Quantum copying can increase the practically available information - art. no. 042304

While it is known that copying a quantum system does not increase the amoun t of information obtainable about the originals, it may increase the amount available in practice, when one is restricted to imperfect measurements. W e present a detection scheme which, using imperfect detectors and possibly noisy quantum copying machines (that entangle the copies), allows one to ex tract more information from an incoming signal than with the imperfect dete ctors alone. The case of single-photon detection with noisy,inefficient det ectors and copiers (single controlled-NOT gates in this case) is investigat ed in detail. The improvement in distinguishability between a photon and va cuum is found to occur for a wide range of parameters, and to be quite robu st to random noise. The properties that a quantum-copying device must have to be useful in this scheme are investigated.