Recent advances in quantum key distribution (QKD) have given rise to systems that operate at transmission periods significantly shorter than the dead times of their component single-photon detectors. As systems continue to increase in transmission rate, security concerns associated with detector dead times can limit the production rate of sifted bits. We present a model of high-speed QKD in this limit that identifies an optimum transmission rate for a system with given link loss and detector response characteristics.
There has been growing research interest in quantum technology in general and in quantum key distribution (QKD) in particular. However, the key generation rate of QKD systems is still limited by the quantum channel losses as well as the practical limitations of the quantum sources and detectors. Here, we focus on one of these limitations, namely, the quantum detector's dead time effect, which happens after each photon detection, limiting the maximum quantum bit transmission rate and consequently the key generation rate of QKD systems. Qubit transmission at the sub-dead-time regime (i.e., faster than the reciprocal of dead time) can introduce different security loopholes. In this paper, we propose the use of a detector array instead of a single detector to measure the quantum state of individual photons in discrete variable QKD systems, showing that it can significantly alleviate the limitations induced by detectors' dead time. A dead-time compensated BB84 scheme is introduced, allowing qubit transmission at the sub-dead-time regime. An
M−dimensional Markov chain model is used to describe the impact of dead time on the operation of the proposed BB84 system, employing detector arrays of size
M. Consequently, novel analytical expressions can be derived for the sifted bit rate (SBR) and secrecy key rate (SKR) of the proposed QKD system, showing excellent match with the Monte Carlo simulation results. A remarkable gain is observed for the BB84 system employing detector arrays, showing a potential
M-fold improvement of SBR at high qubit transmission rates.
The security of a high speed quantum key distribution system with finite detector dead time τ is analyzed. When the transmission rate becomes higher than the maximum count rate of the individual detectors (1/τ), security issues affect the algorithm for sifting bits. Analytical calculations and numerical simulations of the Bennett-Brassard BB84 protocol are performed. We study Rogers et al.'s protocol (introduced in "Detector dead-time effects and paralyzability in high-speed quantum key distribution," New J. Phys. 9 (2007) 319) in the presence of an active eavesdropper Eve who has the power to perform an intercept-resend attack. It is shown that Rogers et al.'s protocol is no longer secure. More specifically, Eve can induce a basis-dependent detection efficiency at the receiver's end. Modified key sifting schemes that are secure in the presence of dead time and an active eavesdropper are then introduced. We analyze and compare these secure sifting schemes for this active Eve scenario, and calculate and simulate their key generation rate. It is shown that the maximum key generation rate is 1/(2τ) for passive basis selection, and 1/τ for active basis selection. The security analysis for finite detector dead time is also extended to the decoy state BB84 protocol.
The security of a high speed quantum key distribution system with finite detector dead time
τ is analyzed. When the transmission rate becomes higher than the maximum count rate of the individual detectors (
1/τ), security issues affect the scheme for sifting bits. Analytical calculations and numerical simulations of the Bennett-Brassard BB84 protocol are performed. We study Rogers et al.'s scheme (further information is available in [D. J. Rogers, J. C. Bienfang, A. Nakassis, H. Xu, and C. W. Clark, New J. Phys.~{\bf 9}, 319 (2007)]) in the presence of an active eavesdropper Eve who has the power to perform an intercept-resend attack. It is shown that Rogers et al.'s scheme is no longer guaranteed to be secure. More specifically, Eve can induce a basis-dependent detection efficiency at the receiver's end. Modified key sifting schemes that are basis-independent and thus secure in the presence of dead time and an active eavesdropper are then introduced. We analyze and compare these secure sifting schemes for this active Eve scenario, and calculate and simulate their key generation rate. It is shown that the maximum key generation rate is
1/(2τ) for passive basis selection, and
1/τ for active basis selection. The security analysis for finite detector dead time is also extended to the decoy state BB84 protocol for one particular secure sifting scheme.