Quantum communication holds the promise of creating disruptive technologies that will play an essential role in future communication networks. For example, the study of quantum communication complexity has shown that quantum communication allows exponential reductions in the information that must be transmitted to solve distributed computational tasks. Recently, protocols that realize this advantage using optical implementations have been proposed. Here we report a proof-of-concept experimental demonstration of a quantum fingerprinting system that is capable of transmitting less information than the best-known classical protocol. Our implementation is based on a modified version of a commercial quantum key distribution system using off-the-shelf optical components over telecom wavelengths, and is practical for messages as large as 100 Mbits, even in the presence of experimental imperfections. Our results provide a first step in the development of experimental quantum communication complexity.

Finding exponential separation between quantum and classical information tasks is like striking gold in quantum information research. Such an advantage is believed to hold for quantum computing but is proven for quantum communication complexity. Recently, a novel quantum resource called the quantum switch-which creates a coherent superposition of the causal order of events, known as quantum causality-has been harnessed theoretically in a new protocol providing provable exponential separation. We experimentally demonstrate such an advantage by realizing a superposition of communication directions for a two-party distributed computation. Our photonic demonstration employs d-dimensional quantum systems, qudits, up to d = 2 13 dimensions and demonstrates a communication complexity advantage, requiring less than (0.696 ± 0.006) times the communication of any causally ordered protocol. These results elucidate the crucial role of the coherence of communication direction in achieving the exponential separation for the one-way processing task, and open a new path for experimentally exploring the fundamentals and applications of advanced features of indefinite causal structures.Computation by separated parties with minimal communication is the focus of communication complexity, which has applications to distributed computing, very-large-scale integration, streaming algorithms, and more [1]. For quantum information, communication complexity is especially exciting as exponential quantum-classical gaps can be proven [2][3][4][5][6][7][8][9]. By contrast, exponential quantum-classical gaps for computation tasks such as factorization [10] depend on the best-known classical algorithm, and thus are strongly believed but not rigorously proven. Experimentally, quantum communication complexity has been studied in proof of principle for the quantum fingerprinting protocol [11][12][13] and beyond [1,14,15].The quantum switch provides a new communication complexity tool that leads to another instance of exponential quantum advantage [16]. The quantum switch is a device where a control qubit determines the order in which two transformations are performed on a target system [17,18]. When the control is in a superposition of logical states, the order of the operations is causally indefinite; i.e., there is a superposition of the ordering of target operations. The quantum switch has broad relevance in the context of quantum causality [19] including applications to studies of quantum gravity [19][20][21][22], communication complexity [16, * These authors contributed equally to this work.2 23], witnessing causality [17,[24][25][26][27][28] and deciding whether a given indefinite causal order is physical [29,30]. In quantum computing, the quantum switch can reduce the query complexity for some tasks compared to causally ordered protocols [18,31] -this advantage has been demonstrated for single-qubit control and single-qubit target circuits [32]. In quantum communication, the quantum switch enhances the communication rate beyond the limits of ...

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...

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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

Measurement-device-independent quantum key distribution (MDI-QKD) can eliminate all detector side channels and it is practical with current technology. Previous implementations of MDI-QKD all use two symmetric channels with similar losses. However, the secret key rate is severely limited when different channels have different losses. Here we report the results of the first highrate MDI-QKD experiment over asymmetric channels. By using the recent 7-intensity optimization approach, we demonstrate >10x higher key rate than previous best-known protocols for MDI-QKD in the situation of large channel asymmetry, and extend the secure transmission distance by more than 20-50 km in standard telecom fiber. The results have moved MDI-QKD towards widespread applications in practical network settings, where the channel losses are asymmetric and user nodes could be dynamically added or deleted. * These authors contributed equally to this work.

We present a robust single photon circular quantum secret sharing (QSS) scheme with phase encoding over 50 km single mode fiber network using a circular QSS protocol. Our scheme can automatically provide a perfect compensation of birefringence and remain stable for a long time. A high visibility of 99.3% is obtained. Furthermore, our scheme realizes a polarization insensitive phase modulators. The visibility of this system can be maintained perpetually without any adjustment to the system every time we test the system.

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