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We present an improved statistical fluctuation analysis for measurement-device-independent quantum key distribution with a three-intensity decoy-state method. Taking the statistical fluctuations for different sources jointly, we present more tightened formulas for some key quantities used in calculating the secure final key. Numerical simulation shows that, given the total number of pulses 1012, our method improves the key rate by about 97% for a distance of 50 km compared with the result given by Xu et al. [F. Xu et al., Phys. Rev. A 89, 052333 (2014)], and improves the key rate by 145% for a distance of 100 km compared with the result from full optimization of all parameters but treating the statistical fluctuations traditionally, i.e., treating the fluctuations for different sources separately. [ABSTRACT FROM AUTHOR]

We present formulas that tighten the upper bound for the phase-flip errors in the decoy-state method by using four intensities. Our result compressed the bound to about a quarter of the known result for measurement-device-independent quantum key distribution. Based on this, we find that the key rate is improved by more than 100% given weak coherent state sources, and even more than 200% with heralded single-photon sources. [ABSTRACT FROM AUTHOR]

Exposed in the nonzero temperature environment, a quantum system can both lose and gain excitations by interacting with the environment. In this work, we show how to optimally protect quantum states and quantum entanglement in such a situation based on measurement reversal from weak measurement. In particular, we present explicit formulas of protection. We find that this scheme can circumvent the entanglement sudden death in certain conditions. [ABSTRACT FROM AUTHOR]

We study the measurement-device-independent quantum key distribution (MDI-QKD) in practice with limited resources, when there are only three different states in implementing the decoy-state method. We present a tighter explicit formula to estimate the lower bound of the yield of two-single-photon pulses sent by Alice and Bob. Moreover, we show that the bounding of this yield and phase flip error of single-photon pulse pairs can be further improved by using other constraints which can be solved by a simple and explicit program. Our methods here can significantly improve the key rate and the secure distance of MDI-QKD with only three intensities. [ABSTRACT FROM AUTHOR]

An arbitrary quantum-optical process (channel) can be completely characterized by probing it with coherent states using the recently developed coherent-state quantum process tomography (QPT) [Lobino et al., Science 322, 563 (2008)]. In general, precise QPT is possible if an infinite set of probes is available. Thus, realistic QPT of infinite-dimensional systems is approximate due to a finite experimentally feasible set of coherent states and its related energy-cutoff approximation. We show with explicit formulas that one can completely identify a quantum-optical Gaussian process just with a few different coherent states without approximations like the energy cutoff. For tomography of multimode processes, our method exponentially reduces the number of different test states, compared with existing methods. [ABSTRACT FROM AUTHOR]

The effects of statistical fluctuation and vacuum source fluctuation on the decoy-state method with an unstable decoy source and signal source are studied. The method is improved by a tightened estimation of the vacuum count. Compared with the existing results with an unstable vacuum source and statistical fluctuation, the results presented here have improved the key rate of the 3-intensity protocol for quantum key distribution. [ABSTRACT FROM AUTHOR]

We show how to calculate the secure final key rate in the four-intensity decoy-state measurement-device-independent quantum key distribution protocol with both source errors and statistical fluctuations with a certain failure probability. Our results rely only on the range of only a few parameters in the source state. All imperfections in this protocol have been taken into consideration without assuming any specific error patterns of the source. [ABSTRACT FROM AUTHOR]

The existing decoy-state measurement-device-independent quantum key distribution (MDI-QKD) theory assumes perfect control of the source states, which is an impossible task for any real setup. In this paper, we study the decoy-state MDI-QKD method with source errors without presuming any unverifiable error patterns. In our method we need the range of only a few parameters in the source state. [ABSTRACT FROM AUTHOR]

In this work, we describe a method to realize a three-dimensional Weyl semimetal by coupling multilayers of a honeycomb optical lattice in the presence of a pair of Raman lasers. The Raman lasers render each isolated honeycomb layer a Chern insulator. With finite interlayer coupling, the bulk gap of the system closes at certain out-of-plane momenta due to Raman assisted tunneling and results in the Weyl semimetal phase. Using experimentally relevant parameters, we show that both one pair and two pairs of Weyl points can be realized by tuning the interlayer coupling strength. We suggest that Landau-Zener tunneling can be used to detect Weyl points and show that the transition probability increases dramatically when the Weyl point emerges. The realization of chiral anomaly by using a magnetic-field gradient is also discussed. [ABSTRACT FROM AUTHOR]

The relatively low key rate seems to be the major barrier to its practical use for the decoy-state measurement-device-independent quantum key distribution (MDI-QKD). We present a four-intensity protocol for the decoy-state MDI-QKD that hugely raises the key rate, especially in the case in which the total data size is not large. Also, calculations show that our method makes it possible for secure private communication with fresh keys generated from MDI-QKD with a delay time of only a few seconds. [ABSTRACT FROM AUTHOR]

In order to improve the key rate of the decoy-state method, we need to jointly study yields of different bases. Given the delicate fact that pulses of the same preparation state can have different counting rates if they are measured in different bases, for example, those vacuum pulses and those single-photon pulses, existing results of decoy-state quantum key distribution using biased bases are actually flawed by assuming that they are equal. We fix this flaw through using the idea that yields of pulses prepared in different bases are the same provided that they are prepared in the same state and also measured in the same basis, for example, those single-photon pulses prepared in different bases but measured in the same basis. Based on this, we present correct formulas for the decoy-state method using biased bases. Taking the effects of statistical fluctuations into account, we then numerically study the key rates of different protocols with all parameters being fully optimized. Our result confirms the prior conclusion that the decoy-state method using biased bases can have an advantage over the symmetric protocol with unbiased bases. We obtain high key rates of our four-intensity protocol without using any vacuum source. [ABSTRACT FROM AUTHOR]

Most of existing protocols for the decoy-state measurement-device-independent quantum key distribution use vacuum sources or extremely weak sources, but the laboratory cannot give real vacuum sources or precise extremely weak sources. We present a four-intensity protocol without using any of them. Our protocol gives a secure key rate close to those in the protocols with vacuum sources. [ABSTRACT FROM AUTHOR]