Ilaria Vagniluca

We experimentally tested an alternative fiber-based setup for 4D-QKD, with time and phase encoding and one-decoy technique. We evaluated the secret key rate achievable in a finite-key scenario and we compared it with the binary-encoded BB84 protocol, which was tested with the same experimental setup. Our 4D-QKD system makes it possible to improve the secret key rate by more than a factor 2 in the saturation-regime of single-photon detectors, without requiring additional expensive resources to the 2D-QKD setup. In comparison to previous works, our scheme allows to measure the 4D states with a simplified and compact receiver, thus making it a cost-effective solution for practical and fiber-based QKD.

The advances in quantum key distribution (QKD) during the last 30 years have been outstanding in terms of reachable distance and key generation rate. However, the integration of quantum systems in real telecommunication networks generates multiple challenges, from technology availability to the design of inter-operable QKD systems, interconnected to key management layers and cyphers, and that can be embedded in existing telecommunication network topologies. We present several use-cases of implementation and integration of our QKD systems, in different contexts and involving Italy and neighboring countries in Europe.

Quantum key distribution (QKD) is introduced to make encryption and transmission of data over any public channel unconditionally secure. A key requirement of such a promise is to have access to an encryption key with a similar length as the message and data itself. While QKD has become mature and the key rate significantly increased over the past 20 years, there is still a notable gap between data transmission and key generation rates. High-dimensional QKD is proposed as a method to respond to this demand. Here, we demonstrate a 4-dimensional path-\&-time encoding QKD system with more than 100\% improvement compared to a standard 2D system in the same test-bed, a 52-km deployed multicore fiber link.

The coexistence of classical and quantum communication within the same fiber optics infrastructure is still an open challenge to be solved. In fact, most of the practical implementations of quantum key distribution (QKD) are accomplished by taking advantage of dark fiber channels, i.e. fiber-optics links totally dedicated to the transmission of quantum signals. This prevents the intense classical light to affect the qubit error rate, but strongly reduces the possibilities for a full deployment of QKD technologies in large-scale and realistic applications. Looking for a solution several approaches have been tested, generally based on multiplexing of different degrees of freedom of photons. In our work we combined a dense-wavelength-division-multiplexing scheme with two different home-made single photon detection stages able to convert C-band photons into photons detectable by a silicon photon counter, by exploiting sum-frequency-generation process in nonlinear crystals. We compared the results with an off-the-shelf InGaAs single-photon detector, equipping it with polarization and wavelength filters, that was tested under the same experimental conditions. Injecting an intense light laser into a different DWDM channel to simulate a real-worl QKD scenario, we demonstrated that our up-conversion based detector makes QKD feasible with a classical launch power of 4 dB higher than the one affordable by the InGaAs detector. This result paves the way to the employment of quantum communication in many realistic situations, by enabling the usage of already existing telecom infrastructures, where the noise levels are not manageable by current single photon avalanche detectors.