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.

In principle, quantum key distribution (QKD) offers information-theoretic security based on the laws of physics. In practice, however, the imperfections of realistic devices might introduce deviations from the idealized models used in security analyses. Can quantum code-breakers successfully hack real systems by exploiting the side channels? Can quantum code-makers design innovative counter-measures to foil quantum code-breakers? This article reviews theoretical and experimental progress in the practical security aspects of quantum code-making and quantum code-breaking. After numerous attempts, researchers now thoroughly understand and are able to manage the practical imperfections. Recent advances, such as the measurement-device-independent protocol, have closed the critical side channels in the physical implementations, paving the way for secure QKD with realistic devices. VI. DetectionSecurity 36 A. Countermeasures against detection attacks 36 B. Measurement-device-independent scheme 36 1. Time-reversed EPR QKD 36 2. MDI-QKD protocol 37 3. Theoretical developments 38 4. Experimental developments 39 C. Twin-field QKD 41 VII. Continuous-Variable QKD 42 A. Protocol and security 43 1. Gaussian-modulated protocol 43 2. Discrete modulated protocol 44 3. Security analysis 44 B. Experimental developments 45 C. Quantum hacking and countermeasures 46 VIII. Other Quantum Cryptographic Protocols 47 A. Device-independent QKD 47 B. Some New QKD implementations 48 1. Round-robin DPS QKD 48 2. High-dimensional QKD 50 3. QKD with wavelength-division multiplexing 51 4. Chip-based QKD 51 C. Other quantum cryptographic protocols 53 1. Quantum bit commitment 53 2. Quantum digital signature 53 3. Other protocols 54 IX. Concluding Remarks 55 Acknowledgement 57 A. General questions to QKD 57 References 58 1 Google Q: research.google/teams/applied-science/quantum 2 IBM Q: www.research.ibm.com/ibm-q 3 Rigetti: www.rigetti.com 4 CAS-Alibaba: quantumcomputer.ac.cn/index.html 1. Concern 1. Since RSA is secure under current computational power, we do not need QKD now.

Based on the theory of quantum mechanics, intrinsic randomness in measurement distinguishes quantum effects from classical ones. From the perspective of states, this quantum feature can be summarized as coherence or superposition in a specific (classical) computational basis. Recently, by regarding coherence as a physical resource, Baumgratz et al. present a comprehensive framework for coherence measures. Here, we propose a quantum coherence measure essentially using the intrinsic randomness of measurement. The proposed coherence measure provides an answer to the open question in completing the resource theory of coherence. Meanwhile, we show that the coherence distillation process can be treated as quantum extraction, which can be regarded as an equivalent process of classical random number extraction. From this viewpoint, the proposed coherence measure also clarifies the operational aspect of quantum coherence. Finally, our results indicate a strong similarity between two types of quantumness -coherence and entanglement.

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...

Quantum key distribution allows remote parties to generate information-theoretic secure keys. The bottleneck throttling its real-life applications lies in the limited communication distance and key generation speed, due to the fact that the information carrier can be easily lost in the channel. For all the current implementations, the key rate is bounded by the channel transmission probability η. Rather surprisingly, by matching the phases of two coherent states and encoding the key information into the common phase, this linear key-rate constraint can be overcome-the secure key rate scales with the square root of the transmission probability, O( √ η), as proposed in twin-field quantum key distribution [Nature (London) 557, 400 (2018)]. To achieve this, we develop an optical-mode-based security proof that is different from the conventional qubit-based security proofs. Furthermore, the proposed scheme is measurement device independent, i.e., it is immune to all possible detection attacks. The simulation result shows that the key rate can even exceed the transmission probability η between two communication parties. In addition, we apply phase postcompensation to devise a practical version of the scheme without phase locking, which makes the proposed scheme feasible with the current technology. This means that quantum key distribution can enjoy both sides of the world-practicality and security.