A fully passive transmitter for decoy-state quantum key distribution

Abstract

A passive quantum key distribution (QKD) transmitter generates the quantum states prescribed by a QKD protocol at random, combining a fixed quantum mechanism and a post-selection step. By circumventing the use of active optical modulators externally driven by random number generators, passive QKD transmitters offer immunity to modulator side channels and potentially enable higher frequencies of operation. Recently, the first linear optics setup suitable for passive decoy-state QKD has been proposed. In this work, we simplify the prototype and adopt sharply different approaches for BB84 polarization encoding and decoy-state parameter estimation. In particular, our scheme avoids a probabilistic post-selection step that is central to the former proposal. On top of it, we elaborate a simple and tight custom-made security analysis.

Security of the Decoy-State BB84 Protocol with Imperfect State Preparation

“…[30,31]. In order to take into account the imperfect polarization state preparation, we apply an idea from the fully-passive source approach [38] and compute the density matrices of mixed states in two polarization bases; then we evaluate the fidelity between them and estimate the upper bound on the phase error rate using the concept of imbalanced quantum coin [6,7,40]. From the analysis of experimental angular distributions, we find that the key length is reduced by less than 9 and 47% for the transmission distances up to 50 and 100 km, respectively.…”

“…Nevertheless, we can use one of the ideas of Refs. [33,38]-the replacement of the source of randomly fluctuating pure states {|ψ i } by an equivalent source emitting the mixed states {ρ i },…”

Bell-State-Exchange-Parity-Based Protocol for Efficient Autocompensation of Quantum Key Distribution Encoded in Polarization or Spatial Modes

“…In this way, QKD protocols can be made robust against both coupling and phase perturbations at a reduced cost. This passive approach towards quantum optical state processing in QKD is not restricted to noise mitigation but also can be applied in other stages of the protocol [16].…”