We obtain bounds on the signal-to-noise ratio for quantum-nondemolition measurement of photon number, using soliton collisions. A combined phase and amplitude quadrature measurement strategy can give almost noise-free results, provided equal amplitude solitons with low relative velocities are used.
We report the quantum-nondemolition measurement of the photon number of an optical soliton. Taking advantage of the conservation properties of quantum solitons, we were able to read out the photon number of a ``signal'' soliton via the phase of a ``probe'' soliton after a soliton collision in a single-mode low-loss optical fiber. Subsequent noise measurements showed the photon number fluctuations of the signal soliton at the shot-noise limit to be correlated to the phase fluctuations of the probe soliton, signifying a backaction-evading measurement of the photon number.
We report the quantum-nondemolition measurement of the photon number of an optical soliton. Taking advantage of the conservation properties of quantum solitons, we were able to read out the photon number of a ``signal'' soliton via the phase of a ``probe'' soliton after a soliton collision in a single-mode low-loss optical fiber. Subsequent noise measurements showed the photon number fluctuations of the signal soliton at the shot-noise limit to be correlated to the phase fluctuations of the probe soliton, signifying a backaction-evading measurement of the photon number.
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.
A fully quantum-mechanical Heisenberg description of the quantum-nondemolition measurements via soliton collision in an optical fiber is provided. Solutions to the equations of motion for meaningful soliton collective operators such as photon number, phase, position, and frequency (momentum) are applied to quantifying the error of inference of photon number and frequency of the signal soliton by performing a measurement on the probe soliton. A detection method based on measuring a four-component quadrature amplitude is proposed and proved to give the best accuracy.
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.
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.