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arXiv:quant-ph/9911103 · Physical Review A

Improving detectors using entangling quantum copiers

Piotr Deuar, William J. Munro

Published Dec 13, 1999• 10 citations
arXivHTMLPDF

Abstract

We present a detection scheme that, using imperfect detectors, and imperfect quantum copying machines (which entangle the copies), allows one to extract more information from an incoming signal than with the imperfect detectors alone.
DOI
10.1103/physreva.61.010306

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While it is known that copying a quantum system does not increase the amount of information obtainable about the originals, it may increase the amount available in practice, when one is restricted to imperfect measurements. We present a detection scheme which, using imperfect detectors and possibly noisy quantum copying machines (that entangle the copies), allows one to extract more information from an incoming signal than with the imperfect detectors alone. The case of single-photon detection with noisy, inefficient detectors and copiers (single controlled-NOT gates in this case) is investigated in detail. The improvement in distinguishability between a photon and vacuum is found to occur for a wide range of parameters, and to be quite robust to random noise. The properties that a quantum-copying device must have to be useful in this scheme are investigated.
We find that very different quantum copying machines are optimal depending on the indicator used to assess their performance. Several quantum copying machine models acting on nonorthogonal input states are investigated, and assessed according to two types of criteria: transfer of (Shannon) information encoded in the initial states to the copies, and fidelity between the copies and the initial states. Transformations that optimize information transfer for messages encoded in qubits are found for three situations: (1) when the message is decoded one state at a time; (2) with simple schemes that allow the message to be encoded using block-coding schemes; and (3) when the copier produces independent copies. If the message is decoded one symbol at a time, information is best copied by a Wootters-Zurek copier.
Quantum telescopy, proposed by Grottesman et al., uses spatially entangled single-photon states to modify HBT stellar interferometry, potentially surpassing Michelson interferometry in resolution without baseline limits. However, finite source bandwidth, non-unit efficiency, detector response times, and celestial light bandwidth all affect its effectiveness. In this work, we consider a spatially entangled singe-photon state generated by beam-splitting a practical anti-bunched photon source from resonance fluorescence, which is a common method of generating single-photon states, and evaluate the signal level in quantum telescopy with inclusion of imperfection of detection system and finite bandwidth of celestial light. Here, given the incoming light, we optimize the parameters of the anti-bunched photon source and the detector system for the best signal-to-noise ratio (SNR), which can match that of the Michelson method in the ideal case but is always smaller in the realistic case. Alternatively, weak coherent states from heavily attenuated lasers can replace the entangled single-photon state but suffer low signal level, similar to HBT method. We find that the SNR with a realistic anti-bunched source for producing the reference fields can still beat the SNR of the coherent state scheme in the weak incoming field limit. The optimized parameters for the anti-bunched source and detection system will be a guide for future experimental endeavor.