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Optics Express

Enhancing vacuum squeezing via magnetic field optimization

Zhifei Yu, Shuqi Liu, Jinxian Guo, Guzhi Bao, Yuan Wu, L. Q. Chen

Published Apr 22, 2022• 4 citations
Publisher

Abstract

In this paper, we report on -3.5±0.2 dB vacuum squeezing (corresponding to -4.2±0.2 dB with loss correction) at 795 nm via the polarization self-rotation (PSR) effect in rubidium vapor by applying a magnetic field, whose direction is perpendicular to the propagation and polarization of the pump light. Compared with the case without the magnetic field, whose optimal squeezing degree is about -1.5 dB, this weak magnetic field can enhance the PSR effect and ultimately increase the squeezing degree. This compact squeezed light source can be potentially utilized in quantum protocols in which atomic ensembles are involved, such as in quantum memory, atomic magnetometers and quantum interferometers.

DOI
10.1364/oe.455071

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In this paper, we report on -3.5±0.2 dB vacuum squeezing (corresponding to -4.2±0.2 dB with loss correction) at 795 nm via the polarization self-rotation (PSR) effect in rubidium vapor by applying a magnetic field, whose direction is perpendicular to the propagation and polarization of the pump light. Compared with the case without the magnetic field, whose optimal squeezing degree is about -1.5 dB, this weak magnetic field can enhance the PSR effect and ultimately increase the squeezing degree. This compact squeezed light source can be potentially utilized in quantum protocols in which atomic ensembles are involved, such as in quantum memory, atomic magnetometers and quantum interferometers.
We reported a compact squeezed light source consisting of an diode laser near resonant on 87Rb optical D1 transition and an warm Rubidium vapor cell. The -4dB vacuum squeezing at 795 nm via nonlinear magneto-optical rotation was observed when applying the magnetic field orthogonal to the propagation direction of the light beam. This compact squeezed light source can be potentially utilized in the quantum information protocols such as quantum repeater and memory, and quantum metrology such as atomic magnetometer.
Nonclassical states of light are of increasing interest due to their applications in the emerging field of quantum information processing and communication. Squeezed light is such a state of the electromagnetic field in which the quantum noise properties are altered compared with those of coherent light. Squeezed light and squeezed vacuum states are potentially useful for quantum information protocols as well as optical measurements, where sensitivities can be limited by quantum noise. We experimentally study a source of squeezed vacuum resulting from the interaction of near-resonant light with both cold and hot Rb atoms via the nonlinear polarization self-rotation effect (PSR). We investigate the optimal conditions for noise reduction in the resulting squeezed states, reaching quadrature squeezing levels of up to 2.6 dB below shot noise, as well as observing noise reduction for a broad range of detection frequencies, from tens of kHz to several MHz. We use this source of squeezed vacuum at 795 nm to further study the noise properties of these states and how they are affected by resonant atomic interactions. This includes the use of a squeezed light probe to give a quantum enhancement to an optical magnetometer, as well as studying the propagation of squeezed vacuum in an atomic medium under conditions of electromagnetically induced transparency (EIT). We also investigate the propagation of pulses of quantum squeezed light through a dispersive atomic medium, where we examine the possibilities for quantum noise signals traveling at subluminal and superluminal velocities. The interaction of squeezed light with resonant atomic vapors finds various potential applications in both quantum measurements and continuous variable quantum memories.
We report on an optical magnetometer enhanced by vacuum-squeezed light, employing an Mx magnetometer based on 87^{87}87Rb vapor in a micrometer-scale cell (~100 μμμm). Using the well-established polarization self-rotation effect in a room-temperature 87^{87}87Rb vapor cell, we achieve -3 dB of vacuum squeezing within the noise spectral window of 100 Hz to several MHz, corresponding to 3.5 dB squeezing when accounting for optical losses. Leveraging this level of squeezing, we demonstrate a magnetic field sensitivity of approx. 1 pT///Hz\sqrt{Hz}Hz​. The combination of vacuum-squeezed light and micrometer-scale vapor cells paves the way for compact, low-power-consumption atomic sensors with enhanced performance.
Abstract With the help of Stokes operator Ŝ 2 squeezed state (also called polarization squeezed state (PSS)) of 795 nm light, rubidium-87 ( 87 Rb) atomic magnetometer based on Faraday rotation (FR) has been implemented and characterized. The PSS of Stokes operator Ŝ 2 of 795 nm light has been prepared by means of coherently combining the polarization coherent state (PCS) of a linearly p-polarized bright 795 nm light beam and a linearly s-polarized squeezed vacuum state (SVS) generated by a 397.5 nm ultraviolet laser pumped sub-threshold optical parametric oscillator (OPO) with a PPKTP bulk crystal inside the OPO cavity. PSS with a squeezing level of −3.7 ± 0.3 dB has been achieved around the analysis frequency of 10 kHz. At different transitions of D 1 line, various frequency detuning, and reasonable atomic vapor cell’s temperature, FR has been measured and compared. To decrease absorption (scattering) losses and the back-action from atomic spin noise to the probe beam’s polarization noise for maintaining the quantum properties of PSS of Stokes operator Ŝ 2 of 795 nm light, we had to run our magnetometer with 87 Rb vapor cell’s temperature below 60 °C, at which the PSS was almost destroyed. The sensitivities of magnetic field measurement were characterized via measuring signal-to-noise ratio of the alternating current calibrated magnetic field signal with a balanced polarimeter. Under the conditions of the atomic number density of ∼5.8 × 10 10 cm −3 ( T ∼ 40 °C) and the probe beam with a detuning of ∼−400 MHz relative to the 5S 1/2 ( F g = 2) − 5P 1/2 ( F e = 1) transition of 87 Rb D 1 line, a typical sensitivity of ∼19.5 pT (Hz 1/2 ) −1 has been achieved employing PSS of Stokes operator Ŝ 2 as the probe, compared with a sensitivity of ∼28.3pT (Hz 1/2 ) −1 using PCS as the probe. We preliminarily demonstrated that the quantum-enhanced sensitivity in a Faraday-rotation-based 87 Rb atomic magnetometer with the help of PSS of 795 nm light.
High precision optical detection is fundamentally limited by quantum noise. This limit can be bypassed with the use of squeezed states of light with modified quantum noise. We study squeezed states of light with a focus on optimization of squeezing generated via polarization self-rotation (PSR) in hot rubidium vapor. The goal of our research is to reduce quantum noise by optimizing cell temperature and beam shape of the input pump field. We find that computerized spatial optimization algorithms (combined with manual optimization of temperature and laser detuning) are successful in improving squeezing levels, with one spatial mask yielding over 1.0 dB of squeezing improvement under certain conditions. We have achieved quantum noise suppression of 2.3 ± 0.1 dB below shot noise.
We study a squeezed vacuum field generated in hot Rb vapor via the polarization self-rotation effect. Our previous experiments showed that the amount of observed squeezing may be limited by the contamination of the squeezed vacuum output with higher-order spatial modes, also generated inside the cell. Here, we demonstrate that the squeezing can be improved by making the light interact several times with a less dense atomic ensemble. With optimization of some parameters we can achieve up to $\ensuremath{-}2.6$ dB of squeezing in the multipass case, which is a 0.6 dB improvement compared to the single-pass experimental configuration. Our results show that, other than the optical depth of the medium, the spatial mode structure and the cell configuration also affect the squeezing level.