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Optics Express

Characterizing two-mode-squeezed light from four-wave mixing in rubidium vapor for quantum sensing and information processing

Luı́s E. E. de Araujo, Zhifan Zhou, M. T. DiMario, Brian E. Anderson, Jie Zhao, K. M. Jones, Paul D. Lett

Published Dec 11, 2023• 16 citations
Publisher

Abstract

We present a study of homodyne measurements of two-mode, vacuum-seeded, quadrature-squeezed light generated by four-wave mixing in warm rubidium vapor. Our results reveal that the vacuum squeezing can extend down to measurement frequencies of less than 1 Hz, and the squeezing bandwidth, similar to the seeded intensity-difference squeezing measured in this system, reaches up to approximately 20 MHz for typical pump parameters. By dividing the squeezing bandwidth into smaller frequency bins, we show that different sideband frequencies represent independent sources of two-mode squeezing. These properties are useful for quantum sensing and quantum information processing applications. We also investigate the impact of group velocity delays on the correlations in the system, which allows us to optimize the useful spectrum.

DOI
10.1364/oe.507727

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We present a study of homodyne measurements of two-mode, vacuum-seeded, quadrature-squeezed light generated by four-wave mixing in warm rubidium vapor. Our results reveal that the vacuum squeezing can extend down to measurement frequencies of less than 1 Hz, and the squeezing bandwidth, similar to the seeded intensity-difference squeezing measured in this system, reaches up to approximately 20 MHz for typical pump parameters. By dividing the squeezing bandwidth into smaller frequency bins, we show that different sideband frequencies represent independent sources of two-mode squeezing. These properties are useful for quantum sensing and quantum information processing applications. We also investigate the impact of group velocity delays on the correlations in the system, which allows us to optimize the useful spectrum.
We experimentally demonstrate the creation of two correlated beams generated by a nondegenerate four-wave-mixing amplifier at λ=795 nm in hot rubidium vapor. We achieve intensity difference squeezing at frequencies as low as 1.5 kHz which is so far the lowest frequency to observe squeezing in an atomic system. The squeezing spans from 5.5 to 16.5 MHz with a maximum squeezing of -5 dB at 1 MHz. We can control the squeezing bandwidth by changing the pump power. Both low frequency and controllable bandwidth squeezing show great potential in sensitivity detection and precise control of the atom optics measurement.
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 review our recent experimental progress in quantum technology employing amplification effect of four-wave mixing in a rubidium vapor. We have produced an intensity difference squeezed light source at frequencies as low as 1.5 kHz which is so far the lowest frequency at which squeezing has been observed in an atomic system. Moreover, we find that the bandwidth of our squeezed light source can be controlled with light intensity pumping. Using our non-classical light source, we have further developed a nonlinear Mach-Zehnder (MZ) interferometer, for which the maximum fringe intensity depends quadratically on the intensity of the phase-sensing field at the high-gain regime, leading to much better sensitivity than a linear MZ interferometer in which the beam splitters have the same phase-sensing intensity. The quantum technologies developed by our group could have great potential in areas such as precision measurement and quantum information.
Quantum correlated states of light, such as squeezed states, are a fundamental resource for the development of quantum technologies, as they are needed for applications in quantum metrology, quantum computation, and quantum communications. It is thus critical to develop compact, efficient, and robust sources to generate such states. Here, we report on a compact, narrowband, fiber-coupled source of two-mode squeezed states of light at 795 nm based on four-wave mixing (FWM) in an 85Rb atomic vapor. The source is designed in a small modular form factor, with two input fiber-coupled beams, the seed and pump beams required for the FWM, and two output fibers, one for each of the modes of the squeezed state. The system is optimized for low pump power (135 mW) to achieve a maximum intensity-difference squeezing of 4.4 dB after the output of fibers at an analysis frequency of 1 MHz. The narrowband nature of the source makes it ideal for atomic-based quantum sensing and quantum networking configurations that rely on atomic quantum memories. Such a source paves the way for a versatile and portable platform for applications in quantum information science.
Entangled multi-spatial-mode fields have interesting applications in quantum information, such as parallel quantum information protocols, quantum computing, and quantum imaging. We study the use of a nondegenerate four-wave mixing process in rubidium vapor at 795 nm to demonstrate generation of quantum-entangled images. Owing to the lack of an optical resonator cavity, the four-wave mixing scheme generates inherently multi-spatialmode output fields. We have verified the presence of entanglement between the multi-mode beams by analyzing the amplitude difference and the phase sum noise using a dual homodyne detection scheme, measuring more than 4 dB of squeezing in both cases. This paper will discuss the quantum properties of amplifiers based on four-wave-mixing, along with the multi mode properties of such devices.