Alberto Dall'Arche

Resource Allocation for Secret Key Agreement Over Parallel Channels With Full and Partial Eavesdropper CSI

We consider the distillation phase of the secret key agreement (SKA) channel model for users connected through parallel additive white Gaussian noise fading channels. Alice sends binary phase shift keying modulated random bits to Bob who selects the subset of bits that have a log-likelihood ratio higher than a given threshold, i.e., they are sufficiently reliable. Alice has either full or partial channel state information (CSI) on her channel to the eavesdropper Eve. The main contributions of this paper are: 1) the derivation of the expression of outage probability when partial CSI is available; 2) the introduction of (outage) secret key throughput (SKT) as a metric in the full (partial) CSI scenario; 3) the derivation of SKT bounds; 4) the proposal of a new power allocation and threshold choice algorithm that maximizes the SKT under an outage probability constraint; and 5) the derivation of a suboptimal version of the allocation algorithm amenable for practical implementation obtained by approximating the objective functions with a closed-form expression. Moreover, we show that when power allocation and thresholds are optimized, per-channel and joint SKA are equivalent.

Intersatellite quantum communication feasibility study

The shift in the Communication paradigm from the bit to the qubit is increasingly exploited in terrestrial long range links and networks, with strong potentials in secure communications, quantum computing and metrology. The space-to-ground quantum key distribution was also considered as feasible. A new different scenario for the quantum communications is that of the intersatellite link. In this study we focus on the extension of intersatellite communications into the quantum domain. The long distances involved and the fast relative motion are severe constraints, partially compensated by the absence of beam degradation due to the propagation in the atmosphere as well as the relatively low background noise level. We address the conception of the optical terminal and the predicted performances in the case of constellations of LEO and MEO satellite including the quantum communications and quantum teleportation.

Resource allocation for secret key agreement by LLR thresholding over parallel channels

We consider a channel-model based secret key agreement (SKA) over parallel channels, where Alice transmits a random sequence of bits, and Bob selects those having an absolute value of log-likelihood ratio (LLR) larger than a given threshold. Distillation and reconciliation processes follow, independently on each channel. Assuming a total transmit power constraint, we aim at choosing both the LLR threshold and the transmit power on each channel, in order to maximize the secret key throughput (SKT), under a full channel state information of both legitimate and eavesdropper channels. A two-step solution is considered. First the LLR threshold is determined on a per-channel basis for a given power allocation by numerical methods. Then we fit the SKT as a function of the transmit power by an exponential function and the power allocation problem is solved in closed form. Extensive simulation results show accuracy of the fitting function and the importance of the optimization, also for practical reconciliation processes.

Turbulent single-photon propagation in the Canary optical link

The role of turbulence for Quantum Communications (QC) has been investigated in a 143 km-long link. The analysis of the received signal temporal domain indicate how to exploit constructively its effects in the case of QC along very long free-space links as well satellite links. Novel applications with relevant background noise may be envisaged.

Experimental Studies Toward the Quantum Communications with Orbiting Terminals

Realization of Quantum Communications in Space requires a deep understanding of issues including link-budget, turbulence mitigation and single-photon terminal synchro-nization. Here we report on supporting novel experiments on very long-distance links and modeling.

Long range beam propagation for quantum communications

The study of the free-space propagation of quantum correlations is necessary for any future application of quantum communication aiming to connect two remote locations. Here we study the propagation of a coherent laser beam over over 144 Km (between Tenerife and La Palma Islands of the Canary archipelagos). By attenuating the beam we also studied the propagation at the single photon level. We investigated the statistic of arrival of the incoming photons and the scintillation of the beam.

Experimental Study of Free-space Beam Propagation for Single-photon Quantum Communications

We report on the study the propagation of a laser beam over a 144 free-space link. We report on the losses of the channel, the temporal scintillation of the intensity and, by attenuating the beam, the statistic of arrival of single photons.

The space as frontier for global quantum communications

The sharing of quantum states among ground and orbiting terminal may be considered as feasible according to present optical technologies. The paradigm shift that Quantum Communications represent vs. classical counterpart allows to envisage the application of global cryptographic key distribution as well as the extension to Space of quantum technologies.

Long range beam propagation for single-photon communications

In the perspective of the long range single-photon communications, we study in this work the propagation of a single or twin optical beams in scale length of several tens to a few hundreds kilometres, introducing in the experiment the collection of the whole beam combined to the measure of local irradiance. The experimental models were realized in different localities of Italian Alps as well as between Tenerife and La Palma Islands of the Canary archipelagos. The whole beam at the receiver was acquired and compared to models including the local meteorological conditions.