We address M -ary communication channels based on entangled two-mode states of radiation in the presence of losses. In particular we focus on channels build by two-mode coherently-correlated (TMC) or twin-beam (TWB) states. Optimized bit discrimination thresholds, as well as the corresponding maximized mutual information, are explicitly evaluated as a function of beam intensities and loss parameters for binary and quaternary alphabets. The evolution of the two entangled support states in lossy channels is analyzed and the joint photon number distribution is evaluated, showing that the beam statistics (either sub-Poissonian for TMC or super-Poissonian for TWB) is not altered by channel losses. The effects of losses on the channel security is discussed.
We address M -ary communication channels based on entangled two-mode states of radiation in the presence of losses. In particular we focus on channels build by two-mode coherently-correlated (TMC) or twin-beam (TWB) states. Optimized bit discrimination thresholds, as well as the corresponding maximized mutual information, are explicitly evaluated as a function of beam intensities and loss parameters for binary and quaternary alphabets. The evolution of the two entangled support states in lossy channels is analyzed and the joint photon number distribution is evaluated, showing that the beam statistics (either sub-Poissonian for TMC or super-Poissonian for TWB) is not altered by channel losses. The effects of losses on the channel security is discussed.
We address binary and quaternary communication channels based on correlated multiphoton two-mode states of radiation in the presence of losses. The protocol are based on photon number correlations and realized upon choosing a shared set of thresholds to convert the outcome of a joint photon number measurement into a symbol from a discrete alphabet. In particular, we focus on channels built using feasible photon-number entangled states (PNES) as two-mode coherently-correlated (TMC) or twin-beam (TWB) states and compare their performances with that of channels built using feasible classically correlated (separable) states. We found that PNES provide larger channel capacity in the presence of loss, and that TWB-based channels may transmit a larger amount of information than TMC-based ones at fixed energy and overall loss. Optimized bit discrimination thresholds, as well as the corresponding maximized mutual information, are explicitly evaluated as a function of the beam intensity and the loss parameter. The propagation of TMC and TWB in lossy channels is analyzed and the joint photon number distribution is evaluated, showing that the beam statistics, either sub-Poissonian for TMC or super-Poissonian for TWB, is not altered by losses. Although entanglement is not strictly needed to establish the channels, which are based on photon-number correlations owned also by separable mixed states, purity of the support state is relevant to increase security. The joint requirement of correlation and purity individuates PNES as a suitable choice to build effective channels. The effects of losses on channel security are briefly discussed.
Communication protocols based on quantum states of light have attracted increasing interest in the recent years, since they offer the possibility of enhancing either the communication capacity or the security by exploiting the very quantum nature of the information carriers. While first theoretical ideas and practical realizations started from discrete variables (DC) coding (practically based on the weak laser beams and SPDC-generated photon pairs), the continuous variables (CV) approach (supposing the use of coherent beams and entangled beam pairs) to the quantum information processing is aimed at the increase of the effectiveness and reliability of the quantum information protocols. Though being the subject to the environmental influence and thus performing always imperfectly, the CV-based quantum systems require the study of the effect the environment has on the quantum information processing schemes which is done by analyzing the perturbance of the corresponding quantum states. In this work we address M-ary communication channels based on photon-number continuous-variable multiphoton entangled states (PNES) in the conditions of the realistic lossy optical media.
Twin-beam (TWB) states generated by parametric down-conversion exhibit strong photon-number correlations that can be exploited for quantum communication. In the mesoscopic regime, these correlations can be directly investigated using photon-number-resolving detectors, such as silicon photomultipliers (SiPMs). Their nonclassical nature is quantified by the noise reduction factor (NRF), which provides a witness of quantum correlations and a means of monitoring channel security. In this work, a quantum communication protocol based on TWB states, originally proposed for binary encoding, is extended to a four-state scheme capable of transmitting two bits per use. Information is encoded by combining two mean photon-number values and two mode numbers of thermal noise superimposed on one arm of the twin beam, defining four distinguishable states in the (m,R) plane. While the mean detected photon number enables the retrieval of one bit, the NRF provides discrimination of the second bit and a security check against intercept-and-resend attacks, which introduce additional noise and degrade the quantum correlations. The protocol is investigated through two experimental measurement campaigns. First, the twin-beam source is characterized in terms of photon-number statistics, NRF, and detection efficiency to establish the operating conditions of the communication channel. Subsequently, the four-state protocol is implemented and evaluated through the analysis of state distributions and their 95% confidence regions, error probability as a function of sample size, and a comparative assessment of machine learning classifiers for state discrimination. Finally, the response of the NRF to an intercept-and-resend attack is investigated to assess the protocol's ability to detect eavesdropping.
We consider the discrimination of lossy bosonic channels and focus to the case when one of the values for the loss parameter is zero, i.e., we address the detection of a possible loss against the alternative hypothesis of an ideal lossless channel. This discrimination is performed by inputting one-mode or two-mode squeezed thermal states with fixed total energy. By optimizing over this class of states, we find that the optimal inputs are pure, thus corresponding to single- and two-mode squeezed vacuum states. In particular, we show that for any value of the damping rate smaller than a critical value there is a threshold on the energy that makes the two-mode squeezed vacuum state more convenient than the corresponding single-mode state, whereas for damping larger than this critical value two-mode squeezed vacua are always better. We then consider the discrimination in realistic conditions, where it is unlikely to have pure squeezing. Thus by fixing both input energy and squeezing, we show that two-mode squeezed thermal states are always better than their single- mode counterpart when all the thermal photons are directed into the dissipative channel. Besides, this result also holds approximately for unbalanced distribution of the thermal photons. Finally, we also investigate the role of correlations in the improvement of detection. For fixed input squeezing (single-mode or two-mode), we find that the reduction of the quantum Chernoff bound is a monotone function of the two-mode entanglement as well as the quantum mutual information and the quantum discord. We thus verify that employing squeezing in the form of correlations (quantum or classical) is always a resource for loss detection whenever squeezed thermal states are taken as input.
We have developed a “digital twin” of the photon entanglement generation process to model the degradation of quantum entangled states produced via spontaneous parametric down-conversion (SPDC) when transmitted through lossy and noisy optical channels. These two-mode entangled states can exhibit entanglement in photon number, polarization, and spectral-temporal modes, making them valuable resources for quantum communication and information processing. Using a density matrix formalism, we perform numerical simulations to model the interaction of each mode with its environment, incorporating the effects of photon loss, incoherent noise processes, and intermodal correlations between polarization and frequency degrees of freedom. Purity and fidelity are key metrics used to quantify the mixedness and coherence of the transmitted state. The simulation results quantify the decoherence as noise and loss increase, highlighting the vulnerability of entanglement to environmental disturbances and the compounding impact of mode-dependent quantum correlations. This study provides insight into the performance limits of SPDC-based entangled photon sources and offers guidance for optimizing quantum systems operating under realistic conditions. The simulation platform serves as a predictive tool, enabling future integration with experimental control systems and benchmarking against measured entangled photon characteristics such as Hong-Ou-Mandel interference and polarization correlation visibility.
In the past twenty years many experiments have demonstrated that quantum states of light can be used for secure data transfer, despite the presence of many noise sources. In this paper we investigate, both theoretically and experimentally, the role played by a statistically-distributed asymmetric amount of loss in the degradation of nonclassical photon-number correlations between the two parties of multimode twin-beam states in the mesoscopic intensity regime. To be as close as possible to realistic scenarios, we consider two different statistical distributions of such a loss, a Gaussian distribution and a log-normal one. The results achieved in the two cases show to what extent the involved parameters, both those connected to loss and those describing the employed states of light, preserve nonclassicality.