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Annual Meeting Optical Society of America

Simple optics for evading backaction noise and generating Schrodinger kittens

Richard E. Slusher, Arthur La Porta, Bernard Yurke

Published Jan 1, 1989• 0 citations
Publisher

Abstract

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.

DOI
10.1364/oam.1989.wh2

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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.
A "Schrödinger cat" state of free-propagating light can be defined as a quantum superposition of well separated coherent states. 1, 2 We demonstrated, theoretically and experimentally, a protocol which allows to generate arbitrarily large squeezed Schrödinger cat states, using a homodyne detection and photon number states as resources. We implemented this protocol experimentally with light pulses containing two photons, producing a squeezed Schrödinger cat state with a negative Wigner function. This state clearly presents several quantum phase-space interference fringes between the "dead" and "alive" components, and it is large enough to become useful for experimental tests of quantum theory and quantum information processing.
We present a detailed experimental analysis of a free-propagating light pulse prepared in a "Schrödinger kitten" state, which is defined as a quantum superposition of "classical" coherent states with small amplitudes. This kitten state is generated by subtracting one photon from a squeezed vacuum beam, and it clearly presents a negative Wigner function. The predicted influence of the experimental parameters is in excellent agreement with the experimental results. The amplitude of the coherent states can be amplified to transform our "Schrödinger kittens" into bigger Schrödinger cats, providing an essential tool for quantum information processing.
Pulsed squeezed light and intensity-correlated twin beams are generated by means of a traveling-wave optical parametric amplifier (OPA) consisting of a type-II phase-matched KTiOPO4 crystal that is pumped by a Q-switched and frequency-doubled Nd:YAG laser. Q switching of the laser results in a high peak pump intensity and a large peak parametric gain. Squeezed light is generated by amplifying the vacuum input field to the OPA, and it is detected by a balanced homodyne configuration that uses the fundamental beam of the laser as a local oscillator.1 Twin beams are generated by supplying a small input signal to the OPA. The amplified signal and idler beams are direct-detected individually and yield quantum-noise reduction when the two photocurrents are subtracted.
We have constructed a mode-locked optical parametric oscillator (OPO), synchronously pumped by the second harmonic of a continuously mode-locked Nd:YAG laser and based on type II phase matching in a KTP crystal. The two subharmonics (signal and idler waves) are nearly degenerate in frequency but are distinct orthogonal polarization states, and the oscillator is thus a nondegenerate OPO. Above threshold, the intensities of the signal and idler output beams should be highly correlated, as recently demonstrated for a similar single-frequency OPO.1 Ideally, the quantum noise on the corresponding photocurrent difference signal is completely suppressed for frequencies that are small compared to the OPO cavity bandwidth. Since this configuration produces intense light beams with potentially very good noise suppression, it is an important prototype for possible applications, e.g., to spectroscopy. Initial results with our OPO show an excess uncorrelated polarization noise, possibly due to depolarized light scattering in the KTP crystal. When the OPO cavity is stabilized with the signal and idler degenerate in frequency, they can be mixed to form a superposition by manipulating their polarization. When the oscillator is pumped below threshold, a pulsed squeezed vacuum state is produced in two such mixtures.2 When the polarizations are mixed inside the OPO cavity by a Faraday rotator, the backaction evasion necessary for a quantum nondemolition (QND) measurement of one of the field quadratures of an externally injected beam is obtained.3 The pulsed local oscillator used in observing the squeezing or QND effects is derived from the fundamental output of the pump laser. Progress in implementing these experiments with our mode-locked OPO is described as well as possible applications to time-domain measurements with noise below the shot-noise limit.
Balanced homodyne detection relies on a beam splitter to superpose the weak signal input and strong local oscillator. However, recent investigation shows that a high gain phase sensitive amplifier (PSA) can be viewed as homodyne detector, in which the strong pump of PSA serves as the local oscillator [1]. Here, we analyze a new method of measuring the continuous variable entanglement by assisting a balanced homodyne detector with the PSA and implement it experimentally. Before measuring quadrature amplitude with the balanced homodyne detectors, two entangled fields generated from a pulse pumped fiber optical parametric amplifier are simultaneously coupled into the PSA. We find that the normalized noise for both the difference and sum of the quadrature amplitudes of the two entangled fields fall below the shot noise limit by about 4.6 dB, which is the record degree of entanglement measured in optical fiber systems. The experimental results illustrate that the advantages of the new measurement method include but not limit to tolerance to detection loss and characterizing entanglement with only one homodyne detector. The influence of mode-mismatching due to multi-mode property of entanglement on the measured noise reduction can also be greatly mitigated, indicating the new method is advantageous over the traditional measurement in multi-mode case.