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arXiv:1204.4215 · Review of Scientific Instruments

A low phase noise microwave source for atomic spin squeezing experiments in 87Rb

Zilong Chen, Justin G. Bohnet, Joshua M. Weiner, James K. Thompson

Published Apr 1, 2012• 23 citations
arXivHTMLPDF

Abstract

We describe and characterize a simple, low cost, low phase noise microwave source that operates near 6.800 GHz for agile, coherent manipulation of ensembles of (87)Rb. Low phase noise is achieved by directly multiplying a low phase noise 100 MHz crystal to 6.8 GHz using a nonlinear transmission line and filtering the output with custom band-pass filters. The fixed frequency signal is single sideband modulated with a direct digital synthesis frequency source to provide the desired phase, amplitude, and frequency control. Before modulation, the source has a single sideband phase noise near -140 dBc/Hz in the range of 10 kHz-1 MHz offset from the carrier frequency and -130 dBc/Hz after modulation. The resulting source is estimated to contribute added spin-noise variance 16 dB below the quantum projection noise level during quantum nondemolition measurements of the clock transition in an ensemble 7 × 10(5) (87)Rb atoms.

DOI
10.1063/1.3700247

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We describe and characterize a simple, low cost, low phase noise microwave source that operates near 6.800 GHz for agile, coherent manipulation of ensembles of (87)Rb. Low phase noise is achieved by directly multiplying a low phase noise 100 MHz crystal to 6.8 GHz using a nonlinear transmission line and filtering the output with custom band-pass filters. The fixed frequency signal is single sideband modulated with a direct digital synthesis frequency source to provide the desired phase, amplitude, and frequency control. Before modulation, the source has a single sideband phase noise near -140 dBc/Hz in the range of 10 kHz-1 MHz offset from the carrier frequency and -130 dBc/Hz after modulation. The resulting source is estimated to contribute added spin-noise variance 16 dB below the quantum projection noise level during quantum nondemolition measurements of the clock transition in an ensemble 7 × 10(5) (87)Rb atoms.
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We present a radio frequency source based on the AD9914 direct digital synthesizer chip, with a frequency range up to 1.4 GHz. The phase noise at an output frequency of 1 GHz is lower than −110 dBc/Hz at a 10 kHz offset. Through an optimized design of a field-programmable gate array, we have achieved the theoretical maximum modulation rate of AD9914 and developed multiple functional modules for cold atom experiments to implement modulation of frequency, phase, and amplitude. The modulation update rate reaches 146 MHz. The phase modulation response latency is 158 ns, and the amplitude modulation response latency is 95.1 ns. We have applied the RF source to a sideband Pound–Drever–Hall locking experiment and achieved frequency stabilization, demonstrating its suitability for cold atom experiments that require precise and complex laser control.
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A compact and robust laser system is essential for mobile atom interferometers. Phase modulation can provide the two necessary phase-coherent frequencies without sophisticated phase-locking between two different lasers. However, the additional laser frequencies generated can perturb the atom interferometer. In this article, we report on a novel method to produce a single high-power laser beam composed of two phase-coherent sidebands without the perturbing carrier mode. Light from a diode laser is phase-modulated by using a fiber-coupled electro-optic modulator driven at 3.4 GHz and passes through a Fabry-Perot cavity with a 6.8 GHz free spectral range. The cavity filters the carrier mode to leave the two first-order sidebands for the two-photon Raman transition between the two hyperfine ground states of 87 Rb. The laser beam is then fed to a single tapered amplifier, and the two sidebands are both amplified without mode competition. The phase noise is lower than that of a state-of-the-art optically phase-locked external-cavity diode laser (−135 dBrad 2 /Hz at 10 kHz) at frequencies above 10 Hz. This technique can be used in all-fiber-based laser systems for future mobile atom interferometers.
We demonstrate a possibility to control the noise spectrum of squeezed vacuum by passing it through Rb vapor under EIT conditions. For this experiment, we generate low sideband frequency (1kHz-2MHz) squeezed vacuum using polarization self-rotation. As an example, we compare the quadrature noise levels to theoretical predictions and observe ‘EIT filtering’ of the noise due to the finite width of the EIT resonance. This work may lead to the development of a low-cost compact squeezing source for applications in gravitational wave interferometers and continuous-variable quantum memory systems.