PAN Jianwei and colleagues PENG Chengzhi and ZHANG Qiang, from University of Science and Technology of China of the Chinese Academy of Sciences (CAS), collaborating with WANG Xiangbin from Tsinghua University and YOU Lixing from Shanghai Institute of Microsystems of CAS, realized the measurement device independent quantum key distribution (MDI-QKD) experiment based on long-distance free space channel for the first time. The study was published online in Physics Review Letter.

Due to the fact that the atmospheric turbulence in free space channel destroys the spatial mode, it is necessary to use single-mode optical fiber for spatial filtering before interferometry. The low coupling efficiency and intensity fluctuation are the two major difficulties in this experiment.

The researchers in this study developed an adaptive optics system with strong turbulence resistance based on the stochastic gradient descent algorithm, which improved the total channel efficiency of dual links by about 4~10 times.

The rapid fluctuation of light intensity challenges clock synchronization and optical frequency comparison methods in the traditional optical fiber system to apply.

To tackle the synchronization problem, the researchers adopted a super stable crystal oscillator as the independent clock source at three experimental points and measuring the real-time feedback of the pulse arrival time, which achieves am accuracy of 32 ps.

The hydrogen cyanide molecular absorption cell was deployed at the both coding ends to calibrate the optical frequency and assured the frequency difference of the interference light was less than 10 MHz, achieving frequency locking.

The above breakthroughs helped to realize the first free space MDI-QKD experiment in Shanghai urban atmospheric channel. The length of the two channels is 7.7 km and 11.5 km respectively and the distance between two communication ends is 19.2 km.

This study provides the possibility of realizing more complex quantum information processing tasks based on long-distance quantum interference in free space channel. It is one step closer to "a global internet invulnerable to hackers may be a ways off," according to an article published in the website of American Physical Society.

 Experiment setup. It shows the top view of the experimental layout at the Pudong area, Shanghai. (Image by CAO Yuan et al.)