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Phase sensitive measurements in a nonlinear interferometer using four-wave mixing in Rubidium-85

Tabitha Colter

Published Jan 1, 2016• 0 citations
OpenAlex

Abstract

Conventional interferometers make precision measurements by analyzing the position of interference fringes while nonlinear interferometers rely upon a sensitive measurement of phase difference. We detect this phase sensitivity using a correlated probe and conjugate beam that have undergone four-wave mixing inside a hot Rubidium-85 vapor cell. By placing a piece of glass into the path of the probe we observe a phase shift that indicates a successful nonlinear interferometer. We now plan to place into the probe path oils with known indices of refraction on plasmonic media and measure the resulting phase shifts. These results will then allow us to back-calculate an unknown material’s index of refraction simply by observing the phase shift with a level of sensitivity greater than conventional interferometers. Continued research in the field of interferometry utilizing nonlinear processes should provide even greater sensitivity in precision measurements and applications across a range of scientific disciplines.

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Conventional interferometers make precision measurements by analyzing the position of interference fringes while nonlinear interferometers rely upon a sensitive measurement of phase difference. We detect this phase sensitivity using a correlated probe and conjugate beam that have undergone four-wave mixing inside a hot Rubidium-85 vapor cell. By placing a piece of glass into the path of the probe we observe a phase shift that indicates a successful nonlinear interferometer. We now plan to place into the probe path oils with known indices of refraction on plasmonic media and measure the resulting phase shifts. These results will then allow us to back-calculate an unknown material’s index of refraction simply by observing the phase shift with a level of sensitivity greater than conventional interferometers. Continued research in the field of interferometry utilizing nonlinear processes should provide even greater sensitivity in precision measurements and applications across a range of scientific disciplines.
Based upon two cascade four-wave mixing processes in two identical hot rubidium vapor cells, a nonlinear interferometer has been experimentally realized [Jing et al., Appl. Phys. Lett. 99, 011110 (2011); Hudelist et al., Nat. Commun. 5, 3049 (2014)]. It has a higher degree of phase sensitivity than a traditional linear interferometer and has many potential applications in quantum metrology. Phase locking of the nonlinear interferometer is needed before it can find its way into applications. In this letter, we investigate the experimental implementation of phase locking of the relative phase between the three beams at different frequencies involved in such a nonlinear interferometer. We have utilized two different methods, namely, beat note locking and coherent modulation locking. We find that coherent modulation locking can achieve much better phase stability than beat note locking in our system. Our results pave the way for real applications of a nonlinear interferometer in precision measurement and quantum manipulation, for example, phase control in phase-sensitive N-wave mixing process, N-port nonlinear interferometer and quantum-enhanced real-time phase tracking.
We have experimentally demonstrated a general method to perform high-precision refractive index spectrum measurement using phase-shifting interferometry in a phase-tunable Mach–Zehnder interferometer. We observed coherent amplification on the refractive spectrum, across the Lamb dips of ground hyperfine states in a Doppler-broadened Rb87 vapor. Most importantly, the spectrum still has a large signal-to-noise ratio when the probe power is in the dark light level under which the method using a single interferogram fails. This experimental scheme is efficient for refractive spectrum measurements and could be extended for 2D phase imaging. It provides many potential applications for optical phase measurements in linear, nonlinear, and quantum optics.
Beamsplitters have played an important role in quantum optics experiments. They are often used to split and combine two beams, especially in the construct of an interferometer. In this letter, we experimentally implement a nonlinear beamsplitter using a phase-sensitive parametric amplifier, which is based on four-wave mixing in hot rubidium vapor. Here we show that, despite the different frequencies of the two input beams, the output ports of the nonlinear beamsplitter exhibit interference phenomena. We make measurements of the interference fringe visibility and study how various parameters, such as the intensity gain of the amplifier, the intensity ratio of the two input beams, and the one and two photon detunings, affect the behavior of the nonlinear beamsplitter. It may find potential applications in quantum metrology and quantum information processing.
A nonlinear interferometer uses nonlinear elements as beam splitters to split and to recombine optical waves for interference. As a result, the interference fringe size has a nonlinear dependence on the intensity of the field for phase sensing and leads to an enhanced phase signal. In this paper, a practical scheme of nonlinear interferometry for precision phase measurement is analyzed with parametric amplifiers as the nonlinear beam splitters. It is found that the signal due to phase shift is enhanced by a factor of the amplification gain as compared to a linear interferometer with the same phase-sensing light intensity while the quantum noise is kept at the vacuum level, thus, effectively increasing the signal-to-noise ratio (SNR) beyond the standard quantum limit. Furthermore, the scheme is not as sensitive to the detection loss as the linear scheme with a squeezed state for noise reduction. However, losses inside the interferometer limit the enhancement factor in SNR. We apply the concept to a Michelson interferometer but with parametric amplifiers involved for gravitational-wave detection. We find that effective power is increased by the gain of the amplifiers without actually increasing the cycling power inside the interferometer. Furthermore, the full benefits with squeezed input and variational output or the combination of a quantum nondemolition interferometer for sensitivity beyond the standard quantum limit apply here with even better results. Such a nonlinear interferometer will find wide applications in precision measurements.