovatively adopted a multi-degree-of-freedom parallel multiplexing storage scheme. For example, if there are M storage modes in the first degree of freedom, N modes in the second degree of freedom, and P modes in the third degree of freedom, the total number of multiplexing modes of the quantum memory is the product of the number of degrees of each degree of freedom. , ie M*N*P. The research team selected the time, space and frequency degrees of freedom of photons for parallel multiplexing, and achieved the first internationally realized multiplexing quantum storage across these three degrees of freedom. Two time modes, two frequency modes and three spatial modes are used in the experiment. The total number of modes reaches 2*2*3=12. The experimental results demonstrate the feasibility of multi-degree-of-freedom parallel multiplexing quantum storage. This new method of increasing the number of quantum storage modes will have important applications in the research of quantum networks and quantum USB flash drives.

Figure 2 Multi-degree-of-freedom multiplexing quantum storage experimental device diagram

In multi-mode multiplexed long-range quantum communication, the working modes of two relay nodes may be different. In order to perform further entanglement switching, different relay nodes must transform the working mode to the same mode, which requires a mode. Transform function. The research team cleverly designed storage schemes and storage devices to show that their multi-degree-of-freedom multiplexed quantum memory has arbitrary mode conversion functions in time and frequency degrees of freedom.

The team further demonstrated that their memories can perform arbitrary pulse operations with time and frequency freedom. Representative operations include pulse sequencing, splitting, frequency division, inter-frequency photon combining, and narrow-band filtering. The experimental results show that in all these operations, the three-dimensional quantum states carried by photons maintain a fidelity of about 89%. The memory device can realize all the operations required for Knill-Laflamme-Milburn type quantum calculation, so the result is expected to find more applications in the field of linear optical quantum computing.

The first author of the thesis is Ph.D. student Yang Tianshu. The work was funded by the Ministry of Science and Technology, the National Natural Science Foundation of China, and the Collaborative Innovation Center for Quantum Information and Quantum Technology of the Chinese Academy of Sciences.

(Original title: China Science and Technology University realizes multi-degree-of-freedom multiplexing multi-function solid-state quantum memory)