Squeezed-light spin noise spectroscopy

Vito Giovanni Lucivero, Ricardo Jiménez-Martínez, Jia Kong, and Morgan W. Mitchell

Phys. Rev. A 93, 053802 – Published 2 May 2016

Abstract

We report quantum enhancement of Faraday rotation spin noise spectroscopy by polarization squeezing of the probe beam. Using natural abundance Rb in 100 Torr of $N_{2}$ buffer gas and squeezed light from a subthreshold optical parametric oscillator stabilized 20 GHz to the blue of the $D_{1}$ resonance, we observe that an input squeezing of 3.0 dB improves the signal-to-noise ratio by 1.5 to 2.6 dB over the combined (power) $\otimes$ (number density) ranges (0.5–4.0 mW) $\otimes (1.5 \times 10^{12} {cm}^{- 3}$ to $1.3 \times 10^{13} {cm}^{- 3}$ ), covering the ranges used in optimized spin noise spectroscopy experiments. We also show that squeezing improves the tradeoff between statistical sensitivity and broadening effects, a previously unobserved quantum advantage.

Received 17 September 2015

DOI:https://doi.org/10.1103/PhysRevA.93.053802

Physics Subject Headings (PhySH)

Quantum optics

Spectroscopy

Atomic, Molecular & Optical

Authors & Affiliations

Vito Giovanni Lucivero^1,*, Ricardo Jiménez-Martínez¹, Jia Kong¹, and Morgan W. Mitchell^1,2

¹ICFO-Institut de Ciencies Fotoniques, Barcelona Institute of Science and Technology, 08860 Castelldefels (Barcelona), Spain
²ICREA-Institució Catalana de Recerca i Estudis Avançats, 08015 Barcelona, Spain

^*vito-giovanni.lucivero@icfo.es

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Issue

Vol. 93, Iss. 5 — May 2016

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Images

Figure 1
Squeezed-light spin noise spectroscopy. (a) Experimental schematic. LO - local oscillator, PBS - polarizing beam splitter, DPD - differential photodetector, FM - flip mirror, HWP - half-wave plate, WP - Wollaston prism, FFT - fast Fourier transform analyzer. (b) SNS spectra. Averaged spin noise spectra at $T = 90^{\circ}$ ( $n = 2.4 \times 10^{12} {cm}^{- 3}$ ) acquired with coherent probe, cyan (light gray), and polarization squeezed probe, red (dark gray), respectively. The spectra shown are averages of 10 spectra, each representing 0.5 s of acquisition organized into bins of width 10 Hz. 0 dB on the power scale corresponds to $- 95.57$ dBV/Hz at the detector. Dashed and continuous smooth curves show fits by Eq. (1) to the coherent and squeezed spectra, respectively. Optical power $P = 2.5 mW$ , magnetic field $B_{x} = 5.6 μ T$ .
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Figure 2
SNR Enhancement. SNR $η$ versus atomic density for coherent probes (empty markers) and polarization squeezed probes (filled markers). We show three optical powers of $P = 0.5 mW$ (squares), $P = 1.5 mW$ (triangles), and $P = 4 mW$ (circles). Predicted $η$ from Eq. (2) is plotted for coherent (dashed lines) and squeezed (continuous lines) probing, taking into account the reduction of squeezing versus density due to absorption at the probe frequency. The discrepancy between theoretical curves and experimental data is due to uncertainty on density estimation (see Appendix).
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Figure 3
Collisional-broadening reduction. (a) SNR $η$ versus optical power for coherent (empty black squares) and polarization squeezed (filled red circles) probing at $n = 0.9 \times 10^{13} {cm}^{- 3}$ and just for squeezed probe (filled blue circles) at lower density $n = 0.5 \times 10^{13} {cm}^{- 3}$ . Theoretical SNR from Eq. (2) is shown for coherent (dashed lines) and squeezed (continuous lines) probing. (b) FWHM linewidth $Δ ν_{85}$ vs probe power for the same conditions of (a). Theoretical FHWM widths from Eq. (3) are plotted for coherent (dashed line) and squeezed (continuous line) probing, respectively.
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Figure 4
Power-broadening reduction. FWHM linewidth $Δ ν_{85}$ vs Rb density for coherent (empty markers) and polarization squeezed (filled markers) probing at optical powers of $P = 4 mW$ (red circles) and $P = 2 mW$ (blue triangles). Theoretical FHWM widths from Eq. (3) are plotted.
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Figure 5
Shot-noise background, $S_{ph}$ , as a function of probe power reaching the detector for coherent probe (hollow symbols) and squeezed probe (filled symbols) at densities of $2.4 \times 10^{12} {cm}^{- 3}$ (blue) and $9.3 \times 10^{12} {cm}^{- 3}$ (red).
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Figure 6
$Var Θ_{F R}^{(85)}$ as a function of atomic density for coherent probe. The solid line shows a linear fit to the data. Dashed lines show the best fit $\pm 3 σ$ statistical uncertainty in the fit offset.
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