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Physical Review Letters

Back-Action Evading Measurements of an Optical Field Using Parametric Down Conversion

Arthur La Porta, Richard E. Slusher, Bernard Yurke

Published Jan 2, 1989• 212 citations
Publisher

Abstract

A simple experimental arrangement demonstrates back-action evasion of noise in a quantum measurement of an optical field. Correlation between the photocurrents from a measured signal beam and a measurement readout beam is 0.6 dB below the shot-noise limit. The measurement is accomplished by use of a single-pass parametric down-conversion process.

DOI
10.1103/physrevlett.62.28

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A simple optical arrangement using polarization rotators and a single-pass parametric amplifier is being used to evade backaction noise in a measurement of a quadrature component of the light field.1 The transmitted field component is left in an eigenstate with an eigenvalue equal to the measured field value. The degree of precision for this eigenvalue can be made arbitrarily high as the optical losses are decreased and the parametric gain is increased. Measurement of the field is accomplished by a homodyne detection of a polarization component orthogonal to the incident and transmitted field. A pulse mode-locked Nd:YAG laser is used as a light source for the measured field and as a pump for the KTP parametric amplifier. If a direct detection measurement is made, instead of homodyne detection, the transmitted light is left in a state which has the properties of a Schrodinger cat,2 or more precisely, a Schrodinger kitten since the feasible experiments will only allow a small number of photons per cat. This catlike state can be transformed by a parametric amplifier into a state that is nearly the superposition of two coherent states. A homodyne measurement can demonstrate the quantum coherence of these two states.

Annual Meeting Optical Society of America•Jan 1, 1989•0 citations
A simple experimental arrangement demonstrates back-action evasion of noise in a quantum measurement of an optical field. Correlation between the photocurrents from a measured signal beam and a measurement readout beam is 0.6 dB below the shot-noise limit. The measurement is accomplished by use of a single-pass parametric down-conversion process.
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.
Backaction-evading measurement of an optical intensity has been observed, using a three-level system in a sodium atomic beam as a nonlinear medium. The principle of this experiment is to use the cross-phase-modulation effect, which in principle allows one to read nondestructively the quantum intensity fluctuations of the ``signal'' beam when detecting the phase fluctuations of the ``meter'' beam. Using a cavity-enhancement scheme, the quantum noise measured when recombining the signal and meter photocurrents is 0.8 dB below the shot-noise level.
We use a lithium iodate nonlinear crystal as a source of two quantum correlated beams at about 830 nm. A liquid-crystal cell is placed in one of the beams and is electrically driven to produce a very small modulation of its transmission coefficient. Low-noise high-efficiency photodiodes detect the light in each beam and the spectrum of the photocurrents, and the spectrum of the difference between the photocurrents, are measured. The difference spectrum shows a reduction of the noise below the shot-noise level of 4 dB compared with the classical difference technique, where the shot noise in the two detectors simply adds. However, the classical noise in one beam is low enough to make a single-beam shot-noise-limited measurement. Using an optimized difference technique, we show a reduction of the noise level compared to the single-beam shot noise of 2 dB.
Electro-optical transducers are utilized for upconversion of radio frequency (RF) signals to the optical frequency domain to study the RF signals with optical tools. The transducers frequently impact the RF system and introduce additional noise, including optical shot noise as well as quantum back action noise, limiting the measurement accuracy. In this paper, we theoretically study a technique based on a high efficiency electro-optical phase modulation effect that allows back action evading detection of an RF field quadrature. The main idea of the proposed method is independent homodyne detection of two (Stokes and anti-Stokes) optical modes. It allows subsequent postprocessing of the detected signals, which, in turn, results in a broadband back action evading measurement leading to the high sensitivity evaluation of the RF signals.
A simple optical arrangement using polarization rotators and a single-pass parametric amplifier is being used to evade backaction noise in a measurement of a quadrature component of the light field.1 The transmitted field component is left in an eigenstate with an eigenvalue equal to the measured field value. The degree of precision for this eigenvalue can be made arbitrarily high as the optical losses are decreased and the parametric gain is increased. Measurement of the field is accomplished by a homodyne detection of a polarization component orthogonal to the incident and transmitted field. A pulse mode-locked Nd:YAG laser is used as a light source for the measured field and as a pump for the KTP parametric amplifier. If a direct detection measurement is made, instead of homodyne detection, the transmitted light is left in a state which has the properties of a Schrodinger cat,2 or more precisely, a Schrodinger kitten since the feasible experiments will only allow a small number of photons per cat. This catlike state can be transformed by a parametric amplifier into a state that is nearly the superposition of two coherent states. A homodyne measurement can demonstrate the quantum coherence of these two states.