Wen-Qi Cai

Decoy-state quantum key distribution with polarized photons over 200 km

We report an implementation of decoy-state quantum key distribution (QKD) over 200 km optical fiber cable through photon polarization encoding. This is achieved by constructing the whole QKD system operating at 320 MHz repetition rate, and developing high-speed transmitter and receiver modules. A novel and economic way of synchronization method is designed and incorporated into the system, which allows to work at a low frequency of 40kHz and removes the use of highly precise clock. A final key rate of 15 Hz is distributed within the experimental time of 3089 seconds, by using super-conducting single photon detectors. This is longest decoy-state QKD yet demonstrated up to date. It helps to make a significant step towards practical secure communication in long-distance scope.We demonstrate the decoy-state quantum key distribution ov er 200 km with photon polarization through optical fiber, by usi ng superconducting single photon detector with a repetition rate of 320 Mega Hz and a dark count rate of lower than 1 Hz. Since we have used the pola rization coding, the synchronization pulses can be run in a low freque ncy. The final key rate is 14.1 Hz. The experiment lasts for 3089 seconds wit h 43555 total final bits.

Satellite-based entanglement distribution over 1200 kilometers

Entangled photons are distributed over vast distances using a satellite-to-ground link. Space calling Earth, on the quantum line A successful quantum communication network will rely on the ability to distribute entangled photons over large distances between receiver stations. So far, free-space demonstrations have been limited to line-of-sight links across cities or between mountaintops. Scattering and coherence decay have limited the link separations to around 100 km. Yin et al. used the Micius satellite, which was launched last year and is equipped with a specialized quantum optical payload. They successfully demonstrated the satellite-based entanglement distribution to receiver stations separated by more than 1200 km. The results illustrate the possibility of a future global quantum communication network. Science, this issue p. 1140 Long-distance entanglement distribution is essential for both foundational tests of quantum physics and scalable quantum networks. Owing to channel loss, however, the previously achieved distance was limited to ~100 kilometers. Here we demonstrate satellite-based distribution of entangled photon pairs to two locations separated by 1203 kilometers on Earth, through two satellite-to-ground downlinks with a summed length varying from 1600 to 2400 kilometers. We observed a survival of two-photon entanglement and a violation of Bell inequality by 2.37 ± 0.09 under strict Einstein locality conditions. The obtained effective link efficiency is orders of magnitude higher than that of the direct bidirectional transmission of the two photons through telecommunication fibers.

Metropolitan all-pass and inter-city quantum communication network

We have demonstrated a metropolitan all-pass quantum communication network in field fiber for four nodes. Any two nodes of them can be connected in the network to perform quantum key distribution (QKD). An optical switching module is presented that enables arbitrary 2-connectivity among output ports. Integrated QKD terminals are worked out, which can operate either as a transmitter, a receiver, or even both at the same time. Furthermore, an additional link in another city of 60 km fiber (up to 130 km) is seamless integrated into this network based on a trusted relay architecture. On all the links, we have implemented protocol of decoy state scheme. All of necessary electrical hardware, synchronization, feedback control, network software, execution of QKD protocols are made by tailored designing, which allow a completely automatical and stable running. Our system has been put into operation in Hefei in August 2009, and publicly demonstrated during an evaluation conference on quantum network organized by the Chinese Academy of Sciences on August 29, 2009. Real-time voice telephone with one-time pad encoding between any two of the five nodes (four all-pass nodes plus one additional node through relay) is successfully established in the network within 60 km.

Field test of a practical secure communication network with decoy-state quantum cryptography.

We present a secure network communication system that operated with decoy-state quantum cryptography in a real-world application scenario. The full key exchange and application protocols were performed in real time among three nodes, in which two adjacent nodes were connected by approximate 20 km of commercial telecom optical fiber. The generated quantum keys were immediately employed and demonstrated for communication applications, including unbreakable real-time voice telephone between any two of the three communication nodes, or a broadcast from one node to the other two nodes by using one-time pad encryption.

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.

Ground-to-satellite quantum teleportation

An arbitrary unknown quantum state cannot be measured precisely or replicated perfectly. However, quantum teleportation enables unknown quantum states to be transferred reliably from one object to another over long distances, without physical travelling of the object itself. Long-distance teleportation is a fundamental element of protocols such as large-scale quantum networks and distributed quantum computation. But the distances over which transmission was achieved in previous teleportation experiments, which used optical fibres and terrestrial free-space channels, were limited to about 100 kilometres, owing to the photon loss of these channels. To realize a global-scale ‘quantum internet’ the range of quantum teleportation needs to be greatly extended. A promising way of doing so involves using satellite platforms and space-based links, which can connect two remote points on Earth with greatly reduced channel loss because most of the propagation path of the photons is in empty space. Here we report quantum teleportation of independent single-photon qubits from a ground observatory to a low-Earth-orbit satellite, through an uplink channel, over distances of up to 1,400 kilometres. To optimize the efficiency of the link and to counter the atmospheric turbulence in the uplink, we use a compact ultra-bright source of entangled photons, a narrow beam divergence and high-bandwidth and high-accuracy acquiring, pointing and tracking. We demonstrate successful quantum teleportation of six input states in mutually unbiased bases with an average fidelity of 0.80 ± 0.01, well above the optimal state-estimation fidelity on a single copy of a qubit (the classical limit). Our demonstration of a ground-to-satellite uplink for reliable and ultra-long-distance quantum teleportation is an essential step towards a global-scale quantum internet.

Experimental free-space quantum key distribution with efficient error correction

We report a 17-km free-space quantum key distribution (QKD) experiment using an engineering model of the space-bound optical transmitter and a ground station for satellite-ground QKD. The final key rate of ~ 0.5 kbps is achieved in this experiment with the quantum bit error rate (QBER) of ~ 3.4%. An efficient error correction algorithm, Turbo Code, is employed. Compared with the current error correction algorithm of Cascade, a high-efficiency error correction is realized by Turbo Code with only one-time data exchange. For a low QBER, with only one-time data exchange, the final key rates based on Turbo code are similar with Cascade. As the QBER increases, Turbo Code gives higher final key rates than Cascade. Our results experimentally demonstrate the feasibility of satellite-ground QKD and show that the efficient error correction based on Turbo Code is potentially useful for the satellite-ground quantum communication.

A Compact Readout Electronics for the Ground Station of a Quantum Communication Satellite

Free-space quantum key distribution (QKD) is being developed for achieving unconditional secure communication over ultra-long distance, which requires higher electronics performance, such as higher time measurement precision, higher data-transfer rate, and higher system integration density. As part of the ground station of the Quantum Science Satellite that will be launched in 2016, we specifically designed a compact PCI-based measurement and control electronics with high time-resolution and high data-transfer-rate. Some necessary modules in the quantum communication experiment such as multi-channel counter, system monitor and experiment control are also integrated in a single board. The electronics performance of this system was tested, with the time precision bin size is 23.9 ps and the time resolution root-mean-square (RMS) is less than 24 ps for 16 channels. The dead time is 30 ns. The data transfer rate to a local computer is up to 35 MBps, and the count rate is up to 30 MHz. The system has been proven to perform well and operate stably through a test of in-door free space QKD experiment.Since the 1990s, there has been a dramatic interest in quantum communication. Free-space quantum communication is being developed to ultra-long distance quantum experiment, which requires higher electronics performance, such as time measurement precision, data-transfer rate, and system integration density. As part of the ground station of quantum experiment satellite that will be launched in 2016, we specifically designed a compact PCI-based multi-channel electronics system with high time-resolution, high data-transfer-rate. The electronics performance of this system was tested. The time bin size is 23.9ps and the time precision root-mean-square (RMS) is less than 24ps for 16 channels. The dead time is 30ns. The data transfer rate to local computer is up to 35 MBps, and the count rate is up to 30M/s. The system has been proven to perform well and operate stably through a test of free space quantum key distribution (QKD) experiment.

A compact PCI-based measurement and control system for satellite-ground quantum communication

Summary form only given. Since the 1990s, there has been a dramatic interest in quantum communication. Quantum communication has become the hotspot of future information technology research because of its unique advantages in improving the security, capability and efficiency of information transmission. Free space quantum communication is being developed to ultra-long-distance satelliteto-ground quantum experiment, which requires higher electronics performance, such as time measurement precision, data transfer rate, and system integration density. As part of ground station of scientific applications system of quantum experiment satellite that will be launched in 2016, a compact PCI-based multi-channel electronics system with high timeresolution, high data-transfer-rate is designed. The main tasks of this system are time measurement, data transmission and experiment control. Based on the compact PCI system, we designed the following functions in a standard 6U size circuit board. A 16-channel high resolution time-to-digital converter (TDC), using the field-programmable-gate-arrays (FPGAs) carry chains, is implemented to detect and record the arrival time of the input signals. The TDC is based on a counter and an interpolator method. Taking full advantage of the FPGAs flexibility, not only the above, but also some necessary modules in the quantum communication experiment such as multi-channel counter, system monitor and experiment control logic are integrated in a single FPGA. The system monitor module is used to measure the voltage and current on board, and FPGA temperature. A GPS signal is used to synchronize clocks over large distance with high precision and accuracy. A physical random generator provides enough random numbers by before the start of communication and stores them in a local memory. A synchronous dynamic random access memory (SDRAM) is used to guarantee no data loss during processing. The data transmission is based on compact peripheral component interconnect (CPCI) to guarantee a high data rate. A complex programmable logic device (CPLD) functions as the interface with the IP core between the FPGA and the PCI bus, and configures the FPGA online via PC.

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.