Parametric Amplification and Noise Squeezing in Room Temperature Atomic Vapors

Abstract

We report on the use of parametric excitation to coherently manipulate the collective spin state of an atomic vapour at room temperature. Signatures of the parametric excitation are detected in the ground-state spin evolution. These include the excitation spectrum of the atomic coherences, which contains resonances at frequencies characteristic of the parametric process. The amplitudes of the signal quadratures show amplification and attenuation, and their noise distribution is characterized by a strong asymmetry, similarly to those observed in mechanical oscillators. The parametric excitation is produced by periodic modulation of the pumping beam, exploiting a Bell-Bloom-like technique widely used in atomic magnetometry. Notably, we find that the noise-squeezing obtained by this technique enhances the signal-to-noise ratio of the measurements up to a factor of 10, and improves the performance of a Bell-Bloom magnetometer by a factor of 3.

Spin noise spectroscopy of a noise-squeezed atomic state

“…This system undergoes indirect pumping, via optical excitation and spin-exchange collisions (SEC), and thus a nonequilibrium ensemble polarization is created. In addition, it can be parametrically excited by proper modulation of the optical pump amplitude, as we have previously demonstrated in [14]. Standard SNS is often a clever diagnostic technique alternative to a pump/probe configuration, however we demonstrate that, in the presence of an active pumping mechanism, SNS can also provide useful, complementary information on the system dynamics.…”

“…Standard SNS is often a clever diagnostic technique alternative to a pump/probe configuration, however we demonstrate that, in the presence of an active pumping mechanism, SNS can also provide useful, complementary information on the system dynamics. In particular, it can assist in identifying the mechanism leading to the formation of noise squeezing, which we previously observed in the transient dynamics of conjugated spin variables following the parametric excitation [14]. As such, on one side our work extends the scope of SNS beyond its current boundaries, which has long been appealing from both a theoretical and experimental point of view [15,16].…”

“…The cell is surrounded by five layers of 2 mm thick mu metal shields to reduce static magnetic background. A small static magnetic field is created by a Hemholtz pair of coils (along the z-axis, see [14] for further experimental details). The optical pumping is performed by a circularly polarised laser beam frequency locked to the caesium 6 2 S 1/2 F=3→ 6 2 P 3/2 F'=2 transition (propagating along the x-axis).…”

“…for modulation frequencies of the pump beam amplitude which are equal to 2ω L /n, being n an integer number. In the model we discussed in [14], the parametric term enters the individual atoms' spin evolution via a sufficiently strong pump, and its macroscopic effect relies on the synchronised atomic response [18]. Also, we recall that in the case of modulation frequency ω M = ω L /n, the spectrum of the evolving ground state coherences contains mainly two components: a steady state oscillation at the modulation frequency ω M , and a transient oscillating at ω L damped on a timescale associated with the rate of spin-exchange collisions.…”

Spin dynamic response to a time dependent field

“…Optical magnetometers are developed for a variety of applications [13] including fundamental research [14][15][16], characterization of magnetic anomalies and of their dynamics in space-physics [17,18], geology [19], archaeology [20,21], material science e.g. to detect diluted magnetic nano-and micro-particles [22] or induced eddy currents [23][24][25][26] (with potential in medical applications, in detection of biomagnetism [27] or in building apparatuses for nuclear magnetic resonance (for spectroscopy or imaging) in the ultra-low-field [28][29][30][31] and zero-to-ultra-low field regimes [32,33].…”