In quantum communication protocols, the use of hybrid devices, giving access to both particle- and wave-like properties of light, could open new perspectives for novel encoding procedures that merge the properties of discrete and continuous variables. In our work, we explore the context of continuous-variable quantum key distribution with binary modulated coherent states over a pure-loss channel with a Silicon-photomultiplier-based hybrid receiver. In particular, we investigate how the key generation rate behaves as a function of the losses affecting the coherent states and the number of data used to send them both in the case of individual and collective attacks. The obtained outcomes encourage the further exploitation of this hybrid receiver in extended networks.
In quantum communication protocols, the use of hybrid devices, giving access to both particle- and wave-like properties of light, could open new perspectives for novel encoding procedures that merge the properties of discrete and continuous variables. In our work, we explore the context of continuous-variable quantum key distribution with binary modulated coherent states over a pure-loss channel with a Silicon-photomultiplier-based hybrid receiver. In particular, we investigate how the key generation rate behaves as a function of the losses affecting the coherent states and the number of data used to send them both in the case of individual and collective attacks. The obtained outcomes encourage the further exploitation of this hybrid receiver in extended networks.
In quantum communication protocols, the use of photon-number-resolving detectors could open new perspectives by broadening the way to encode and decode information and merging the properties of discrete and continuous variables. In this work, we consider a quantum channel exploiting a silicon-photomultiplier-based receiver and evaluate its performance for quantum communication protocols under three possible configurations, defined by different post-processing of the detection outcomes. We investigate two scenarios: information transmission over the channel, quantified by the mutual information, and continuous-variable quantum key distribution, quantified by the key generation rate. The preliminary results encourage further use of this detection scheme in extended networks.
Nowadays, quantum communications provide a vast field of research in rapid expansion, with a huge potential impact on the future developments of quantum technologies. In particular, continuous variable systems, employing coherent-state encoding and quadrature measurements, represent a suitable platform, due to their compatibility with both the modulation and detection systems currently employed in standard fiber-optical communications. In this work, we address some relevant aspects of the field, and provide innovative results being also experimentally oriented. In particular, we focus on two relevant paradigms: quantum decision theory and continuous variable quantum key distribution (CVQKD). In the former case, we address the problem of coherent-state discrimination and design new hybrid receivers for binary phase-shift keying discrimination, obtaining a quantum advantage over conventional detection schemes, being also robust against typical experimental imperfections. In the latter scenario, we proceed in two different directions. On the one hand, we design new CVQKD protocols employing discrete modulation of coherent states, being a feasible solution compatible with the state of the art in optical communications technologies. On the other hand, we address the more fundamental problem of performing channel losses mitigation to enhance existing protocols, and investigate the role of optical amplifiers for the task. Finally, we make a first step towards a fully non-Gaussian CVQKD scheme by proposing, for the first time, the adoption of an optimized state-discrimination receiver, commonly adopted for quantum decision theory, within the context of CVQKD, obtaining a genuine quantum enhancement over conventional protocols.
Over the past decade, binary phase-shift-keying encoding has been used as a benchmark to test the performance of different detection strategies to address the problem of state discrimination. In this context, hybrid devices, giving access to both particle- and wave-like properties of light, could offer the possibility to better discriminate the sent signals. In this work, we consider a quantum channel exploiting a hybrid receiver embedding Silicon photomultipliers as photon-number-resolving detectors. We retrieve the discrimination error probability and the mutual information between sender and receiver as functions of some relevant experimental parameters in the case of binary phase-shifted coherent states. Our promising results, supported also with numerical simulations and theoretical analysis, foster further using this kind of hybrid receiver in more complex detection schemes. • We address state discrimination by a Silicon photomultiplier-based hybrid receiver. • The hybrid receiver gives access to particle- and wave-like properties of light. • We consider error probability and mutual information in a BPSK scenario.
We present a discretely modulated continuous-variable quantum key distribution system in free space by using strong coherent states. The amplitude noise in the laser source is suppressed to the shot-noise limit by using a mode cleaner combined with a frequency shift technique. Also, it is proven that the phase noise in the source has no impact on the final secret key rate. In order to increase the encoding rate, we use broadband homodyne detectors and the no-switching protocol. In a realistic model, we establish a secret key rate of 46.8 kbits/s against collective attacks at an encoding rate of 10 MHz for a 90 channel loss when the modulation variance is optimal.
Quantum conditional entropies play a fundamental role in quantum information theory. In quantum key distribution, they are exploited to obtain reliable lower bounds on the secret-key rates in the finite-size regime, against collective attacks and coherent attacks under suitable assumptions. Here we consider continuous-variable communication protocols, where the sender Alice encodes information using a discrete modulation of phase-shifted coherent states, and the receiver Bob decodes by homodyne or heterodyne detection. We compute the Petz-Rényi and sandwiched Rényi conditional entropies associated with these setups, assuming either a passive eavesdropper or one that injects thermal photons into the channel, who gathers the quantum information leaked through a lossy communication line of known or bounded transmittance. Whereas our results do not directly provide reliable key-rate estimates, they do represent useful ballpark figures. We obtain analytical or semianalytical expressions that do not require intensive numerical calculations. These expressions serve as bounds on the key rates that may be tight in certain scenarios. We compare different estimates, including known bounds that have already appeared in the literature and bounds that we determine. The latter are found to be tighter for very short block sizes.
We consider discrete-modulation protocols for continuous-variable quantum key distribution (CV-QKD) that employ a modulation constellation consisting of a finite number of coherent states and that use a homodyne- or a heterodyne-detection receiver. We establish a security proof for collective attacks in the asymptotic regime, and we provide a formula for an achievable secret-key rate. Previous works established security proofs for discrete-modulation CV-QKD protocols that use two or three coherent states. The main constituents of our approach include approximating a complex, isotropic Gaussian probability distribution by a finite-size Gauss-Hermite constellation, applying entropic continuity bounds, and leveraging previous security proofs for Gaussian-modulation protocols. As an application of our method, we calculate secret-key rates achievable over a lossy thermal bosonic channel. We show that the rates for discrete-modulation protocols approach the rates achieved by a Gaussian-modulation protocol as the constellation size is increased. For pure-loss channels, our results indicate that in the high-loss regime and for sufficiently large constellation size, the achievable key rates scale optimally, i.e., proportional to the channel's transmissivity.