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Twin field quantum key distribution promises high key rates at long distance to beat the rate distance limit. Here, applying the sending or not sending TF QKD protocol, we experimentally demonstrate a secure key distribution breaking the absolute key rate limit of repeaterless QKD over 509 km, 408 km ultra-low loss optical fibre and 350 km standard optical fibre. Two independent lasers are used as the source with remote frequency locking technique over 500 km fiber distance; Practical optical fibers are used as the optical path with appropriate noise filtering; And finite key effects are considered in the key rate analysis. The secure key rates obtained at different distances are more than 5 times higher than the conditional limit of repeaterless QKD, a bound value assuming the same detection loss in the comparison. The achieved secure key rate is also higher than that a traditional QKD protocol running with a perfect repeaterless QKD device and even if an infinite number of sent pulses. Our result shows that the protocol and technologies applied in this experiment enable TF QKD to achieve high secure key rate at long distribution distance, and hence practically useful for field implementation of intercity QKD.Introduction.-Channel loss seems to be the most severe limitation on the practical application of long distance quantum key distribution (QKD) [1-3], given that quantum signals cannot be amplified. Much efforts have been made towards the goal of a longerdistance for QKD [4][5][6]. Theoretically, the decoy-state method [7][8][9] can improve the key rate of coherent-state based QKD from scaling quadratically to a linear with the channel transmittance, as what behaves of a perfect single-photon source. This method can beat the photonnumber-splitting attack to the imperfect single-photon source and the coherent state is used as if only those single-photon pulses were used for key distillation, and hence it can reach the key rate to a level comparable with that of a perfect single-photon source.

Randomness is critical for many information processing applications, including numerical modeling and cryptography [1,2]. Device-independent quantum random number generation [3] (DIQRNG) based on the loophole free violation of Bell inequality [4][5][6][7] produces unpredictable genuine randomness without any device assumption and is therefore an ultimate goal in the field of quantum information science [8][9][10]. However, due to formidable technical challenges, there were very few reported experimental studies of DIQRNG [11][12][13][14], which were vulnerable to the adversaries. Here we present a fully functional DIQRNG against the most general quantum adversaries [15][16][17]. We construct a robust experimental platform that realizes Bell inequality violation with entangled photons with detection and locality loopholes closed simultaneously. This platform enables a continuous recording of a large volume of data sufficient for security analysis against the general quantum side information and without assuming independent and identical distribution.Lastly, by developing a large Toeplitz matrix (137.90 Gb × 62.469 Mb) hashing technique, we demonstrate that this DIQRNG generates 6.2469 × 10 7 quantum-certified random bits in 96 hours (or 181 bits/s) with uniformity within 10 −5 . We anticipate this DIQRNG may have profound impact on the research of quantum randomness and information-secured applications.

Measurement-device-independent quantum key distribution (MDIQKD) protocol is immune to all attacks on detection and guarantees the information-theoretical security even with imperfect single-photon detectors. Recently, several proof-of-principle demonstrations of MDIQKD have been achieved. Those experiments, although novel, are implemented through limited distance with a key rate less than 0.1  bit/s. Here, by developing a 75 MHz clock rate fully automatic and highly stable system and superconducting nanowire single-photon detectors with detection efficiencies of more than 40%, we extend the secure transmission distance of MDIQKD to 200 km and achieve a secure key rate 3 orders of magnitude higher. These results pave the way towards a quantum network with measurement-device-independent security.

In quantum key distribution (QKD), the bit error rate is used to estimate the information leakage and hence determines the amount of privacy amplification -making the final key private by shortening the key. In general, there exists a threshold of the error rate for each scheme, above which no secure key can be generated. This threshold puts a restriction on the environment noises. For example, a widely used QKD protocol -BB84 -cannot tolerate error rates beyond 25%. A new protocol, round-robin differential phase shifted (RRDPS) QKD, essentially removes this restriction and can in principle tolerate more environment disturbance. Here, we propose and experimentally demonstrate a passive RRDPS QKD scheme. In particular, our 500 MHz passive RRDPS QKD system is able to generate a secure key over 50 km with a bit error rate as high as 29%. This scheme should find its applications in noisy environment conditions.The uncertainty principle guarantees that whenever an eavesdropper, Eve wants to learn key information in the quantum channel, she would inevitably introduce disturbances, which could be detected by the two authorized parties, Alice and Bob. In reality, the quantum channel may suffer from environment disturbance, which could cause errors and even more vitally conceal Eve's attack.The amount of leaked key information, which is quantified by a phase error e p , can be inferred from the channel disturbance, which is quantified by a bit error e b . The final key rate is given by [1],where H(e) = −e log e − (1 − e) log(1 − e) is the binary Shannon entropy function. The bit error can be directly computed from the experimental data, whereas the phase error needs to be estimated or bounded. In the BB84 protocol with strong symmetries, one can show that e p = e b in the long key length limit. In other protocols, normally there is a relation between the two error rates. In the end, when the error rate e b goes beyond some threshold level, no secure key can be generated. For example, with the Shor-Preskill security proof [1, 2], the BB84 protocol can maximally tolerate 11% error rate. For any security analysis, a simple intercept-and-resend attack [3] shows that the BB84 protocol cannot tolerate more than 25% error rate. This threshold puts a stringent requirement on the system environment, which makes some practical implementations challenging. Recently, Sasaki et al. proposed a round-robin differential phase-shift (RRDPS) QKD protocol [4]. The sender Alice encodes a random phase, chosen from {0, π}, on each of L pulses, with an average photon number of µ. Upon receiving the L-pulse block, the receiver Bob implements a single-photon interference with an MachZehnder interferometer (MZI), as shown in Fig. 1a. The key point is that Bob can randomly adjust the length difference of the two arms of the MZI. After obtaining a detection click, Bob first identifies which two pulses interfere and then announces the corresponding indices i, j to Alice. Alice can derive the relative phase between the two pulses as the raw key, and ...

Channel loss seems to be the most severe limitation on the practical application of long distance quantum key distribution. The idea of twin-field quantum key distribution can improve the key rate from the linear scale of channel loss in the traditional decoy-state method to the square root scale of the channel transmittance. However, the technical demanding is rather tough because it requests single photon level interference of two remote independent lasers. Here, we adopt the technology developed in the frequency and time transfer to lock two independent lasers' wavelengths and utilize additional phase reference light to estimate and compensate the fiber fluctuation. Further with a single photon detector with high detection rate, we demonstrate twin field quantum key distribution through the sending-or-not-sending protocol with realistic phase drift over 300 km optical fiber spools. We calculate the secure key rates with finite size effect. The secure key rate at 300 km (1.96 × 10 −6 ) is higher than that of the repeaterless secret key capacity (8.64 × 10 −7 ).Introduction.-Although quantum key distribution (QKD) can in principle offer secure private communication [1][2][3][4][5][6][7], there are still some technical limitations on practical long distance quantum communication. Perhaps the most severe of these is channel loss, given that quantum signals cannot be amplified. Much efforts have been made towards QKD over longer-distance. Theoretically, the decoy-state method [8][9][10] can improve the key rate of coherent-state based QKD from scaling quadratically to linearly with the channel transmittance, as what behaves of a perfect single-photon source. This method can defeat the photon-number-splitting attack to the imperfect source and the coherent state is used as if only the single-photon pulses were used for key distillation, and hence it can reach a key rate in the linear scale of channel loss, as the perfect single-photon source does. Remarkable theoretical progress was made toward achieving practical, secure QKD over longer distance with the proposal of twin-field QKD [11], which improves the key rate scaling to follow the square root of the channel transmittance. It shows that, the coherent-state source can actually be an advantage over the single-photon source because the post-selection of phase coherence of the twin fields from Alice and Bob can potentially lead to secure QKD with the encoding state of single-photon and vacuum, and their linear super-positions.This method has the potential to achieve a key rate that scales with the square root of channel transmittance, and can by far break the known distance limit of existing protocols in practical QKD [12][13][14][15][16][17][18][19][20]. The theoretical secure key rate can be even higher than the repeaterless secret key capacities, known as the Takeoka-Guha-Wilde (TGW) bound [19] and the Pirandola-Laurenza-Ottaviani-Bianchi (PLOB) bound [20]. However, considerable work still remains to make this a reality.First, there is the theoretical challenge ...

Quantum communication has historically been at the forefront of advancements, from fundamental tests of quantum physics to utilizing the quantum-mechanical properties of physical systems for practical applications. In the field of communication complexity, quantum communication allows the advantage of an exponential reduction in the transmitted information over classical communication to accomplish distributed computational tasks. However, to date, demonstrating this advantage in a practical setting continues to be a central challenge. Here, we report a proof-of-principle experimental demonstration of a quantum fingerprinting protocol that for the first time surpasses the ultimate classical limit to transmitted information. Ultralow noise superconducting single-photon detectors and a stable fiber-based Sagnac interferometer are used to implement a quantum fingerprinting system that is capable of transmitting less information than the classical proven lower bound over 20 km standard telecom fiber for input sizes of up to 2 Gbits. The results pave the way for experimentally exploring the advanced features of quantum communication and open a new window of opportunity for research in communication complexity and testing the foundations of physics. DOI: 10.1103/PhysRevLett.116.240502 The quantum-communication network [1] is believed to be the next-generation platform for remote information processing tasks. So far, however, only one protocolquantum key distribution (QKD) [2,3]-has been widely investigated and deployed in commercial applications. The extension of the practically available quantum communication protocols beyond QKD in order to fully understand the potential of large-scale quantum communication networks is therefore highly important. Significant progress has been made in this direction [4][5][6][7][8][9], but the rich class of quantum communication complexity (QCC) protocols [10-12] remains largely undemonstrated, except for a few proof-of-principle implementations [13][14][15][16]. The field of QCC explores quantum-mechanical properties in order to determine the minimum amount of information that must be transmitted to solve distributed computational tasks [11]. It not only has many connections to the foundational issues of quantum mechanics [12,17], but also has important applications for the design of communication systems, green communication techniques, computer circuits, and data structures [18]. For instance, QCC essentially connects the foundational physics questions regarding nonlocality with those of communication complexity studied in theoretical computer science [12]. Quantum fingerprinting, proposed by Buhrman, Cleve, Watrous, and Wolf, is the most appealing protocol in QCC [19]. Specifically, the simultaneous message-passing model [10] corresponds to the scenario where two parties, Alice and Bob, respectively, receive inputs x a ; x b ∈ f0; 1g n and send messages to a third party, Referee, who must determine whether x a equals x b or not, with a small error probability ϵ. This model has...

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