Quantum Non-Demolition (QND) Measurements


Stroboscopic Backaction Evasion in a Dense Alkali-Metal Vapor

We explore experimentally quantum nondemolition measurements of atomic spin in a hot potassium vapor in the presence of spin-exchange relaxation. We demonstrate a new technique for backaction evasion by stroboscopic modulation of the probe light. With this technique we study spin noise as a function of polarization for atoms with spin greater than 1/2 and obtain good agreement with a simple theoretical model. We point that, in a system with fast spin exchange, where the spin-relaxation rate is changing with time, it is possible to improve the long-term sensitivity of atomic magnetometry by using quantum nondemolition measurements.

FIG. 1. (a) Experimental apparatus for a QND rf magnetometer. (b) A schematic of the stroboscopic Faraday rotation measurement showing a rotating squeezed spin uncertainty distribution of the vector F. (c) Measured PSD for unpolarized (dashed line) and highly polarized atoms (solid line). Both curves were taken with the same probe intensity and 10% duty cycle. The atomic density was 1014 cm-3.

Experimental Apparatus
Experimental Apparatus

Measurement of Spin Variance
FIG. 2. Measurement of spin variance for unpolarized and polarized (P~85%) atomic ensembles as a function of stroboscopic frequency. The shaded area of the noise dip can be compared with the theory.
Experimental measurement of Fx 
and Calculated Variance
Experimental measurement of Fx 
and Calculated Variance

FIG. 3. (a) Experimental measurement of Fx at high density (n ~ 6x1013 cm-3) following a short magnetic field pulse, showing changes in the transverse relaxation rate. For these data the probe beam scattering rate was increased. (b) Calculated variance in the estimate of the magnetic field relative to the SQL as a function of the optical density for various ratios of spin-exchange rate to spin-destruction rate. Dashed lines, single-pulse measurement; solid lines, two-pulse measurement with spin squeezing.


High Bandwidth Atomic Magnetometery with Continuous Quantum Nondemolition Measurements

We describe an experimental study of spin-projection noise in a high sensitivity alkali-metal magnetometer and demonstrate a fourfold improvement in the measurement bandwidth of the magnetometer using continuous quantum nondemolition measurements. Operating in the scalar mode with a measurement volume of 2 cm^3 we achieve magnetic field sensitivity of 22 fT/Hz^1/2 and a bandwidth of 1.9 kHz with a spin polarization of only 1%. Our experimental arrangement is naturally backaction evading and can be used to realize sub-fT sensitivity with a highly polarized spin-squeezed atomic vapor.

FIG. 1. Apparatus for high-bandwidth QND scalar magnetometry.

Experimentally Observed Magnetic Field Noise Spectrum   
Experimental Apparatus

FIG. 2. Experimentally observed magnetic field noise spectral density corrected for the frequency response of the magnetometer (solid line). The spikes are due to narrow-band magnetic noise. The dashed line shows expected magnetometer sensitivity for a demolition measurement assuming a flat noise spectrum. The shaded area represents improvement in the magnetometer sensitivity at high frequencies as a result of QND measurements.


Very Large Optical Rotation Generation in a Multi-Pass Cell

Paramagnetic Faraday rotation is a powerful technique for atom sensing widely used in quantum non-demolition measurements, fundamental symmetry tests, and other precision measurements. We demonstrate the use of a multi-pass optical cell for Faraday rotation spectroscopy and observe polarization rotation in excess of 100 radians from spin-polarized Rb vapor. Unlike optical cavities, multi-pass cells have a deterministic number of light passes and can be used to measure large optical rotations. We also observe a 10-fold suppression of transverse spin relaxation when Rb atoms are placed in a coherent superposition state immune to spin-exchange collisions.

Experimental Apparatus
FIG. 1. Schematic of the experimental apparatus and the multi-pass cell, the inset shows a photo of the beam spots on the entrance mirror.
Optical Rotation
FIG. 2. "Unwrapped" optical rotation signal for large initial polarization (a) and small initial polarization (inset b). The instantaneous transverse spin relaxation rate is shown in (d), determined from decay of a few periods as shown in inset (c).


Relevant papers



QND Experimental Apparatus (First Generation, 2010)

Multi-Pass Rb Cell with Internal Mirrors (Second Generation, 2011)