Quantum Key Distribution Based On Generating More Than One Secret Bit Per Shared EPR Pair

Given a shared EPR pair, two cooperative parties can obtain one bit of shared secret key by separately measuring the pair along the same basis. Apparently, the Holevo bound [1] suggests that they cannot obtain more than one secret bit per shared EPR pair. Here we show that they can do so by independently putting each of their EPR pairs in a Hilbert subspace which is unknown to the eavesdropper. We also prove the unconditional security of a practical prepare-and-measure quantum key distribution scheme based on this principle and derive its key rate in the one-way communication setting.