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arXiv:quant-ph/0312211 · Physical Review A

Conditional unitary transformation on biphotons

G. Brida, Maria V. Chekhova, Marco Genovese, Marco Gramegna, Leonid A. Krivitsky, S. P. Kulik

Published Sep 30, 2004• 16 citations
arXivHTMLPDF

Abstract

A conditional unitary transformation ($90\ifmmode^\circ\else\textdegree\fi{}$ polarization rotation) is performed at single-photon level. The transformation is realized by rotating polarization for one of the photons of a polarization-entangled biphoton state (signal photon) by means of a Pockel cell triggered by the detection of the other (idler) photon after polarization selection. As a result, the state of the signal photon is losslessly changed from being completely unpolarized to being partially polarized, so that the final polarization degree is given by the idler detector quantum efficiency. This experiment can be used for developing a different method of absolute quantum efficiency calibration.

DOI
10.1103/physreva.70.032332

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A conditional unitary transformation (90\ifmmode∘\else\textdegree\fi90\ifmmode^\circ\else\textdegree\fi{}90\ifmmode∘\else\textdegree\fi polarization rotation) is performed at single-photon level. The transformation is realized by rotating polarization for one of the photons of a polarization-entangled biphoton state (signal photon) by means of a Pockel cell triggered by the detection of the other (idler) photon after polarization selection. As a result, the state of the signal photon is losslessly changed from being completely unpolarized to being partially polarized, so that the final polarization degree is given by the idler detector quantum efficiency. This experiment can be used for developing a different method of absolute quantum efficiency calibration.
We propose a new scheme for measuring the quantum efficiency of a single-photon detection apparatus by using polarization-entangled states. The scheme consists of measuring the polarization of a member of a polarization-entangled pair after a 90° polarization rotation conditional on the detection of the correlated photon after polarization selection. We present experimental results obtained with this scheme compared with traditional biphoton calibration. Our results show the interesting potentiality of the suggested scheme.
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Our last experimental results on the realization of a measurement-conditional unitary operation at single photon level are presented. This gate operates by rotating by 90° the polarization of a photon produced by means of Type-II Parametric Down Conversion conditional to a polarization measurement on the correlated photon. We then propose a new scheme for measuring the quantum efficiency of a single photon detection apparatus by using this set-up. We present experimental results obtained with this scheme compared with traditional biphoton calibration. Our results show the interesting potentiality of the suggested scheme.
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