We demonstrate the first implementation of polarization encoding measurement-deviceindependent quantum key distribution (MDI-QKD), which is immune to all detector side-channel attacks. Active phase randomization of each individual pulse is implemented to protect against attacks on imperfect sources. By optimizing the parameters in the decoy state protocol, we show that it is feasible to implement polarization encoding MDI-QKD over large optical fiber distances. A 1600-bit secure key is generated between two parties separated by 10 km of telecom fibers. Our work suggests the possibility of building a MDI-QKD network, in which complicated and expensive detection system is placed in a central node and users connected to it can perform confidential communication by preparing polarization qubits with compact and low-cost equipment. Since MDI-QKD is highly compatible with the quantum network, our work brings the realization of quantum internet one step closer. Quantum key distribution (QKD) allows two parties, normally referred to as Alice and Bob, to generate a private key even with the presence of an eavesdropper, Eve [1,2]. With perfect single photon sources and single photon detectors, the security of QKD is guaranteed by quantum mechanics [3]. However, the aforementioned perfect devices are not available today and the security of QKD cannot be guaranteed in real life implementation. For example, attenuated coherent laser pulses are commonly used in practical QKD setups, which makes the QKD system vulnerable to the photon number splitting (PNS) attack [4]. Fortunately, it has been shown that the unconditional security of QKD can still be assured with phase randomized weak coherent pulses [5]. Furthermore, by applying decoy state techniques [6], secure key rate can be dramatically increased in practical implementations [7]. Nonetheless, other imperfections in practical QKD systems still present loopholes that can be exploited by Eve to steal the secret key [8,9]. We remark that most of the identified security loopholes are due to imperfections in the detection systems [8].Much effort has been put to build loophole-free QKD systems with practical devices. On one hand, people have been trying to build a better model to understand all the imperfections in a QKD detection system [10], but it is almost impossible to guarantee that all the loopholes have been fixed. On the other hand, full device-independent QKD (DI-QKD) has been proposed to close all the loopholes due to devices' imperfections [11]. The security of DI-QKD relies on the violation of Bell's inequality and does not require any knowledge of how practical QKD devices work. However, the demand for single photon detectors with near unity detection efficiency and the low key rate make this protocol highly impractical [12].Fortunately, measurement-device-independent QKD (MDI-QKD), which removes all loopholes in detec- arXiv:1306.6134v2 [quant-ph]

This document provides supplementary information to "Silicon photonic transmitter for polarization-encoded quantum key distribution," http://dx.doi.org/10.1364/optica.3.001274. Details on the refractive index and absorption change as well as the electroluminescence in the forward-biased silicon (Si) PIN diodes are described. © 2016 Optical Society of America http://dx.doi.org/10.1364/optica.3.001274.s001 REAL AND IMAGINARY REFRACTIVE INDEX CHANGE IN SI WAVEGUIDE PIN DIODESThe modulation of the refractive index in silicon (Si) is typically through the plasma dispersion effect. The carrier density changes both the real and imaginary parts of the refractive index, which can cause, for example, a reduced extinction ratio in an optical attenuator or modulator, and polarization dependent loss in a polarization controller. According to [1], the changes in the refractive index Δn and absorption Δα of Si near a wavelength of 1550 nm arewhere ΔN e and ΔN h are changes in the free electron density and free hole density measured in cm −3 . By incorporating both Δn and Δα into a mode-solver, the coupled changes in the real and imaginary parts of the effective index as a function of carrier density can be modelled. Experimentally, for the Si waveguide PIN diode with the cross-section illustrated in Fig. S1(a), which is similar to the one used in the present work, the measured phase-shift and attenuation as function of the applied forward bias voltage are shown in Fig. S1(b) and Fig. S1(c), respectively [2]. The length of the Si PIN diode used for these measurements was 500 μm. The measured differential phase-shift was about −7.3π/(mm · V) and the corresponding differential absorption change was about 20 dB /(mm · V). These figures have been reproduced from [2]. The Si PIN diodes we have used in the current work had P++ and N++ regions that were 700 nm away from the waveguide core compared to 800 nm in Fig. S1. The reduced separation would lead to a slightly lower series resistance. ELECTROLUMINESCENCE FROM SI PIN DIODESWe observed that the Si waveguide PIN diodes in forward bias could generate weak electroluminescence. Fig. S2(a) shows the electroluminescence spectrum of a 1000 μm-long PIN diode at several forward bias voltages. The electroluminescence is broadband and centered near a wavelength of 1150 nm, close to the bandgap energy of Si (1.1 eV = 1130 nm). This electroluminescence is not power efficient due to the indirect bandgap of Si. Fig. S2(b) shows the current vs. voltage relationship of the diode. Fig. S2(c) shows the total optical power collected

Measurement-device-independent quantum key distribution (MDI-QKD), which is immune to all detector side-channel attacks, is the most promising solution to the security issues in practical quantum key distribution systems. Though several experimental demonstrations of MDI-QKD have been reported, they all make one crucial but not yet verified assumption, that is there are no flaws in state preparation. Such an assumption is unrealistic and security loopholes remain in the source. Here we present, to our knowledge, the first MDI-QKD experiment with the modulation error taken into consideration. By applying a security proof by Tamaki et al (Phys. Rev. A 90, 052314 (2014)), we distribute secure keys over fiber links up to 40 km with imperfect sources, which would not have been possible under previous security proofs. By simultaneously closing loopholes the detectors and a critical loophole -modulation error in the source, our work shows the feasibility of secure QKD with practical imperfect devices.PACS numbers: 03.67. Dd, 03.67.Hk, 42.50.Ex Quantum key distribution (QKD), in principle, offers unconditional security based on the laws of quantum physics rather than computational complexity [1]. However, it has been realized that, due to the gap between the security proof and real-life implementations, practical QKD systems are vulnerable to various attacks [2].Device-independent QKD (DI-QKD) [3], was proposed to remove all assumptions of the internal working of devices of QKD. The security of DI-QKD is based on the loophole-free Bell test. Despite a number of recent experimental demonstrations of loophole-free Bell test [4], DI-QKD is impractical at practical distances (20-30 km of telecom fiber) due to its low key rate of about 10 −10 bit per pulse [5]. Fortunately a protocol, namely the Measurement-Device-Independent QKD (MDI-QKD), whose security is built on the time-reversed entanglement QKD [6] , has been proposed [7] to remove all potential security loopholes in the detection side, the most vulnerable part of a QKD system (See also [8] It is conceivable that MDI-QKD [7] will be widely adopted in the near future. Since MDI-QKD is intrinsically immune to all detector side-channel attacks, eavesdroppers will shift their focus from hacking the detectors to hacking the sources, which are not protected in MDI-QKD. Several theoretical studies on MDI-QKD with imperfect sources have been reported [17].A crucial assumption in discrete-variable MDI-QKD is that the source employed must be trusted. An ideal trusted source need to satisfy two conditions: first, the source only emits single photons; second, information should be encoded without flaws. However, these two conditions cannot be satisfied perfectly with today's technology. First, phase-randomized weak coherent pulses (WCPs) rather than single-photon sources are widely used in most QKD (including BB84 and MDI-QKD) demonstrations. Fortunately, it has been shown that unconditional security can still be achieved with phase-randomized WCPs [18]. Furthermore, the perfo...

Decoy-state quantum key distribution (QKD) is a standard technique in current quantum cryptographic implementations. Unfortunately, existing experiments have two important drawbacks: the state preparation is assumed to be perfect without errors and the employed security proofs do not fully consider the finite-key effects for general attacks. These two drawbacks mean that existing experiments are not guaranteed to be secure in practice. Here, we perform an experiment that for the first time shows secure QKD with imperfect state preparations over long distances and achieves rigorous finite-key security bounds for decoy-state QKD against coherent attacks in the universally composable framework. We quantify the source flaws experimentally and demonstrate a QKD implementation that is tolerant to channel loss despite the source flaws. Our implementation considers more real-world problems than most previous experiments and our theory can be applied to general QKD systems. These features constitute a step towards secure QKD with imperfect devices.Comment: 12 pages, 4 figures, updated experiment and theor

We experimentally demonstrate the direct generation of polarization-entangled photon pairs in an optical fiber at room temperature by exploiting type-II phase-matched spontaneous parametric down-conversion. A second-order nonlinearity is artificially induced in the 17-cm-long weakly birefringent step-index fiber through the process of thermal poling, and quasi-phase-matching allows for the generation of entangled photons in the 1.5-micron telecom band when the fiber is pumped at 775 nm. A greater-than 80:1 coincidence-to-accidental ratio is achieved, limited mainly by multiphoton pair generation. Without the need to subtract accidentals or to compensate for walk-off, the raw two-photon interference visibility is found to be better than 95%, and violation of Bell's inequality is observed by more than 18 standard deviations. This makes for a truly alignment-free, plug-and-play source of polarization-entangled photon pairs.

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