Burning the Trojan Horse: Defending against Side-Channel Attacks in QKD

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Abstract

The discrete-variable QKD protocols based on BB84 are known to be secure against an eavesdropper, Eve, intercepting the flying qubits and performing any quantum operation on them. However, these protocols may still be vulnerable to side-channel attacks. We investigate the Trojan-Horse side-channel attack where Eve sends her own state into Alice's apparatus and measures the reflected state to estimate the key. We prove that the separable coherent state is optimal for Eve amongst the class of multi-mode Gaussian attack states. We describe how Alice may defend against this by adding thermal noise to the system and give an analytic expression of the resulting secret key rate. We also provide a bound on the secret key rate in the case where Eve may use any separable state, and describe an active defense system based on optical modulators.

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Long-distance quantum communication with atomic ensembles and linear optics

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Quantum communication holds a promise for absolutely secure transmission of secret messages and faithful transfer of unknown quantum states. Photonic channels appear to be very attractive for physical implementation of quantum communication. However, due to losses and decoherence in the channel, the communication fidelity decreases exponentially with the channel length. We describe a scheme that allows to implement robust quantum communication over long lossy channels. The scheme involves laser manipulation of atomic ensembles, beam splitters, and single-photon detectors with moderate efficiencies, and therefore well fits the status of the current experimental technology. We show that the communication efficiency scale polynomially with the channel length thereby facilitating scalability to very long distances.