How to remove detector side channel attacks has been a notoriously hard problem in quantum cryptography. Here, we propose a simple solution to this problem-measurement device independent quantum key distribution. It not only removes all detector side channels, but also doubles the secure distance with conventional lasers. Our proposal can be implemented with standard optical components with low detection efficiency and highly lossy channels. In contrast to the previous solution of full device independent QKD, the realization of our idea does not require detectors of near unity detection efficiency in combination with a qubit amplifier (based on teleportation) or a quantum non-demolition measurement of the number of photons in a pulse. Furthermore, its key generation rate is many orders of magnitude higher than that based on full device independent QKD. The results show that long-distance quantum cryptography over say 200km will remain secure even with seriously flawed detectors.Quantum key distribution (QKD) allows two parties (typically called Alice and Bob) to generate a common string of secret bits, called a secret key, in the presence of an eavesdropper, Eve [1]. This key can be used for tasks such as secure communication and authentication. Unfortunately, there is a big gap between the theory and practice of QKD. In principle, QKD offers unconditional security guaranteed by the laws of physics [2-4]. However, real-life implementations of QKD rarely conform to the assumptions in idealized models used in security proofs. Indeed, by exploiting security loopholes in practical realizations, especially imperfections in the detectors, different attacks have been successfully launched against commercial QKD systems [5, 6], thus highlighting their practical vulnerabilities.To connect theory with practice again, several approaches have been proposed. The first one is the presumably hard-verifiable problem of trying to characterize real devices fully and account for all side channels. The second approach is a teleportation trick [2,7]. The third solution is (full) device independent QKD (DI-QKD) [9]. This last technique does not require detailed knowledge of how QKD devices work and can prove security based on the violation of a Bell inequality. Unfortunately, DI-QKD is highly impractical because it needs near unity detection efficiency together with a qubit amplifier or a quantum non-demolition (QND) measurement of the number of photons in a pulse, and even then generates an extremely low key rate (of order 10 −10 bits per pulse) at practical distances [10].

Decoy states have recently been proposed as a useful method for substantially improving the performance of quantum key distribution. Here, we present a general theory of the decoy state protocol based on only two decoy states and one signal state. We perform optimization on the choice of intensities of the two decoy states and the signal state. Our result shows that a decoy state protocol with only two types of decoy states--the vacuum and a weak decoy state-asymptotically approaches the theoretical limit of the most general type of decoy state protocols (with an infinite number of decoy states). We also present a one-decoy-state protocol. Moreover, we provide estimations on the effects of statistical fluctuations and suggest that, even for long distance (larger than 100km) QKD, our two-decoy-state protocol can be implemented with only a few hours of experimental data. In conclusion, decoy state quantum key distribution is highly practical.

Quantum key distribution (QKD) promises unconditional security in data communication and is currently being deployed in commercial applications. Nonetheless, before QKD can be widely adopted, it faces a number of important challenges such as secret key rate, distance, size, cost and practical security. Here, we survey those key challenges and the approaches that are currently being taken to address them. For thousands of years, human beings have been using codes to keep secrets. With the rise of the Internet and recent trends to the Internet of Things, our sensitive personal financial and health data as well as commercial and national secrets are routinely being transmitted through the Internet. In this context, communication security is of utmost importance. In conventional symmetric cryptographic algorithms, communication security relies solely on the secrecy of an encryption key. If two users, Alice and Bob, share a long random string of secret bits-the key-then they can achieve unconditional security by encrypting their message using the standard one-time-pad encryption scheme. The central question then is: how do Alice and Bob share a secure key in the first place? This is called the key distribution problem. Unfortunately, all classical methods to distribute a secure key are fundamentally insecure because in classical physics there is nothing preventing an eavesdropper, Eve, from copying the key during its transit from Alice to Bob. On the other hand, standard asymmetric or public-key cryptography solves the key distribution problem by relying on computational assumptions such as the hardness of factoring. Therefore, such schemes do not provide information-theoretic security because they are vulnerable to future advances in hardware and algorithms, including the construction of a large-scale quantum computer.

Quantum key distribution (QKD) systems can send signals over more than 100 km standard optical fiber and are widely believed to be secure. Here, we show experimentally for the first time a technologically feasible attack, namely the time-shift attack, against a commercial QKD system. Our result shows that, contrary to popular belief, an eavesdropper, Eve, has a non-negligible probability (∼4%) to break the security of the system. Eve's success is due to the well-known detection efficiency loophole in the experimental testing of Bell inequalities. Therefore, the detection efficiency loophole plays a key role not only in fundamental physics, but also in technological applications such as QKD.PACS numbers: 03.67.Dd Quantum key distribution (QKD) [1,2] provides a method to share a secret key between legitimate users called "Alice" (the sender) and "Bob" (the receiver). The unconditional security of QKD has ben rigorously proved based on the laws of physics [3,4]. Even imperfect practical QKD systems have also been proved secure assuming some semi-realistic models [5,6]. The decoy method [7] was proposed to dramatically improve the performance of a practical QKD system. Our group has implemented the decoy method experimentally over 15km and 60km of telecom fibers [8]. Incidentally, QKD has found real-life applications in a recent Swiss election [9].Recently, there has been a lot of theoretical interest on the connection between the security of QKD and fundamental physical principles such as the violation of Bell's inequality and the no-signaling constraint [10] on spacelike observables. An ultimate goal, which has not yet been achieved [11], is to construct a device-independent security proof. As is well-known, the experimental testing of Bell's inequality often suffers from the detection efficiency loophole. A fair sampling assumption may save the day. However, as we will demonstrate below, rather surprisingly, the low detection efficiency of practical detectors not only violates the fair sampling assumption that would be needed in security proofs based on Bellinequality violation, but also gives Eve (an eavesdropper) a powerful handle to break the security of a practical QKD system. Therefore, the detection efficiency loophole is of both conceptual and practical interest.Our work is an illustration of general physical limitations, rather than a particular technological weakness. Indeed, a practical QKD system often includes two or more detectors. It is virtually impossible to manufacture identical detectors in practice. As a result, the two detectors of the same QKD system will exhibit different detection efficiencies as functions of either one or a combination of variables in the time, frequency, polarization, and/or spatial domains. If Eve manipulates a signal in these variables, she could effectively exploit the detection efficiency loophole to break the security of a QKD system. In our experiment, we consider Eve's manipulation of the time variable. Our work demonstrates the general problem of detection efficie...

The possibility that single-cell organisms undergo programmed cell death has been questioned in part because they lack several key components of the mammalian cell death machinery. However, yeast encode a homolog of human Drp1, a mitochondrial fission protein that was shown previously to promote mammalian cell death and the excessive mitochondrial fragmentation characteristic of apoptotic mammalian cells. In support of a primordial origin of programmed cell death involving mitochondria, we found that the Saccharomyces cerevisiae homolog of human Drp1, Dnm1, promotes mitochondrial fragmentation/degradation and cell death following treatment with several death stimuli. Two Dnm1-interacting factors also regulate yeast cell death.

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...

Continuous-variable quantum key distribution (CV-QKD) protocols based on coherent detection have been studied extensively in both theory and experiment. In all the existing implementations of CV-QKD, both the quantum signal and the local oscillator (LO) are generated from the same laser and propagate through the insecure quantum channel. This arrangement may open security loopholes and limit the potential applications of CV-QKD. In this paper, we propose and demonstrate a pilot-aided feedforward data recovery scheme that enables reliable coherent detection using a "locally" generated LO. Using two independent commercial laser sources and a spool of 25-km optical fiber, we construct a coherent communication system. The variance of the phase noise introduced by the proposed scheme is measured to be 0.04 (rad 2 ), which is small enough to enable secure key distribution. This technology also opens the door for other quantum communication protocols, such as the recently proposed measurement-device-independent CV-QKD, where independent light sources are employed by different users.

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]