To increase dramatically the distance and the secure key generation rate of quantum key distribution (QKD), the idea of quantum decoys-signals of different intensities -has recently been proposed. Here, we present the first experimental implementation of decoy state QKD. By making simple modifications to a commercial quantum key distribution system, we show that a secure key generation rate of 165 bit=s, which is 1=4 of the theoretical limit, can be obtained over 15 km of a telecommunication fiber. We also show that with the same experimental parameters, not even a single bit of secure key can be extracted with a non-decoy-state protocol. Compared to building single photon sources, decoy state QKD is a much simpler method for increasing the distance and key generation rate of unconditionally secure QKD. DOI: 10.1103/PhysRevLett.96.070502 PACS numbers: 03.67.Dd, 42.50.Dv Quantum key distribution (QKD) [1,2] was proposed as a method of achieving perfectly secure communications. Any eavesdropping attempt by a third party will necessarily introduce an abnormally high quantum bit error rate in a quantum transmission and thus be caught by the users. With a perfect single photon source, QKD provides proven unconditional security guaranteed by the fundamental laws of quantum physics [3,4].Most current experimental QKD setups are based on attenuated laser pulses which occasionally give out multiphotons. Therefore, any security proofs must take into account the possibility of subtle eavesdropping attacks, including the photon-number splitting attack [5]. A hallmark of those subtle attacks is that they introduce a photonnumber dependent attenuation to the signal. Fortunately, it is still possible to obtain unconditionally secure QKD, even with (phase randomized) attenuated laser pulses, as theoretically demonstrated in [6] and by Gottesman-Lo-Lütkenhaus-Preskill (GLLP) [7]. However, one must pay a steep price by placing severe limits on the distance and the key generation rate. See also [8].A key question is this: How can one extend the distance and key generation rate of secure QKD? A brute force solution to this problem would be to use a (nearly) perfect single photon source. Despite much experimental effort [9], reliable perfect single photon sources are far from practical.Another solution to increase the transmission distance and key generation rate is to employ decoy states, using extra states of different average photon number to detect photon-number dependent attenuation. It has attracted great recent interest. The decoy method was first discovered by Hwang [10]. In [11], we presented the first rigorous security proof of decoy state QKD. We showed that the decoy state method can be combined with the standard GLLP result to achieve dramatically higher key generation rates and distances. Moreover, we proposed practical protocols with vacua or weak coherent states as decoys. Subsequently, the security of practical protocols have been analyzed by Wang [12] and us [13]. See also [14]. In particular, we [13] demonstr...

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]

A light-storage experiment with a total (storage and retrieval) efficiency η=56% is carried out by enclosing a sample, with a single-pass absorption of 10%, in an impedance-matched cavity. The experiment is carried out using the atomic frequency comb (AFC) technique in a praseodymium-doped crystal (0.05%Pr(3+):Y2SiO5) and the cavity is created by depositing reflection coatings directly onto the crystal surfaces. The AFC technique has previously by far demonstrated the highest multimode capacity of all quantum memory concepts tested experimentally. We claim that the present work shows that it is realistic to create efficient, on-demand, long storage time AFC memories.

We present a high-speed random number generation scheme based on measuring the quantum phase noise of a single-mode laser operating at a low intensity level near the lasing threshold. A delayed self-heterodyning system has been developed to measure the random phase fluctuation. By actively stabilizing the phase of the interferometer, a random number generation rate of 500 Mbit/s has been demonstrated and the generated random numbers have passed all the DIEHARD tests.

Twin-field (TF) quantum key distribution (QKD) is highly attractive because it can beat the fundamental limit of secret key rate for point-to-point QKD without quantum repeaters. Many theoretical and experimental studies have shown the superiority of TFQKD in long-distance communication. All previous experimental implementations of TFQKD have been done over optical channels with symmetric losses. But in reality, especially in a network setting, the distances between users and the middle node could be very different. In this paper, we perform a first proof-of-principle experimental demonstration of TFQKD over optical channels with asymmetric losses. We compare two compensation strategies, that are (1) applying asymmetric signal intensities and (2) adding extra losses, and verify that strategy (1) provides much better key rate. Moreover, the higher the loss, the more key rate enhancement it can achieve. By applying asymmetric signal intensities, TFQKD with asymmetric channel losses not only surpasses the fundamental limit of key rate of point-to-point QKD for 50 dB overall loss, but also has key rate as high as 2.918 × 10 −6 for 56 dB overall loss. Whereas no keys are obtained with strategy (2) for 56 dB loss. The increased key rate and enlarged distance coverage of TFQKD with asymmetric channel losses guarantee its superiority in long-distance quantum networks.Quantum key distribution (QKD) enables remote users to share secret keys with information-theoretic security [1,2]. However, due to the unavoidable losses of optical channels, there exists a fundamental limit on the achievable secret key rate of long distance QKD. Without using quantum repeaters, the upper bound (also called repeaterless bound in this paper) of the secret key rate of QKD scales linearly with the channel transmittance η [3,4]. Remarkably, a new type of QKD, called twin-field (TF) QKD, has been proposed [5] and can practically overcome the repeaterless bound. In TFQKD, like in the measurement-deviceindependent (MDI) QKD [6], two users (Alice and Bob) send two coherent states to an un-trusted intermediate node, i.e. Charlie, who performs the measurement. Because TFQKD employs single photon interference, rather than two-photon interference in MDIQKD, the secret key rate of TFQKD scales as √ η, allowing for unprecedented distance coverage. Plenty of variations and security analysis of TFQKD [7-12] have been studied, followed by multiple experimental demonstrations [13][14][15][16]. More recently, TFQKD has been successfully implemented over more than 500 km fibers [17,18]. It has been shown that TFQKD is one of the most promising and practical solutions to long distance QKD. However, all the above mentioned studies only consider TFQKD over optical channels with symmetric losses between each of the users and intermediate node, and let Alice and Bob use identical sets of operations in preparing their signals. However, this assumption on channel symmetry is seldom true in reality. TFQKD over asymmetric channels is important not only for practical po...

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We discuss excess noise contributions of a practical balanced homodyne detector in Gaussianmodulated coherent-state (GMCS) quantum key distribution (QKD). We point out the key generated from the original realistic model of GMCS QKD may not be secure. In our refined realistic model, we take into account excess noise due to the finite bandwidth of the homodyne detector and the fluctuation of the local oscillator. A high speed balanced homodyne detector suitable for GMCS QKD in the telecommunication wavelength region is built and experimentally tested. The 3 dB bandwidth of the balanced homodyne detector is found to be 104 MHz and its electronic noise level is 13 dB below the shot noise at a local oscillator level of 8.5×10 8 photon per pulse.The secure key rate of a GMCS QKD experiment with this homodyne detector is expected to reach Mbits/s over a few kilometers. PACS numbers: 03.67.Dd * Electronic address: l.qian@utoronto.ca. If we assume E(W n ) = 0, E(W n W n+2 ) = 0 (only consecutive pulse value has a non-zero expectation), CC = a. In GMCS QKD, the variance of Bob's measurement of individual pulse will be contributed by the variances of its adjacent pulses. With the quadrature modulation of the coherent state prepared by Alice V , the excess noise due to the overlap between pulses ε overlap referring to

A balanced homodyne detector for highrate Gaussian-modulated coherent-state quantum key distribution Yue-Meng Chi, Bing Qi, Wen Zhu et al.Quantum key distribution and 1 Gbps data encryption over a single fibre P Eraerds, N Walenta, M Legré et al. Abstract. In this paper, we study the feasibility of conducting quantum key distribution (QKD) together with classical communication through the same optical fiber by employing dense-wavelength-division-multiplexing (DWDM) technology at telecom wavelength. The impact of classical channels on the quantum channel has been investigated for both QKD based on single-photon detection and QKD based on homodyne detection. Our studies show that the latter can tolerate a much higher level of contamination from classical channels than the former. This is because the local oscillator used in the homodyne detector acts as a 'mode selector', which can suppress noise photons effectively. We have performed simulations based on both the decoy BB84 QKD protocol and the Gaussian-modulated coherent state (GMCS) QKD protocol. While the former cannot tolerate even one classical channel (with a power of 0 dBm), the latter can be multiplexed with 38 classical channels (0 dBm power per channel) and still has a secure distance around 10 km. A preliminary experiment has been conducted based on a 100 MHz bandwidth homodyne detector.