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Conference on Lasers and Electro-Optics

Using photonic entanglement in quantum measurements

Geoff J. Pryde, Jeremy L. O’Brien, Andrew G. White, Timothy C. Ralph, Stephen D. Bartlett, Nathan K. Langford

Published Jan 1, 2005• 0 citations
Publisher

Abstract

The use of entanglement allows a wide variety of quantum measurements outside the usual case of projective measurement. We describe several experimental photonic quantum measurement results relying on entangled measurement, such as weak measurements.
DOI
10.1109/iqec.2005.1561114

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The ability to post-select the outcomes of an experiment is a useful theoretical concept and experimental tool. In the context of weak measurements, post-selection can lead to surprising results such as complex weak values outside the range of eigenvalues. Usually post-selection is realized by a projective measurement, which is hard to implement in ensemble systems such as NMR. We demonstrate the first experiment of a weak measurement with post-selection on an NMR quantum information processor. Our setup is used for measuring complex weak values and weak values outside the range of eigenvalues. The scheme for overcoming the problem of post-selection in an ensemble quantum computer is general and can be applied to any circuit-based implementation. This experiment paves the way for studying and exploiting post-selection and weak measurements in systems where projective measurements are hard to realize experimentally.
We experimentally determine weak values for a single photon's polarization, obtained via a weak measurement that employs a two-photon entangling operation, and postselection. The weak values cannot be explained by a semiclassical wave theory, due to the two-photon entanglement. We observe the variation in the size of the weak value with measurement strength, obtaining an average measurement of the S1 Stokes parameter more than an order of magnitude outside of the operator's spectrum for the smallest measurement strengths.