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We present a simple method to obtain an upper bound on the achievable secretkey rate in quantum key distribution (QKD) protocols that use onlyunidirectional classical communication during the public-discussion phase. Thismethod is based on a necessary precondition for one-way secret keydistillation; the legitimate users need to prove that there exists no quantumstate having a symmetric extension that is compatible with the availablemeasurements results. The main advantage of the obtained upper bound is that itcan be formulated as a semidefinite program, which can be efficiently solved.We illustrate our results by analysing two well-known qubit-based QKDprotocols: the four-state protocol and the six-state protocol. Recent resultsby Renner et al., Phys. Rev. A 72, 012332 (2005), also show that the givenprecondition is only necessary but not sufficient for unidirectional secret keydistillation.Comment: 11 pages, 1 figur

One-way quantum key distribution: Simple upper bound on the secret key rate