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arXiv:1902.10293 · Chinese Physics B

A two-mode squeezed light based on a double-pump phase-matching geometry*

Xuan-Jian He, Jun Fang Jia, Gao-Feng Jiao, L. Q. Chen, Chun-Hua Yuan, Weiping Zhang

Published Apr 18, 2020• 1 citation
arXivHTMLPDF

Abstract

We theoretically investigate the frequency-nondegenerate and frequency degenerate squeezed lights with a four-wave mixing process (4WM) driven by two pump fields crossing at a small angle. Different from a 4WM process driven by a single pump field, the refractive index of the corresponding probe field, n p, can be converted to a value that is greater than 1 or less than 1 by an angle adjustment. In the new region with n p < 1, the bandwidth of the gain is relatively large due to the slow change in the refractive index with the two-photon detuning. In this region with an exchange of the roles of the pump and probe beams, the frequency degenerate and spatial nondegenerate twin beams can be generated, which has potential application in quantum information and quantum metrology.

DOI
10.1088/1674-1056/ab8abd

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•May 1, 2014•1 citation
We theoretically investigate the frequency-nondegenerate and frequency degenerate squeezed lights with a four-wave mixing process (4WM) driven by two pump fields crossing at a small angle. Different from a 4WM process driven by a single pump field, the refractive index of the corresponding probe field, n p, can be converted to a value that is greater than 1 or less than 1 by an angle adjustment. In the new region with n p < 1, the bandwidth of the gain is relatively large due to the slow change in the refractive index with the two-photon detuning. In this region with an exchange of the roles of the pump and probe beams, the frequency degenerate and spatial nondegenerate twin beams can be generated, which has potential application in quantum information and quantum metrology.
We demonstrate a new phase-matching geometry for four-wave mixing processes in hot Rb85 vapor, in which all four fields propagate in different directions but two of them are degenerate in frequency. When used as a parametric amplifier with an injected seed, two types of quantum mechanically correlated twin-beam states, either frequency degenerate or nondegenerate, can be generated. The quantum noise reduction in the intensity difference is almost 7 dB for the nondegenerate type and nearly 5 dB for the degenerate type. The spatial nondegeneracy of the four waves allows a variety of configurations of parametric processes, leading to flexible control for both phase insensitive and sensitive parametric amplification. The spatially nondegenerate but frequency degenerate four-wave mixing process will find wide applications in quantum metrology, quantum communication, and quantum information of continuous variables.
Using a nondegenerate four-wave mixing (FWM) process based on a double-Λ scheme in hot cesium vapor, we demonstrate a compact diode-laser-pumped quantum light source for the generation of quantum correlated twin beams with a maximum squeezing of 6.5 dB. The squeezing is observed at a Fourier frequency in the audio band down to 0.7 kHz which, to the best of our knowledge, is the first observation of sub-kilohertz intensity-difference squeezing in an atomic system so far. A phase-matching condition is also investigated in our system, which confirms the spatial-multi-mode characteristics of the FWM process. Our compact low-frequency squeezed light source may find applications in quantum imaging, quantum metrology, and the transfer of optical squeezing onto a matter wave.
We study a variety of mechanisms that introduce noise into squeezed light generated by a four-wave mixing (4WM) process in Rb vapor. The noise from the seeding beam itself is a general noise that appears in any squeezed light generated from a seeding process. This noise dominates in the squeezed light at acoustic and lower measurement frequencies. A second excess noise source is observed in the twin beams pumped by either a diode laser system or a Ti:sapphire laser system. This excess noise is much stronger in the diode laser systems. It is present in the twin beams at measurement frequencies when the 4WM gain is reduced toward unity. Most of this excess noise can be removed with a dual-seeded 4WM scheme. A third noise source we examine is from a two-beam coupling that degrades the squeezing of the dual-seeded 4WM process at low frequencies of the order of the atomic transition linewidth. This noise can be avoided by seeding skew rays in the 4WM gain region. This gives us an insight to solve this "cross talk" problem by imaging the source in the 4WM gain region. In addition to studying noise sources, we propose a gain-independent calibration scheme that relies on higher order correlation function for the absolute calibration of photodiodes. Having low frequency squeezing is really important if we record quantum images with a CCD camera, which has a slow shutter speed. Also, it's been very difficult for people to get low-frequency squeezing. We obtain a record level of low-frequency squeezing using a simple dual-seeding technique. With this study of noise sources we are closer to having a portable quantum light source using diode lasers.
We demonstrate, to the best of our knowledge, a new four-wave mixing geometry based on structured light. By utilizing near-field diffraction through a narrow slit, the pump beam is asymmetrically structured to modify the phase-matching condition, generating multimode output in both the spatial and frequency domains. We show that the frequency parameter enables selection of various spatial-mode outputs, including a twin-beam geometry, which preserves relative intensity squeezing shared between the two beams. The results suggest that the engineering of atomic states via structured light may provide a pathway to a diverse set of quantum resources based on multimode squeezed light.
Nondegenerate forward four-wave mixing in hot atomic vapors has been shown to produce strong quantum correlations between twin beams of light [McCormick et al., Opt. Lett. 32, 178 (2007)], in a configuration which minimizes losses by absorption. In this paper, we look at the role of the phase-matching condition in the trade-off that occurs between the efficiency of the nonlinear process and the absorption of the twin beams. To this effect, we develop a semiclassical model by deriving the atomic susceptibilities in the relevant double-$\ensuremath{\Lambda}$ configuration and by solving the classical propagation of the twin-beam fields for parameters close to those found in typical experiments. These theoretical results are confirmed by a simple experimental study of the nonlinear gain experienced by the twin beams as a function of the phase mismatch. The model shows that the amount of phase mismatch is key to the realization of the physical conditions in which the absorption of the twin beams is minimized while the cross coupling between the twin beams is maintained at the level required for the generation of strong quantum correlations. The optimum is reached when the four-wave mixing process is not phase matched for fully resonant four-wave mixing.
We show that an optomechanical cavity pumped by a bichromatic light beam can generate a signal whose frequency lies halfway between the two driving frequencies. This process can be understood as a degenerate four-wave mixing, in which two pump photons (one from each frequency) are combined to yield two identical signal photons. This process takes place between a lower and an upper threshold in terms of the pump intensity, which depend on the pump frequency difference. Close to the signal oscillation threshold a clear noise reduction in one of its quadratures is shown numerically.