Backaction evading measurements for quantum nondemolition detection and quantum optical tapping

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Abstract
Backaction evading (BAE) measurements are described for optical parametric amplification with polarization mixing. Quantum correlations between the ‘‘signal’’ and ‘‘meter’’ outputs from the BAE device lead to an observed photocurrent variance 30% (1.5 dB) below the signal shot-noise level and to an inferred variance 44% below the vacuum-state level for determination of the signal field given the meter reading. Together with its efficient information transfer, this BAE detector functions as a quantum optical tap and is suitable for implementing quantum nondemolition detection.