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Frontiers in Optics

Decoherence-free subspaces in quantum key distribution

Zachary Walton, Ayman F. Abouraddy, Alexander V. Sergienko, Bahaa E. A. Saleh, Malvin Carl Teich

Published Jan 1, 2003• 12 citations
Publisher

Abstract

We demonstrate that two recent innovations in the field of practical quantum key distribution (noise-immunity and passive detection) are closely related to the methods developed to protect quantum computations from decoherence. We present a new scheme that combines these advantages, and propose a practical implementation of this scheme that is feasible using existing technology.

DOI
10.1364/fio.2003.thkk4

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We demonstrate that two recent innovations in the field of practical quantum key distribution (noise-immunity and passive detection) are closely related to the methods developed to protect quantum computations from decoherence. We present a new scheme that combines these advantages, and propose a practical implementation of this scheme that is feasible using existing technology.
A quantum key distribution with decoherence-free subspaces has been proposed to overcome collective noise to the polarization modes of photons flying in quantum channels. Prototypes of this scheme have also been achieved with a parametric down-conversion source. However, the photon-number-splitting attack we propose will make practical implementations of this scheme insecure since the parametric down-conversion source may occasionally emit multiphoton pairs. We propose a decoy-state method to make these implementations immune to this attack. With this decoy-state method, both the security distance and key bit rate will be increased.
Decoherence-free subspace (DFS) is a valid solution to realize quantum communication over a collective noise channel, and has been widely studied. Generally speaking, replacing a qubit with a DFS state will cause the reduction of communication efficiency. However, in this letter, it is shown that some kinds of noises may not lower the transmission rate of quantum key distribution. To illustrate it, we propose two quantum key distribution protocols based on Bell states. Here, two nonorthogonal and unbiased sets in a DFS are constructed by linear combination of particles at different positions. Since n−1n-1n−1 classical bits are distributed by using 2n2n2n qubits in our protocols, the transmission rate is close to that of noiseless BB84 protocol. Furthermore, when considering the cost of transmitting classical bits, the efficiencies of these protocols are even higher than that of BB84 protocol.
Quantum key distribution (QKD) is a promising technology to enable secure cryptography after quantum computers have been developed. It allows for a key growing protocol that permits creating absolutely random keys to be used in the onetime pad codification scheme. Enabling a global QKD network is one of the final goals of the field. However, to do this with conventional optical fibres presents a fundamental limitation due to their intrinsic loss. Free-space, and specifically satellite links, have been proposed as an alternative and have gathered a lot of interest in recent years. They are considered one of the best candidates to enable a global network. Free-space QKD implementations are dominated by polarisation encoding protocols due to the relative transparency of the atmosphere to polarization. Nonetheless, time-bin and phase codifications offer some advantages and can be practical thanks to new passive interferometer designs. In this paper, the first free-space Coherent One-Way (COW) implementation is reported, some design considerations are commented, and the results of the experiment are shown. These show how time-bin/phase codifications are interesting candidates for free-space QKD.
We construct a new quantum key distribution scheme and prove its security against the most general type of attacks and the most general type of noises allowed by quantum physics. The novel technique we use is reduction from a quantum scheme to a classical scheme. We first show that, rather surprisingly, the proof of security of our quantum key distribution in the error-free case can be reduced to that of a classical verification scheme. In other words, the quantum verification procedure employed by us has a classical interpretation. By proving the security of the classical scheme, the security of our quantum key distribution scheme in the error-free case follows immediately. The security against the most general types of noises (channel noises, storage errors and computational errors) is then proven by using fault-tolerant quantum computation.