Squeezed-Light Optical Magnetometry

We demonstrate a light-shot-noise-limited magnetometer based on the Faraday effect in a hot unpolarized ensemble of rubidium atoms. By using off-resonant, polarization-squeezed probe light, we improve the sensitivity of the magnetometer by 3.2 dB. The technique could improve the sensitivity of the most advanced magnetometers and quantum nondemolition measurements of atomic spin ensembles.

Figure 1

Experimental apparatus. Rb cell, rubidium vapor cell with magnetic coil and magnetic shielding; OPO, optical parametric oscillator; PPKTP, phase-matched nonlinear crystal; LO, local oscillator beam; PBS, polarizing beam splitter; HWP, half-wave plate; SMF, single-mode fiber; PD, photodiode.

Figure 2

Polarization squeezing after the atomic vapor cell. Polarization noise power as the phase of the local oscillator is scanned. Center frequency 1 MHz, zero-span mode, $RBW=30\mathrm{kHz}$, $VBW=30\mathrm{Hz}$. Horizontal trace shows noise with a polarized (but not squeezed) probe, i.e., with OPO off, and is taken as the reference 0 dB. Oscillating trace shows noise with OPO on, including regions below the shot-noise level.

Figure 3

Faraday rotation measurement. Power of the polarization signal as center frequency is scanned, $RBW=3\mathrm{kHz}$, $VBW=30\mathrm{Hz}$. The (upper) black curve shows the applied magnetic signal at 120 kHz above the shot-noise background of a polarized (but not squeezed) probe. The (lower) green line depicts the same signal with polarization-squeezing. A zoomed view around the calibration peak at 120 kHz is shown in the inset.