Quantum entanglement between memories such as China University of Science and Technology

[ Instrument Network Instrument R & D ] Pan Jianwei, Bao Xiaohui, Zhang Qiang, etc. of University of Science and Technology of China cooperated with Jinan Institute of Quantum Technology and Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences to make breakthroughs in the direction of quantum relay and quantum networks. By developing high-brightness light and atom entangled sources, low-noise and high-efficiency single-photon frequency conversion technology, and long-range single-photon precision interference technology, they successfully entangled two quantum memories that are 50 kilometers apart from each other to build a quantum based on quantum relay. The network has laid the foundation. The results were recently published in the international academic journal Nature.
Constructing a global quantum network and realizing quantum communication based on this is one of the goals of quantum information research. The development path of quantum communication networks widely used in the international academic community is to achieve wide-area large-scale coverage through satellite-based free space channels. Optical fiber network to achieve urban and inter-city ground coverage. However, due to the exponential attenuation of optical signals in the fiber, the farthest point-to-point ground safety communication distance is only on the order of hundreds of kilometers. The long-distance point-to-point transmission is changed to segment transmission, and quantum relay technology is used for cascading, which is expected to further greatly expand the safe communication distance and make it possible to build an all-quantum network.
A quantum network is a type of physical device that performs high-speed mathematical and logical operations, stores, and processes quantum information in accordance with the laws of quantum mechanics. When a device processes and calculates quantum information and runs a quantum algorithm, it is a quantum network. The concept of quantum networks originates from the study of reversible computers. The purpose of studying reversible computers is to solve the problem of energy consumption in computers.
Quantum communication combines the research results of modern physics and optical communication technology. The basic principles of physics guarantee the unconditional security of the key distribution process. Quantum key distribution can be divided into measurement-based and entangled-state based on the characteristics of the quantum state used. Quantum communication based on entangled states uses the quantum entanglement effect when transmitting information, that is, when two microscopic particles are coupled, when one particle's state changes, the other immediately changes accordingly.
However, limited by technical bottlenecks such as low entanglement of light and atoms, mismatch of atomic memory wavelength with communication fiber, and long-range single photon interference, the farthest fiber quantum relay was only on the order of kilometers. In response to the above technical problems, the team has carried out technical research in three areas: first, the use of ring cavity enhancement technology to improve the coupling between single photons and the atomic system, and optimize the transmission efficiency of the optical path, and improve the brightness of the previous entanglement of light and atoms. An order of magnitude; secondly, because the optical wavelength corresponding to the atomic memory has a loss of about 3.5 dB / km in the fiber, the optical signal will be attenuated to one billionth of a billion (10 ^ -17.5) in a 50 km fiber, making quantum communication Unable to achieve, the team independently developed a periodically polarized lithium niobate waveguide, and converted the light wavelength of the memory from near infrared (795 nm) to the communication band (1342 nm) through a non-linear difference frequency process. After 50 km of optical fiber, it was only attenuated to More than one percent, the efficiency has been improved by 16 orders of magnitude compared with the previous one. Finally, in order to achieve long-range single photon interference, the team designed and implemented a double phase locking scheme, which successfully transmitted the optical path difference caused by the transmission of 50 kilometers of fiber. Controlled around 50nm.
Remote quantum entanglement is the core resource for realizing long-range quantum communication, distributed quantum computing, and quantum precision measurement. However, due to the exponential loss of photons in the fiber with distance, the distance of quantum entanglement distribution is limited to the order of hundreds of kilometers. Limited by the efficiency of light sources, memories, and detectors, the expected transmission rate of quantum networks is very low. There are two important methods to increase the transmission rate, namely high-dimensional encoding of quantum states, or the use of multi-mode quantum memories, but the research progress is not satisfactory.
Based on previous research, Li Chuanfeng et al. Used light's orbital angular momentum to encode, first developed a narrow-band high-dimensional entangled light source, and then stored this entangled source in a solid-state quantum memory. The results show the storage fidelity of the three-dimensional entangled state Reached 99.1%.
The research team combined the above technologies, and finally realized the two-node entanglement via a 50km fiber transmission, and demonstrated the two-node entanglement via a 22km external field fiber. This work was highly praised by the reviewers of "Nature": "This result is very outstanding and a significant step towards the realization of quantum relay." In the current experiment, two quantum memories are located in the same laboratory. The team will then develop technologies such as phase synchronization of independent lasers to achieve truly remote two-node experiments. The above work is consistent with the multi-node entanglement technology previously implemented by the team (Nature Photonics, 13, 210, 2019), the deterministic entanglement technology based on Rydberg (Phys. Rev. Lett. 123, 140504, 2019), and 100 millisecond storage Technology (Nature Photonics. 10, 381, 2016), etc., is expected to promote the experimental research of quantum relay and all quantum networks.
Source: University of Science and Technology of China, Encyclopedia

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