Type of non-reciprocity in a fiber Sagnac interferometer induced by geometric phases.

The non-reciprocity of a Sagnac interferometer provides ultra-high sensitivity for parameter estimation and offers a wide range of applications, especially for optical fiber sensing. In this work, we study a new type of non-reciprocity existing in an optical fiber Sagnac interferometer where the polarization dependent loss is taken into consideration. In particular, this non-reciprocity is irrelevant to the physical effects that have been considered in previous studies, which originates from the geometric phases induced by a continuous-weak-measurement. Thus, it has a unique phenomenon of sudden phase transition, which may open a new way for the future design of high precision optical fiber sensors.

Field and long-term demonstration of a wide area quantum key distribution network.

A wide area quantum key distribution (QKD) network deployed on communication infrastructures provided by China Mobile Ltd. is demonstrated. Three cities and two metropolitan area QKD networks were linked up to form the Hefei-Chaohu-Wuhu wide area QKD network with over 150 kilometers coverage area, in which Hefei metropolitan area QKD network was a typical full-mesh core network to offer all-to-all interconnections, and Wuhu metropolitan area QKD network was a representative quantum access network with point-to-multipoint configuration. The whole wide area QKD network ran for more than 5000 hours, from 21 December 2011 to 19 July 2012, and part of the network stopped until last December. To adapt to the complex and volatile field environment, the Faraday-Michelson QKD system with several stability measures was adopted when we designed QKD devices. Through standardized design of QKD devices, resolution of symmetry problem of QKD devices, and seamless switching in dynamic QKD network, we realized the effective integration between point-to-point QKD techniques and networking schemes.

Proof-of-principle experiment of reference-frame-independent quantum key distribution with phase coding.

We have demonstrated a proof-of-principle experiment of reference-frame-independent phase coding quantum key distribution (RFI-QKD) over an 80-km optical fiber. After considering the finite-key bound, we still achieve a distance of 50 km. In this scenario, the phases of the basis states are related by a slowly time-varying transformation. Furthermore, we developed and realized a new decoy state method for RFI-QKD systems with weak coherent sources to counteract the photon-number-splitting attack. With the help of a reference-frame-independent protocol and a Michelson interferometer with Faraday rotator mirrors, our system is rendered immune to the slow phase changes of the interferometer and the polarization disturbances of the channel, making the procedure very robust.