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We investigate the consequences of space-time being curved on space-based quantum communication protocols. We analyze tasks that require either the exchange of single photons in a certain entanglement distribution protocol or beams of light in a continuous-variable quantum key distribution scheme. We find that gravity affects the propagation of photons, therefore adding additional noise to the channel for the transmission of information. The effects could be measured with current technology.

Measurement-device-independent quantum key distribution with a finite number of decoy states is analyzed under finite-data-size assumption. By accounting for statistical fluctuations in parameter estimation, we investigate vacuum+weak-and vacuum+two-weak-decoy-state protocols. In each case, we find proper operation regimes, where the performance of our system is comparable to the asymptotic case for which the key size and the number of decoy states approach infinity.Our results show that practical implementations of this scheme can be both secure and efficient. * xma@tsinghua.edu.cnQuantum key distribution (QKD) [1,2] is one of the most successful applications of quantum information processing, which allows two distant parties, Alice and Bob, to grow secret keys with information-theoretic security [3][4][5][6][7][8]. Conventional security proofs of QKD assume certain physical models for the employed devices -source and detection units. Forinstance, the squashing model is widely assumed for the measurement [9-11] in a standard security analysis [12]. Practical implementations, however, could fall short of meeting all requirements set by the models, hence security could be compromised in reality. In fact, side channels have been identified and exploited to break QKD security. These side-channel attacks include the fake-state attack [13,14], the time-shift attack [15,16], the phaseremapping attack [17,18], and the detector-blinding attack [19,20].Several approaches have been proposed to counter the side-channel attacks. One way is to sufficiently characterize the behavior of the devices and analyze the security by taking into account all device parameters [21][22][23]. This, however, can be difficult to implement in practice. A second approach that can defeat all side-channel attacks is device-independent QKD [24][25][26], in which the security can be proven without knowing the specifications of the devices used. Security, in this case, is derived from nonlocal correlations by violating Bell's inequality [27,28]. In order to avoid the detection efficiency loophole [29], however, a large fraction of the transmitted signals must be detected by the receiver, resulting in impractical requirements for the transmission efficiency (e.g., 82.8% [30] for the Clauser-Horne-Shimony-Holt (CHSH) inequality [28]).Instead of full device independence, a detection-device independent QKD scheme is proposed [31,32], in which the detection system is assumed to be untrusted. Since most of practical hacking strategies focus on the detection site, and the source site is relatively simple for characterization, such a scheme can close most loopholes in a QKD system. Unfortunately, these schemes still need stringent requirements on the transmission efficiency of more than 50% [31]. Recently, Lo, Curty, and Qi [33] proposed efficient schemes that are measurement-device independent (MDI). Alice and Bob send some signals to a willing participant who can even be an eavesdropper, Eve. Eve performs a Bell-state measurement (BSM) and announces the...

Practical schemes for measurement-device-independent quantum key distribution using phase and path or time encoding are presented. In addition to immunity to existing loopholes in detection systems, our setup employs simple encoding and decoding modules without relying on polarization maintenance or optical switches. Moreover, by employing a modified sifting technique to handle the dead-time limitations in single-photon detectors, our scheme can be run with only two singlephoton detectors. With a phase-postselection technique, a decoy-state variant of our scheme is also proposed, whose key generation rate scales linearly with the channel transmittance. *

A protocol with the potential of beating the existing distance records for conventional quantum key distribution (QKD) systems is proposed. It borrows ideas from quantum repeaters by using memories in the middle of the link, and that of measurement-device-independent QKD, which only requires optical source equipment at the userʼs end. For certain memories with short access times, our scheme allows a higher repetition rate than that of quantum repeaters with single-mode memories, thereby requiring lower coherence times. By accounting for various sources of nonideality, such as memory decoherence, dark counts, misalignment errors, and background noise, as well as timing issues with memories, we develop a mathematical framework within which we can compare QKD systems with and without memories. In particular, we show that with the state-of-the-art technology for quantum memories, it is potentially possible to devise memory-assisted QKD systems that, at certain distances of practical interest, outperform current QKD implementations.Keywords: quantum key distribution, quantum memory, measurement device independent, quantum repeaters, quantum networks IntroductionDespite all commercial [1] and experimental achievements in quantum key distribution (QKD) [2-10], reaching arbitrarily long distances is still a remote objective. The fundamental solution to this problem, i.e., quantum repeaters, has been known for over a decade. From early proposals by Briegel et al [11] to the latest no-memory versions [12][13][14], quantum repeaters, typically, rely on highly efficient quantum gates comparable to what we may need for future quantum computers. While the progress on that ground may take some time before such systems become functional, another approach based on probabilistic gate operations was proposed by Duan and co-workers [15], which could offer a simpler way of implementing quantum repeaters for moderate distances of up to around 1000 km. The latter systems require quantum memory (QM) modules with high coupling efficiencies to light and with coherence times exceeding the transmission delays, which are yet to be achieved together. In this paper, we propose a protocol that, although is not as scalable as quantum repeaters, for certain classes of memories, relaxes, to some extent, the harsh requirements on memories' coherence times, thereby paving the way for the existing technologies to beat the highest distance records achieved for no-memory QKD links [2]. The idea behind our protocol was presented in [16], and independent work has also been reported in [17]. This work proposes additional practical schemes and rigorously analyses them under realistic conditions. Our protocol relies on concepts from quantum repeaters, on the one hand, and the recently proposed measurement-device-independent QKD (MDI-QKD), on the other. The original MDI-QKD [18] relies on sending encoded photons by the users to a middle site at which a Bell-state measurement (BSM) is performed. One major practical advantage of MDI-QKD is that this BSM can be...