Assessing a binary quantum channel exploiting a silicon photomultiplier based hybrid receiver.

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.

Quantum Probes for the Characterization of Nonlinear Media.

Active optical media leading to interaction Hamiltonians of the form H=λ˜(a+a†)ζ represent a crucial resource for quantum optical technology. In this paper, we address the characterization of those nonlinear media using quantum probes, as opposed to semiclassical ones. In particular, we investigate how squeezed probes may improve and estimation of the nonlinear coupling λ˜ and of the nonlinearity order ζ.

Squeezing Phase Diffusion.

We address the use of optical parametric oscillator (OPO) to counteract phase diffusion, and demonstrate phase-noise reduction for coherent signals traveling through a suitably tuned OPO. In particular, we theoretically and experimentally show that there is a threshold value on the phase noise, above which OPO can be exploited to "squeeze" phase noise. The threshold depends on the energy of the input coherent state, and on the relevant parameters of the OPO, i.

Homodyne-like detection for coherent state-discrimination in the presence of phase noise.

We propose a homodyne-like detection scheme involving photon-number-resolving detectors to discriminate between two coherent states affected by either uniform or gaussian phase noise. A proof-of-principle experiment is performed employing two hybrid photodetectors, whose outputs are used in post processing to calculate the shot-by-shot photon-number differences. The performance of the strategy is quantified in terms of the error probability in discriminating the noisy coherent signals as a function of the characteristic noise parameters.

Quantum state transfer via Bloch oscillations.

The realization of reliable quantum channels, able to transfer a quantum state with high fidelity, is a fundamental step in the construction of scalable quantum devices. In this paper we describe a transmission scheme based on the genuinely quantum effect known as Bloch oscillations. The proposed protocol makes it possible to carry a quantum state over different distances with a minimal engineering of the transmission medium and can be implemented and verified on current quantum technology hardware.

Phase-reference monitoring in coherent-state discrimination assisted by a photon-number resolving detector.

Phase estimation represents a crucial challenge in many fields of Physics, ranging from Quantum Metrology to Quantum Information Processing. This task is usually pursued by means of interferometric schemes, in which the choice of the input states and of the detection apparatus is aimed at minimizing the uncertainty in the estimation of the relative phase between the inputs. State discrimination protocols in communication channels with coherent states also require the monitoring of the optical phase.

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.

Fidelity matters: the birth of entanglement in the mixing of Gaussian states.

We address the interaction of two Gaussian states through bilinear exchange Hamiltonians and analyze the correlations exhibited by the resulting bipartite systems. We demonstrate that entanglement arises if and only if the fidelity between the two input Gaussian states falls under a threshold value depending only on their purities, first moments, and the strength of the coupling. Our result clarifies the role of quantum fluctuations (squeezing) as a prerequisite for entanglement generation and provides a tool to optimize the generation of entanglement in linear systems of interest for quantum technology.

Optical phase estimation in the presence of phase diffusion.

The measurement problem for the optical phase has been traditionally attacked for noiseless schemes or in the presence of amplitude or detection noise. Here we address the estimation of phase in the presence of phase diffusion and evaluate the ultimate quantum limits to precision for phase-shifted Gaussian states. We look for the optimal detection scheme and derive approximate scaling laws for the quantum Fisher information and the optimal squeezing fraction in terms of the total energy and the amount of noise.