  Continuous-variable quantum key distribution (CV-QKD) has many practical
advantages including compatibility with current optical communication
technology. Implementation using heterodyne measurements is particularly
attractive since it eliminates the need for active phase locking of the remote
pair of local oscillators, but the full security of CV QKD with discrete
modulation was only proved for a protocol using homodyne measurements. Here we
propose an all-heterodyne CV-QKD protocol with binary modulation and prove its
security against general attacks in the finite-key regime. Although replacing a
homodyne measurement with a heterodyne measurement would be naively expected to
incur a 3-dB penalty in the rate-distance curve, our proof achieves a key rate
with only a 1-dB penalty.
