  We propose a scheme for performing quantum key distribution (QKD) which has
the potential to beat schemes based on the direct transmission of photons
between the communicating parties. In our proposal, the communicating parties
exchange photons with two quantum memories placed between them. This is a very
simple quantum repeater scheme and can be implemented with currently available
technology. Ideally, its secret key rate scales as the square root of the
transmittivity of the optical channel, which is superior to QKD schemes based
on direct transmission because key rates for the latter scale at best linearly
with transmittivity. Taking into account various imperfections in each
component of our setup, we present parameter regimes in which our protocol
outperforms protocols based on direct transmission.
