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Entangled photons can be used to make measurements with an accuracy beyond that possible with classical light. While most implementations of quantum metrology have used states made up of a single color of photons, we show that entangled states of two colors can show supersensitivity to optical phase and path length by using a photonic crystal fiber source of photon pairs inside an interferometer. This setup is relatively simple and robust to experimental imperfections. We demonstrate sensitivity beyond the standard quantum limit and show superresolved interference fringes using entangled states of two, four, and six photons.
Bell, B., Kannan, S., McMillan, A., Clark, A. S., Wadsworth, W. J., & Rarity, J. G. (2013). Multicolor quantum metrology with entangled photons. Physical Review Letters, 111(9), . https://doi.org/10.1103/PhysRevLett.111.093603