In the quantum version of a Trojan-horse attack, photons are injected into the optical modules of a quantum key distribution system in an attempt to read information direct from the encoding devices. To stop the Trojan photons, the use of passive opt
ical components has been suggested. However, to date, there is no quantitative bound that specifies such components in relation to the security of the system. Here, we turn the Trojan-horse attack into an information leakage problem. This allows us quantify the system security and relate it to the specification of the optical elements. The analysis is supported by the experimental characterization, within the operation regime, of reflectivity and transmission of the optical components most relevant to security.
Information-theoretical security of quantum key distribution (QKD) has been convincingly proven in recent years and remarkable experiments have shown the potential of QKD for real world applications. Due to its unique capability of combining high key
rate and security in a realistic finite-size scenario, the efficient version of the BB84 QKD protocol endowed with decoy states has been subject of intensive research. Its recent experimental implementation finally demonstrated a secure key rate beyond 1 Mbps over a 50 km optical fiber. However the achieved rate holds under the restrictive assumption that the eavesdropper performs collective attacks. Here, we review the protocol and generalize its security. We exploit a map by Ahrens to rigorously upper bound the Hypergeometric distribution resulting from a general eavesdropping. Despite the extended applicability of the new protocol, its key rate is only marginally smaller than its predecessor in all cases of practical interest.
We analyse the finite-size security of the efficient Bennett-Brassard 1984 protocol implemented with decoy states and apply the results to a gigahertz-clocked quantum key distribution system. Despite the enhanced security level, the obtained secure k
ey rates are the highest reported so far at all fibre distances.
Modern single-photon detectors based on avalanche photodiodes offer increasingly higher triggering speeds, thus fostering their use in several fields, prominently in the recent area of Quantum Key Distribution. To reduce the probability of an afterpu
lse, these detectors are usually equipped with a circuitry that disables the trigger for a certain time after a positive detection event, known as dead time. If the acquisition system connected to the detector is not properly designed, efficiency issues arise when the triggering rate is faster than the inverse of detectors dead-time. Moreover, when this happens with two or more detectors used in coincidence, a security risk called self-blinding can jeopardize the distribution of a secret quantum key. In this paper we introduce a trigger-disabling circuitry based on an FPGA-driven feedback loop, so to avoid the above-mentioned inconveniences. In the regime of single-photon-attenuated light, the electronics dynamically accept a trigger only after detectors complete recovery from dead-time. This technique proves useful to work with detectors at their maximum speed and to increase the security of a quantum key distribution setup.
In the propagation of optical pulses through dispersive media, the frequency degree of freedom acts as an effective decohering environment on the polarization state of the pulse. Here we discuss the application of open-loop dynamical-decoupling techn
iques for suppressing such a polarization decoherence in one-way communication channels. We describe in detail the experimental proof of principle of the bang-bang protection technique recently applied to flying qubits in [Damodarakurup et al., Phys. Rev. Lett. 103, 040502]. Bang-bang operations are implemented through appropriately oriented waveplates and dynamical decoupling is shown to be potentially useful to contrast a generic decoherence acting on polarization qubits propagating in dispersive media like, e.g., optical fibers.
