Quantum bit regeneration

Decoherence and loss will limit the practicality of quantum cryptography and computing unless successful error correction techniques are developed. To this end, we have discovered a new scheme for perfectly detecting and rejecting the error caused by loss (amplitude damping to a reservoir at T = 0), based on using a dual-rail representation of a quantum bit. This is possible because (1) balanced loss does not perform a ''which-path'' measurement in an interferometer, and (2) balanced quantum nondemolition measurement of the ''total'' photon number can be used to detect loss-induced quantum jumps without disturbing the quantum bit's coherence.