The production of conditional quantum states and quantum operations based on the result of measurement is now seen as a key tool in quantum information and metrology. We propose a new type of photon number detector. It functions non-deterministically, but when successful, it has high fidelity. The detector, which makes use of an n-photon auxiliary Fock state and high efficiency Homodyne detection, allows a tunable tradeoff between fidelity and probability. By sacrificing probability of operation, an excellent approximation to a photon number detector is achieved.
@article{arxiv.quant-ph/0305185,
title = {Photon Added Detection},
author = {A. M. Branczyk and Tobias J. Osborne and Alexei Gilchrist and T. C. Ralph},
journal= {arXiv preprint arXiv:quant-ph/0305185},
year = {2009}
}