A team of scientists led by Prof. PAN Jianwei and his colleagues PENG Chengzhi and ZHANG Qiang from the University of Science and Technology of China demonstrated a 53-km free-space quantum key distribution (QKD) during the daytime, the ground-based experiments addressing high channel loss and noise of daylight, thus confirms the feasibility of satellite constellation (SC)-based global quantum communication network. This research, entitled “Long-distance free-space quantum key distribution in daylight towards inter-satellite communication”, has just been published in Nature Photonics (doi:10.1038/nphoton.2017.116).
Satellite-based quantum communication is known as the most feasible way to achieve global quantum communication. Following the launch of the world’s first quantum experimental satellite Micius, subsequent satellite-based quantum communication experiments have opened up bright prospects for world-wide practical quantum communications network research. Due to solar background noise, however, Micius could only work during the night, as daylight would induce an around 50 dB higher noise . With a single satellite of this kind, providing a worldwide connectivity would take a period of three days.
To provide global real-time quantum communication connectivity, a feasible solution is the building of a satellite constellation (SC), composed of multiple quantum satellites operating in Low-Earth-Orbits (LEO) or High-Earth-orbits (HEO) such as geosynchronous orbit (GEO) satellites.
The construction of SC requires technologies to overcome high channel losses and sunlight background noise. The total channel loss between a LEO satellite and the Earth and between LEO satellites is typically around 40 − 45 dB, and the probability of a satellite being in sunlight increases rapidly with the increasing orbit height. Micius, e.g. operating at a height of 500 km from the Earth, has a 68% probability of being in the sunlight area, while that of GEO satellites with orbit heights over 36,000 km rise to ∼ 99%. The background noise during daytime is typically 5 orders of magnitude greater than that during night time.
Figure 1: Satellite-constellation based global quantum network. a) Transmittance spectra from visible to near infrared light in atmosphere at selected zenith angles (the transmission efficiency is slightly higher at 1,550 nm than at 800 nm); b) Solar radiation spectrum from visible to near infrared light (the sunlight intensity at 1,550 nm is around five times weaker than it is at 800 nm).
Aiming to reduce sunlight background noise, major technological breakthroughs were made in three aspects: working wavelength selection, spectrum filtering and spatial filtering. Unlike all previous experiments, which selected the working wavelength between 700 − 800 nm, the Chinese scientists selected 1550 nm, known as another atmospheric window in the solar spectrum and less noise influenced by scattering. The measured noise of 1550 nm light in the daylight case was smaller by ~22.5, one order of magnitude lower, than that of 850 nm light. Also, the team developed compact up-conversion single-photon detectors (SPD), narrow filtering was realized while maintaining the high efficiency of single photon detecting, thus reduce the noise by two orders of magnitude. In addition, the group used a single mode fiber (SMF) to couple the single photons, with a field of view (FOV) of less than 10 μrad, so the noise introduced by stray light was reduced by almost two orders of magnitude. With the integrated application of the above technologies, the scientists conducted a QKD field test between two locations distancing 53 km across Qinghai Lake in daytime. The experiment showed through a 48 dB loss channel (greater than that of satellite-to-ground loss and inter-satellite loss), the error rate of signal state was ~1.65% and the key rate reached ~150 bps. Moreover, 1,550 nm is the telecom-band wavelength and is widely used for fiber optical communication. The work proves the feasibility of a LEO quantum SC which works mostly in the daylight and offers an optimal option for a global quantum network consisting of a quantum SC and the existing ground fiber networks.
Referees of Nature Photonics commented the research as “addresses an important challenge for day-time operation of free-space QKD” and “a remarkable achievement”.
This research is funded by the Chinese Academy of Sciences, National Natural Foundation of China, the Ministry of Science and Technology of China and the Ministry of Education of China.
(Jane Zhou, LIU Yu, Synergetic Innovation Center of Quantum Information and Quantum Physics, Hefei National Laboratory for Physical Sciences at Microscale, School of Physical Sciences, Science and Technology Research Office )