Jincai Wu

Direct and full-scale experimental verifications towards ground-satellite quantum key distribution

Full-scale verifications for establishing quantum cryptography communication via satellites are reported. Three independent experiments using a hot-air balloon are performed: on a rapidly moving platform over a distance of 40 km, on a floating platform over a distance of 20 km, and over 96 km in air with a huge loss.

Satellite-to-ground quantum key distribution

Quantum key distribution (QKD) uses individual light quanta in quantum superposition states to guarantee unconditional communication security between distant parties. However, the distance over which QKD is achievable has been limited to a few hundred kilometres, owing to the channel loss that occurs when using optical fibres or terrestrial free space that exponentially reduces the photon transmission rate. Satellite-based QKD has the potential to help to establish a global-scale quantum network, owing to the negligible photon loss and decoherence experienced in empty space. Here we report the development and launch of a low-Earth-orbit satellite for implementing decoy-state QKD—a form of QKD that uses weak coherent pulses at high channel loss and is secure because photon-number-splitting eavesdropping can be detected. We achieve a kilohertz key rate from the satellite to the ground over a distance of up to 1,200 kilometres. This key rate is around 20 orders of magnitudes greater than that expected using an optical fibre of the same length. The establishment of a reliable and efficient space-to-ground link for quantum-state transmission paves the way to global-scale quantum networks.

Detection and compensation of basis deviation in satellite-to-ground quantum communications

Basis deviation is the reference-frame deviation between a sender and receiver caused by satellite motion in satellite-to-ground quantum communications. It increases the quantum-bit error ratio of the system and must be compensated for to guarantee reliable quantum communications. We present a new scheme for compensating for basis deviation that employs a BB84 decoding module to detect basis deviation and half-wave plate to provide compensation. Based on this detection scheme, we design a basis-deviation compensation approach and test its feasibility in a voyage experiment. Unlike other polarization-correction schemes, this compensation scheme is simple, convenient, and can be easily implemented in satellite-to-ground quantum communications without increased burden to the satellite.

Real time basis-deviation measurement system based on BB84 module

The purpose of this paper is to present a real time basis-deviation measurement system based on BB84 module. As BB84 module is the essential module in QKD receiver system, the basis-deviation measurement system can be directly implanted into the QKD receiver system to detect the polarization of photon current in real time during quantum key distribution. BB84 module distributes the incident photon current into four photon currents with the polarization of H (Horizontal), V (Vertical), + (+45°) and - (-45°). Their energies can be detected by four APD photon-detectors. Basis-deviation compute equation is deduced with the Stocks-vector of the optical devices path in BB84 module. The energies of the four distributed photon currents are collected in real time and then input to basis-deviation compute equation to calculate the basis-deviation. There is error bears on the effects produced by the optical elements in the BB84 module, so we built a set of software module to foundation the process of the real time polarization measurement system working. Thus we can see how all the parameters of the optical elements effects the calculation results. At last, we built a polarization photon current generator which can produce photon current with continuous changing polarization and a real time basis-deviation measurement system based on BB84 module in laboratory.

Space-to-Ground Quantum Key Distribution Using a Small-Sized Payload on Tiangong-2 Space Lab*

Quantum technology establishes a foundation for secure communication via quantum key distribution (QKD). In the last two decades, the rapid development of QKD makes a global quantum communication network feasible. In order to construct this network, it is economical to consider small-sized and low-cost QKD payloads, which can be assembled on satellites with different sizes, such as space stations. Here we report an experimental demonstration of space-to-ground QKD using a small-sized payload, from Tiangong-2 space lab to Nanshan ground station. The 57.9-kg payload integrates a tracking system, a QKD transmitter along with modules for synchronization, and a laser communication transmitter. In the space lab, a 50 MHz vacuum + weak decoy-state optical source is sent through a reflective telescope with an aperture of 200 mm. On the ground station, a telescope with an aperture of 1200 mm collects the signal photons. A stable and high-transmittance communication channel is set up with a high-precision bidirectional tracking system, a polarization compensation module, and a synchronization system. When the quantum link is successfully established, we obtain a key rate over 100 bps with a communication distance up to 719 km. Together with our recent development of QKD in daylight, the present demonstration paves the way towards a practical satellite-constellation-based global quantum secure network with small-sized QKD payloads.

Design of ground testing systems which are compatible with two kinds of communication terminals

Space laser communication and space quantum communication are all space optical communication, they are similar in communication links, high accuracy tracking and pointing, ground testing etc. The characters of space optical communication determine the necessity of testing and verifying communication terminals, for example, the laser beam reach diffraction limit in space laser communication, weak light detection or single photon detection should be carried out in space quantum communication, communication terminals are relatively moving refer to the ground station, and the terminals should have high quality in tracking target and pointing laser beam, so good system should be used in testing and verifying communication terminals. In this paper we did research on ground testing and verifying systems, we developed optical paths and two ground testing and verifying systems which are compatible with space laser communication and space quantum communication, the two systems contain one laboratory used system and one portable outfield used system. The laboratory used system mainly contains optical communication terminal, two dimensional (2D) simulation turntable, collimator and rear optical path, it can simulate the moving environment, transmission channel and relative motion of the satellite to ground or inter-satellite optical communication, it can also fully test the condition and performance of the optical communication terminals. The portable outfield used system mainly contains opto-mechanical subsystem, 2D turntable, electronic cabinet. The opto-mechanical subsystem is installed on the 2D turntable, it contains front and rear optical path. Electronic cabinet contains industry computer, turntable controller, GPS radio and optical controller, it mainly executes data acquisition, receiving and transmitting command. The portable outfield used system can be used in outfield, to help testing and verifying space-borne equipment. The design breaks through several difficulties, and improved the integrated degree of the system. Theory analysis and experiment shows the system can work properly.