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We investigate theoretically and experimentally thermal light ghost imaging where the light transmitted through the object as the seed light is amplified by an optical parametric amplifier (OPA). In conventional lens imaging systems with OPA, the spectral bandwidth of OPA dominates the image resolution. Theoretically, we prove that in ghost imaging via optical parametric amplification (GIOPA) the bandwidth of OPA will not affect the image resolution. The experimental results show that for weak seed light the image quality in GIOPA is better than that of conventional ghost imaging. Our work may be valuable in remote sensing with ghost imaging technique, where the light passed through the object is weak after a long-distance propagation. [ABSTRACT FROM AUTHOR]

We derive an exact formula of geometric measure of quantum discord (GMQD) for an arbitrary 3-qubit state. The dynamics of GMQD in a quantum spin system coupled to a thermal bath is studied by' simulation and analytical approximation. It is found that tripartite quantum discord has distinct time behaviors within two different eigenspaces. Further, we extend the square norm distance to define tripartite geometric quantifiers of total and classical correlations. We obtain their explicit expressions and discuss the possibility of having a closed additive relation among various kinds of correlations during the dynamical evolution [ABSTRACT FROM AUTHOR]

A wavelength-multiplexing ghost-imaging (WMGI) technique is performed in order to improve long data acquisition time in conventional ghost imaging. In WMGI, a source has more than two colored light beams which are independently modulated and projected onto an object at the same time. Then the reflected light from the object is recorded with wavelength-selected detectors. We analyze the contrast-to-noise ratio (CNR) of the reconstructed images of each single-wavelength ghost imaging and their superposition and demonstrate the validity of the WMGI technique. [ABSTRACT FROM AUTHOR]

The noise in optical source preparation needs to be characterized for the security of practical continuous-variable quantum key distribution (CVQKD). Two feasible schemes, based on either an active optical switch or a passive beamsplitter, are proposed to monitor the variance of the source noise. We derive the security bounds for both schemes against collective attacks in the asymptotical limit and find that the passive scheme performs better. The finite-size effect is also discussed briefly for both schemes. [ABSTRACT FROM AUTHOR]

A continuous-variable measurement-device-independent quantum key distribution (CV-MDI QKD) protocol using squeezed states is proposed where the two legitimate partners send Gaussian-modulated squeezed states to an untrusted third party to realize the measurement. Security analysis shows that the protocol can not only defend all detector side channels, but also attain higher secret key rates than the coherent-state-based protocol. We also present a method to improve the squeezed-state CV-MDI QKD protocol by adding proper Gaussian noise to the reconciliation side. It is found that there is an optimal added noise to optimize the performance of the protocol in terms of both key rates and maximal transmission distances. The resulting protocol shows the potential of long-distance secure communication using the CV-MDI QKD protocol. [ABSTRACT FROM AUTHOR]

We propose a continuous-variable measurement-device-independent quantum key distribution (CV-MDIQKD) protocol, in which detection is conducted by an untrusted third party. Our protocol can defend all detector side channels, which seriously threaten the security of a practical CV QKD system. Its security analysis against arbitrary collective attacks is derived based on the fact that the entanglement-based scheme of CV-MDI QKD is equivalent to the conventional CV QKD with coherent states and heterodyne detection. We find that the maximal total transmission distance is achieved by setting the untrusted third party close to one of the legitimate users. Furthermore, an alternate detection scheme, a special application of CV-MDI QKD, is proposed to enhance the security of the standard CV QKD system. [ABSTRACT FROM AUTHOR]

The four-state continuous-variable quantum key distribution protocol has a long practical secure distance [A. Leverrier and P. Grangier, Phys. Rev. Lett. 102, 180504 (2009)], while it poses the difficulty of parameter estimation. We propose an improved four-state protocol where the covariance matrix can be estimated from experimental data without using the linear channel assumption, thus ensuring its unconditional security in the asymptotical limit. Our scheme maintains the advantage of high reconciliation efficiency of the four-state protocol, which ensures long practical secure distance its. Our scheme can be implemented with the current technology. [ABSTRACT FROM AUTHOR]

Source noise affects the security of continuous-variable quantum key distribution (CV QKD) and is difficult to analyze. We propose a model to characterize Gaussian source noise through introducing a neutral party (Fred) who induces the noise with a general unitary transformation. Without knowing Fred's exact state, we derive the security bounds for both reverse and direct reconciliations and show that the bound for reverse reconciliation is tight. [ABSTRACT FROM AUTHOR]

Open quantum systems interacting with structured environments is important and manifests non-Markovian behavior, which was conventionally studied using a quantum trajectory stochastic method. In this paper, by dividing the effects of the environment into two parts, we propose a deterministic method without using a quantum trajectory. This method is more efficient and accurate than the stochastic method in most Markovian and non-Markovian cases. We also extend this method to the generalized Lindblad master equation. [ABSTRACT FROM AUTHOR]

Photon subtraction can enhance the performance of continuous-variable quantum key distribution (CV QKD). However, the enhancement effect will be reduced by the imperfections of practical devices, especially the limited efficiency of a single-photon detector. In this paper, we propose a non-Gaussian postselection method to emulate the photon substraction used in coherent-state CV QKD protocols. The virtual photon subtraction not only can avoid the complexity and imperfections of a practical photon-subtraction operation, which extends the secure transmission distance as the ideal case does, but also can be adjusted flexibly according to the channel parameters to optimize the performance. Furthermore, our preliminary tests on the information reconciliation suggest that in the low signal-to-noise ratio regime, the performance of reconciliating the postselected non-Gaussian data is better than that of the Gaussian data, which implies the feasibility of implementing this method practically. [ABSTRACT FROM AUTHOR]