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

TL;DR

This paper presents a simple, low-cost microwave source with ultra-low phase noise at 6.8 GHz, enabling precise control for atomic spin squeezing experiments with 87Rb atoms, achieving noise levels below quantum projection noise.

Contribution

The paper introduces a novel low phase noise microwave source using direct multiplication and filtering, optimized for atomic physics experiments involving 87Rb.

Findings

Achieves phase noise near -140 dBc/Hz at 10 kHz to 1 MHz offset.

Contributes spin-noise variance 16 dB below quantum projection noise.

Demonstrates suitability for quantum nondemolition measurements in atomic ensembles.

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 87Rb. Low phase noise is achieved by directly multiplying a low phase noise 100 MHz crystal to 6.8 GHz using a non-linear 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 to 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 7x10^5 87Rb atoms.