  We develop an improvement to the weak laser pulse BB84 scheme for quantum key
distribution, which utilizes entanglement to improve the security of the scheme
and enhance its resilience to the photon-number-splitting attack. This protocol
relies on the non-commutation of photon phase and number to detect an
eavesdropper performing quantum non-demolition measurement on number. The
potential advantages and disadvantages of this scheme are compared to the
coherent decoy state protocol.
