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We experimentally demonstrate long-distance continuous-variable quantum key distribution over a 50km standard optical fiber based on continuous-variable Einstein-Podolsky-Rosen entangled states. The entanglement survives despite being distributed over a high-loss optical fiber channel. At a channel excess noise level of 0.01 shot-noise units, we achieve an asymptotic secret key rate of 0.03 bit per sample, which is superior to the optimized coherent-state protocol. The superiority is even more evident at a highchannel excess noise level of 0.1 shot-noise units. Our work paves the way toward practical applications of continuous-variable quantum key distribution under high amounts of excess channel noise.

Measurement-device-independent quantum key distribution (MDI-QKD) can remove all side-channel attacks on detectors. In the context of the dramatic progress of discrete-variable MDI-QKD and twin-field QKD, owing to the critical challenge of continuous-variable (CV) Bell-state measurement (BSM) of two remote independent quantum states, experimental demonstration of CV-MDI-QKD over optical fiber has remained elusive. To solve this problem, a technology for CV-BSM of remote independent quantum states is developed that consists of optical phase locking, phase estimation, real-time phase feedback, and quadrature remapping in the present work. With this technology, CV-BSM is accurately implemented, and the first CV-MDI-QKD over optical fiber is demonstrated, to our knowledge. The achieved secret key rates are 0.43 (0.19) bits per pulse over a 5-km (10-km) optical fiber. Our work shows that it is feasible to build a CV-MDI-QKD system over optical fiber. Further, the results pave the way towards realization of a high secret key rate and low-cost metropolitan MDI-QKD network, and serve as a stepping stone to a CV quantum repeater.

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Continuous-variable entangled optical beams at the degenerate wavelength of 0.8 µm or 1.5 µm have been investigated extensively, but separately. The two-color entangled states of these two useful wavelengths, with sufficiently high degrees of entanglement, still lag behind. In this work, we analyze the various limiting factors that affect the entanglement degree. On the basis of this, we successfully achieve 6 dB of two-color quadrature entangled light beams by improving the escape efficiency of the nondegenerate optical amplifier, the stability of the phase-locking servo system, and the detection efficiency. Our entangled source is constructed only from a single ring optical resonator, and thus is highly compact, which is suitable for applications in long-distance quantum communication networks.