David Fainsin

Continuous variable encoding of quantum information requires the deterministic generation of highly correlated quantum states of light in the form of quantum networks, which, in turn, necessitates the controlled generation of a large number of squeezed modes. In this work, we present an experimental source of multimode squeezed states of light at telecommunication wavelengths. Generation at such wavelengths is especially important as it can enable quantum information processing, communication, and sensing beyond the laboratory scale. We use a single-pass spontaneous parametric down-conversion process in a non-linear waveguide pumped with the second harmonic of a femtosecond laser. We demonstrate multiparty entanglement by measuring the state’s covariance matrix. Our measurements reveal significant squeezing in more than 21 frequency modes, with a maximum squeezing value exceeding 2.5 dB. We finally present a frequency-multiplexed quantum key distribution protocol and the expected key rates in bipartite and in multipartite scenarii.

Quantum key distribution (QKD) contrasts with classical cryptographic methods because it provides information-theoretical security on the distilled key. In some security demanding contexts, the perfect confidentiality that can be obtained with QKD combined with One-Time-Pad, may be insufficient. This will be in particular the case if the mere existence of communication can divulge crucial information. Performing QKD covertly solves this by insuring low probability of detection for the QKD signal states. We study here for the first time Covert continuous variable (CV) QKD. We establish, in the general case of a thermal noise channel, that the covertness conditions impose drastic limits to the performance of such protocols. We then propose an original solution to overcome this limitation by performing a computationally-secure coherent block encoding, analogous to spread spectrum, to the signal pulses of a Gaussian modulated coherent state CV-QKD protocol. The resulting protocol provides covertness, under computational assumptions while preserving the information-theoretical security on the final QKD key. We show that our method enables QKD over realistic WDM environments such as a 30 km optical backbone populated by 25 standard channels.