Phase-randomized Coherent States Research Articles

Experimental investigation of heralded Gaussification of phase-randomized coherent states of light

Experimental investigation of heralded Gaussification of phase-randomized coherent states of light

Sending-or-not-sending twin-field quantum key distribution with multiphoton states

The twin-field quantum key distribution (TF-QKD) protocol has led to extensive investigation for its capacity of overcoming the rate-distance limit. One variant for TF-QKD named sending-or-not-sending (SNS) TF-QKD has also been proposed. The SNS-TF-QKD encodes the classical keys 0 and 1 in sending or not sending the phase-randomized coherent states, which helps it to tolerate large misalignment errors due to optical phase fluctuations. Nevertheless, the original SNS-TF-QKD scheme utilizes only single-photon interference in generating secret keys, which seems quite different from other TF-QKD protocols. In this paper, we prove that multiphoton interference in the SNS TF-QKD protocol can also contribute to key generation. Accordingly, an improved SNS-TF-QKD scheme with higher key rate is proposed, where events of not only single-photon but also two-photon emissions are considered as effective events.

Improved analysis of measurement-device-independent quantum key distribution with non-phase-randomized coherent states.

Measurement-device-independent quantum key distribution (MDI-QKD) can remove all detector side-channel attacks, which can be implemented with phase-randomized coherent states (PRCS) or non-phase-randomized coherent states (NPRCS). In this paper, we focus on the MDI-QKD protocol with NPRCS and provide an improved analysis. In contrast with the original MDI-QKD with NPRCS which modulates the same intensity of coherent states in the key and test bases, we propose to modulate different intensities of coherent states in the key and test bases. Simulation results show that the secret key rate and transmission distance of MDI-QKD with NPRCS can be significantly improved. Furthermore, it is noteworthy that the modulation of different intensities does not bring extra complexity for experimental researchers, which can be easily done by adding an intensity modulator.

Application of a Discrete Phase-Randomized Coherent State Source in Round-Robin Differential Phase-Shift Quantum Key Distribution**Supported by the National Basic Research Program of China under Grant No 2013CB338002, and the National Natural Science Foundation of China under Grant Nos 11304397 and 61505261.

Recently, a novel kind of quantum key distribution called the round-robin differential phase-shift (RRDPS) protocol was proposed, which bounds the amount of leakage without monitoring signal disturbance. The protocol can be implemented by a weak coherent source. The security of this protocol with a simply characterized source has been proved. The application of a common phase shift can improve the secret key rate of the protocol. In practice, the randomized phase is discrete and the secret key rate is deviated from the continuous case. In this study, we analyze security of the RRDPS protocol with discrete-phase-randomized coherent state source and bound the secret key rate. We fix the length of each packet at 32 and 64, then simulate the secret key rates of the RRDPS protocol with discrete-phase randomization and continuous-phase randomization. Our simulation results show that the performance of the discrete-phase randomization case is close to the continuous counterpart with only a small number of discrete phases. The research is practically valuable for experimental implementation.

Probing single-photon state tomography using phase-randomized coherent states

Quantum processes involving single-photon states are of broad interest in particular for quantum communication. Extending to continuous values a recent proposal by Yuan et al \cite{YUAN16}, we show that single-photon quantum processes can be characterized using phase randomized coherent states (PRCS) as inputs. As a proof of principle, we present the experimental investigation of single-photon tomography using PRCS. The probability distribution of field quadratures measurements for single-photon states can be accurately derived from the PRCS data. As a consequence, the Wigner function and the density matrix of single-photon states are reconstructed with good precision. The sensitivity of the reconstruction to experimental errors and the number of PRCS used is addressed.

Universal optimal estimation of the polarization of light with arbitrary photon statistics

A universal and optimal method for the polarimetry of light with arbitrary photon statistics is presented. The method is based on the continuous maximum-likelihood positive operator-valued measure (ML-POVM) for pure polarization states over the surface of the Bloch sphere. The success probability and the mean fidelity are used as the figures of merit to show its performance. The POVM is found to attain the collective bound of polarization estimation with respect to the mean fidelity. As demonstrations, explicit results for the N photon Fock state, the phase-randomized coherent state (Poisson distribution), and the thermal light are obtained. It is found that the estimation performances for the Fock state and the Poisson distribution are almost identical, while that for the thermal light is much worse. This suggests that thermal light leaks less information to an eavesdropper and hence could potentially provide more security in polarization-encoded quantum communication protocols than a single-mode laser beam as customarily considered. Finally, comparisons against an optimal adaptive measurement with classical communications are made to show the better and more stable performance of the continuous ML-POVM.

EXPERIMENTAL QUANTIFICATION OF NON-GAUSSIANITY OF PHASE-RANDOMIZED COHERENT STATES

We present the experimental investigation of the non-Gaussian nature of some mixtures of Fock states by reconstructing their Wigner function and exploiting two recently introduced measures of non-Gaussianity. In particular, we demonstrate the consistency between the different approaches and the monotonicity of the two measures for states belonging to the class of phase-randomized coherent states. Moreover, we prove that the exact behavior of one measure with respect to the other depends on the states under investigation and devise possible criteria to discriminate which measure is more useful for the characterization of the states in realistic applications.

Manipulating the non-Gaussianity of phase-randomized coherent states

We experimentally investigate the non-Gaussian features of the phase-randomized coherent states, a class of states exploited in communication channels and in decoy state-based quantum key distribution protocols. In particular, we reconstruct their phase-insensitive Wigner functions and quantify their non-Gaussianity. The measurements are performed in the mesoscopic photon-number domain by means of a direct detection scheme involving linear detectors.

General theory for decoy-state quantum key distribution with an arbitrary number of intensities

We develop a general theory for quantum key distribution (QKD) in both the forward error correction and the reverse error correction cases when the QKD system is equipped with phase-randomized coherent light with an arbitrary number of decoy intensities. For this purpose, generalizing Wang's expansion, we derive a convex expansion of the phase-randomized coherent state. We also numerically check that the asymptotic key generation rates are almost saturated when the number of decoy intensities is three.