Cheng-Zhi Peng

We report a quantum key distribution system that is able to generate key at a record high key rate of 115.8 Mb/s over 10-km standard fibre. This attributes to a high-efficiency multi-pixel superconducting nanowire detector, a low-error integrated transmitter, and a fast post-processing algorithm.

Quantum key distribution (QKD) can provide information-theoretic security governed by the law of quantum physics. Toward real-life applications, secret key rate is a key figure of merit of the QKD system. Here we demonstrate a 2.5-GHz polarization-encoding QKD system with an integrated silicon photonic transmitter that is able to generate a secret key rate of 2.42±0.04 Mbps over 101-km standard telecom fibers (19.6-dB loss). Such high rate attributes to the high clock-rate transmission and the ultra-low quantum bit error rate of 0.49%. The scalability, miniaturization and stability offered by silicon photonic technologies along with high-key-rate performance indicate that our system is a promising solution for large-scale deployment of QKD.

Measurement-device-independent quantum key distribution (MDI-QKD), based on two-photon interference, is immune to all attacks against the detection system and allows a QKD network with untrusted relays. Since the MDI-QKD protocol was proposed, fiber-based implementations aimed at longer distance, higher key rates and network verification have been rapidly developed. However, owing to the effect of atmospheric turbulence, MDI-QKD over free-space channel remains experimentally challenging. Herein, by developing a robust adaptive optics system, high-precision time synchronization and frequency locking between independent photon sources located far apart, we realized the first free-space MDI-QKD over a 19.2-km urban atmospheric channel, which well exceeds the effective atmospheric thickness. Our experiment takes the first step towards satellite-based MDI-QKD. Moreover, the technology developed herein opens the way to quantum experiments in free space involving long-distance interference of independent single photons.

Measurement-device-independent quantum key distribution (MDI-QKD) removes all detector side channels and enables secure QKD with an untrusted relay. It is suitable for building a star-type quantum access network, where the complicated and expensive measurement devices are placed in the central untrusted relay and each user requires only a low-cost transmitter, such as an integrated photonic chip. Here, we experimentally demonstrate a 1.25 GHz silicon photonic chip-based MDI-QKD system using polarization encoding. The photonic chip transmitters integrate the necessary encoding components for a standard QKD source. We implement random modulations of polarization states and decoy intensities, and demonstrate a finite-key secret rate of 31 bps over 36 dB channel loss (or 180 km standard fiber). This key rate is higher than state-of-the-art MDI-QKD experiments. The results show that silicon photonic chip-based MDI-QKD, benefiting from miniaturization, low-cost manufacture and compatibility with CMOS microelectronics, is a promising solution for future quantum secure networks.

In this presentation, I will introduce two of our recent works: experimental realization of measurement-device-independent (MDI) quantum key distribution (QKD) [arXiv:1209.6178] and unambiguous-state-discrimination (USD) attack on a decoy-state QKD system without phase randomization [arXiv:1304.2541]. On one hand, the MDI-QKD is able to shield all practical attacks realized so far. We experimentally demonstrate the MDI-QKD protocol by implementing high-speed and low-noise up-conversion single photon detectors. The security of MDI-QKD relies on a trusted source scenario, where the decoy-state method is assumed. On the other hand, phase randomization is commonly ignored from the decoy-state method. We demonstrate a USD attack on a decoy-state QKD system when the phase randomization is ignored.

Hong-Ou-Mandel interference is essentially a quantum mechanical phenomenon, which is a key part in quantum communication protocols and linear optical quantum computation, especially for measurement-device-independent quantum key distribution. The main challenge of the two-photon interference is to keep the indistinguishability of the two photons. This requirement becomes much more crucial on a large scale and even in free space, where both fluctuations and photon loss overwhelm the fragile interference phenomenon. Here we report a field test of the Hong-Ou-Mandel interference with one arm being a 220 m free-space path. Our experiment is a step towards long distance free-space measurement-device-independent quantum key distribution.