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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Quantum nondemolition measurements in optical fibers

Ofer Shapiro, Moshe Tur

Published Sep 22, 1997• 0 citations
Publisher

Abstract

Quantum non-demolition (QND) measurements are of great interest in recent years as a method to increase the information capacity of communication systems. Common implementations of QND measurements in optical fiber are based on the Kerr effect: a probe beam acquires phase modulation which mimics the amplitude modulation of a signal beam. This phase is typically small and needs to be measured at high frequencies for practical fiber-optic communication systems. Here we present a self reflexive interferometer scheme adequate for this kind of measurements and describe its performance. Using this system we achieved fairly open eye-diagrams at 140 Mbit/sec.

DOI
10.1117/12.281416

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Quantum non-demolition (QND) measurements are of great interest in recent years as a method to increase the information capacity of communication systems. Common implementations of QND measurements in optical fiber are based on the Kerr effect: a probe beam acquires phase modulation which mimics the amplitude modulation of a signal beam. This phase is typically small and needs to be measured at high frequencies for practical fiber-optic communication systems. Here we present a self reflexive interferometer scheme adequate for this kind of measurements and describe its performance. Using this system we achieved fairly open eye-diagrams at 140 Mbit/sec.
We analyze schemes for performing quantum nondemolition (QND) measurements in optical cavities. We consider three schemes: (1) measurement of a quadrature phase amplitude using a parametric process, (2) measurement of a quadrature phase amplitude using the optical Kerr effect in a nonlinear fiber, and (3) measurement of the photon number also using the Kerr effect in a fiber. We show that in the second scheme an enhancement of the QND effect may be obtained by making the cavity finesse for the signal larger than that for the probe.
Conventional devices for measuring the amplitude of a light wave absorb the light, thus destroying it. Recently, quantum nondemolition (QND) methods based on the optical Kerr effect have been shown capable of measuring optical amplitude fluctuations with near quantum-limited sensitivity.1 In these experiments, amplitude fluctuations of a signal beam are transferred to the phase fluctuation quadrature of a probe beam which co-propagates with the signal beam, by the intensity-dependent refractive index, without absorption of the signal beam. A pure Kerr nonlinearity can cause only phase modulation of the light wave, and the amplitude of the signal wave is preserved. Such methods should more properly be called back-action evading measurement schemes since the quantity being measured is not perturbed by the back-action that quantum mechanics requires for any measurement. The signal-to-noise ratio achieved in these experiments was limited by the small magnitude of the Kerr nonlinearity relative to the excess phase noise imposed on the probe beam by thermal light scattering processes in the glass fibers that were employed.
To try to demonstrate a quantum nondemolition (QND) measurement of photon number,1 we built a nonlinear interferometer system with a 10-km polarization-maintaining fiber. (In theory, this fiber length is nearly optimum for 1.5-μm operation.) The optical Kerr effect allows a signal beam intensity to change a probe beam phase, which can then be read out by the interferometer. A modified ring interferometer structure and feedback stabilization enabled the 1,32-μm probe to read the intensity of the 1.06-μm signal beam. We measured a χ(3) value in this fiber of3.6× 10-33(1.4×10-33)MKS when the polarization of the signal beam is coincident with (perpendicular to) that of the probe beam. The interferometer output spectrum displayed large noise peaks due to guided acoustic-wave Brillouin scattering (GAWBS)2 in the fiber. GAWBS was also observed in 5-km conventional single-mode fibers in various interferometer configurations. The coexistence of two different kinds of GAWBS (polarized and depolarized) makes it particularly difficult to obtain QND measurements. We conclude that a QND measurement system free from GAWBS noise requires differential phase detection of two pulses having the same polarization state.
Summary form only given. Quantum-nondemolition (QND) measurements are those that do not disturb the quantum distribution of the variable being measured. It has been experimentally demonstrated that QND measurement of the photon number an be achieved by use of wavelength-multiplexed soliton collisions. A quantum analysis of the experiment has also been developed. It was found that the phase spreading of the probe as a result of self-phase modulation can contribute significant noises. In this paper we propose a new and better QND scheme using cross-polarized soliton collisions in a fiber loop interferometer. The scheme requires only a single wavelength laser source and can be fully fiber-integrated, and, most importantly, the noise caused by self-phase modulation is automatically suppressed by detection of the squeezed quadrature of the probe soliton.
The collision of solitons in optical fibers is a promising scheme for the quantum-nondemolition measurement of the photon number using the cross Kerr effect. In a realistic experimental situation, the sensitivity of the measurement is limited not only by self-phase-modulation noise of the probe but also by the phase noise added due to the vibrational excitations of the fiber. Former schemes proposed to overcome either of these problems are not compatible with each other. In this paper the sensitivity of different type of detection is carefully studied. We propose a solution based on the use of a phase-shifting cavity. We show that this provides a conceptually and practically simple solution eliminating simultaneously both sources of excess noise.
The collision of two optical solitons, one a probe and the other a signal, writes out the energy of the signal soliton as a phase shift of the probe soliton. Consequently, a readout of the probe phase shift gives the photon number of the signal soliton. If the collision takes place in a low-loss optical fiber with the usual Kerr nonlinearity, the photon number of the signal soliton is preserved, and the measurement can be a quantum nondemolitional (QND) measurement. We describe experimental progress towards soliton collision QND measurements. We have collided two solitons at 1.460 nm (probe) and 1.463 nm (signal) and with the help of a delayed reference soliton (identical to the probe) in a homodyne detector system, we have measured the resulting probe soliton phase shifts. Toward a QND measurement, we correlate the shot noise fluctuations of the signal soliton with the corresponding phase noise fluctuations written on the probe. We present our current results.