Cavity enhanced atom interferometry

Nourshargh, Rustin ORCID: 0000-0001-7076-5082 (2021). Cavity enhanced atom interferometry. University of Birmingham. Ph.D.

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Abstract

Atom Interferometry is a technique offering unparalleled sensitivity to a wide range of applications. Their sensitivity is currently limited by the available laser power and flatness of the optical wavefronts. We explore two solutions to these problems, high power high bandwidth operation and optical cavity enhancement.

We have demonstrated a laboratory based, high bandwidth atom interferometer instrument and have performed an incipient gravity measurement with a fractional statistical uncertainty of \(\sigma_g / g = 4.4\times 10^{-6}\) where g is the acceleration due to gravity. We have designed, constructed, and optimised a powerful Raman laser (12 W at 780nm) which will allow large momentum transfer beamsplitters to be implemented at high bandwidth for the first time.

Cavity enhancement offers the ability to dramatically increase the laser power available for experiments, as well as filtering the spatial modes, thus improving the wavefront flatness. Difficulties in simultaneously realising large modes and spatial mode filtering, and accommodating Doppler shifts have limited the use of cavity enhancement thus far. We have designed and demonstrated a Doppler compensated optical cavity for atom interferometer enhancement. This cavity circumvents the Doppler shift limit whilst enabling mode filtering, power enhancement, and a large beam diameter simultaneously. Our novel design combines a magnified linear cavity with an intracavity Pockels cell. The Pockels cell introduces a voltage tunable birefringence, which is used to match the cavity resonances to the laser frequencies as they chirp to track Doppler shifts in the interferometer. The magnified linear geometry produces a large, 5.04mm 1/e\(^2\), diameter beam waist in a 0.69m long cavity and allows the Gouy phase to be tuned, suppressing higher order spatial modes and avoiding regions of instability. These improvements address central limitations of current cavity enhanced systems.

We propose refinements to this design, further improving performance, and allowing cavity enhancement of high bandwidth systems.

The remaining limitation on cavity systems is the lifetime limit. Long, high finesse cavities have long photon lifetimes, causing severe distortion of short pulses. In proposed large scale atom interferometer based gravitational wave detectors, this appears to preclude the use of cavity enhancement. We propose a scheme for optical cavity enhanced atom interferometry, using circulating, spatially resolved pulses, and intracavity frequency modulation to overcome the cavity lifetime limit. We present parameters for the experimental realisation of the target \(10^4 \hbar k\) momentum separation in a 1km interferometer using the 698nm clock transition in \(^{87}\)Sr, and describe potential performance enhancements in 10m scale devices operating on the 689nm intercombination line in \(^{87}\)Sr. Our scheme satisfies the most challenging requirements of these sensors and paves the way for the next generation of high sensitivity, large momentum transfer atom interferometers.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):