Remote quantum distribution on the ground is limited because of the loss of photon in optical fibers. One solution for remote quantum communication lies in quantum memories: photons are stored in the long-lived quantum memory (quantum flash drive) and then quantum information is transmitted by the transportation of the quantum memory. Given the speed of aircrafts and  high-speed trains, it is critical to increase the storage time of the quantum memories to the order of hours.

A research team led by Prof. LI Chuanfeng and Prof. ZHOU  Zongquan from University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS) extended the storage time of the optical memories to over one hour. They broke the record of one minute achieved by German researchers in 2013 and made a great stride towards the application of quantum memories. The work was published in Nature Communications on April 22nd.

In the attempt to achieve optical storage in a  zero-first-order-Zeeman (ZEFOZ) magnetic field, the complicated and unknown energy level structures in both the ground and excited states have challenged  researchers for a long time. Recently, researchers used the spin Hamiltonians to predict the level structures. However, an error may occur in the theoretical  prediction.

To overcome the problem, researchers from USTC adopted the spin wave atomic frequency comb (AFC) protocol in a ZEFOZ field, namely ZEFOZ-AFC method, successfully implementing the long-lived storage of light signals. In this work, dynamical decoupling (DD) is used to protect the spin coherence and extend the storage time. The coherent nature of this device is verified by implementing a time-bin-like interference experiment after 1h storage with a fidelity of 96.4%. The result shows that the great storage capacity of the coherent light and its potential in quantum memories.

Experimental setup of 1hour One-hour coherent optical storage (Image by MA Yu et al.)

This work expands the optical storage time from the order of  minutes to the order of hours. It meets the basic requirements of the optical storage lifetime for quantum memories. Through optimizing the storage efficiency and signal-to-noise ratios (SNR), researchers are expected to transmit quantum information by classical carriers in a new quantum channel.

Paper link: https://www.nature.com/articles/s41467-021-22706-y

(Written by CAI Bohan, edited by LU Hongyu, USTC News Center)