Through collaboration with German researchers, PAN Jianwei's group from Hefei National Laboratory for Physical Sciences at the Microscale (HFNL) has recently realized high-performance quantum memory in which long life time and high retrieval efficiency meet for the first time, taking a crucial step towards scalable long-distance quantum communication and scalable optical quantum computing. This work has been published on May 20 in the journal of Nature Physics.
The main purpose of quantum memory is to coherently store single quantum state and convert it back on demand, so as to realize time synchronization between different quantum operations. Quantum memory is the key element of quantum repeater and large-scale optical quantum computing. It has been realized with different physical systems, such as cold atomic ensemble, hot atomic ensembles, single neutral atoms, cryogenic solid state systems, etc. Judging from the two most important parameters, storage lifetime and retrieval efficiency, cold atomic ensembles perform the best so far and show a nice perspective towards scalable quantum communication and optical quantum computing.
In the past experimental studies, efforts towards extension of memory lifetime and retrieval efficiency have been conducted separately, which results that long storage lifetime and high retrieval efficiency have never been met in a single experiment. Specifically, in the quantum memory experiments claiming long lifetime, even though millisecond regime storage time has been realized, the retrieval efficiency is merely ~20%; in the experiments claiming high retrieval efficiency, even though retrieval efficiency has be improved to ~70%, the storage lifetime is merely several microseconds. The incapability of accomplishing long lifetime and high retrieval efficiency in a single quantum memory strongly limits its applications in more advanced quantum information tasks.
In extending storage lifetime, PAN's group has found in 2008 that the random movement induced spin wave dephasing is the dominant decoherence mechanism limiting millisecond regime storage. In improving retrieval efficiency, according to related experimental studies, it is considered as an effective method to use an optical cavity to enhance the retrieval efficiency. Therefore, how to incorporate the methods and techniques developed in these two lines of researches, is of key importance to realize long lifetime storage and high efficiency retrieval simultaneously in cold atomic ensembles.
Fig 1: Structure of high-performance quantum memory/ Copyright by PAN's group
To increase the spin wave wavelength in order to have long storage lifetime, it is required to configure the setup in a colinear configuration. To separate forward scattered photons from back scattered photons and enhance the retrieval efficiency, it is required to use a ring cavity. Combination of these techniques results in a difficulty to realize quadruple resonance between the cavity and the optical modes. By making use of a delicate experimental design, the researchers alleviate the quadruple resonance problem into a double resonance problem, and relieve the experimental difficulty. Through overcoming a series of other technical difficulties, they finally achieved an excellent result of 3.2 ms storage lifetime and 73% retrieval efficiency. The referee thinks that this work "represents a very significant achievement towards scalable quantum information processing protocols", and "will most likely open the door to new demonstrations of more complex quantum protocols involving several atomic ensembles".
(LV Lingjuan, Hefei National Laboratory for Physical Sciences at the Microscale, USTC News Center )