Low phase noise microwave source for high fidelity quantum control of cesium (Cs) atoms

Atom interferometers are extremely accurate quantum sensors that perform highly precise measurements of fundamental physics and mobile inertial sensing. However, they are particularly sensitive to phase noise of the lasers that drive the atomic beamsplitter transitions. Preserving quantum coherence requires microwave sources with low phase noise in the Hz–kHz band and good long-term stability. We describe a microwave system for quantum control of Cs atoms that blends a variety of commercial parts. We use a dielectric resonator oscillator (DRO) to provide a tunable microwave source at the Cs hyperfine frequency of 9.2 GHz. We achieve improved long-term stability by referencing the DRO to a 10 MHz Wenzel crystal (-143dBc/Hz phase noise at 10 Hz) multiplied to 4.6 GHz through a combination of a 100 MHz phase-locked oscillator and a nonlinear transmission line. This stable reference signal is mixed down with a portion of the DRO signal that had its frequency divided by two (near 4.59 GHz) to create an error signal. The error signal feeds into a servo loop that we use to stabilize the DRO phase at sub-MHz scale and over long term. A direct digital synthesizer (DDS) is incorporated into the error signal and provides tunability of the DRO frequency by as much as 4 MHz, as needed to match the atom phase shift during free fall (gravity ramp). A portion of the DRO signal drives an eletro-optical modulator for performing two-photon Raman beamsplitter transitions in the quantum state of the Cs atoms. This microwave scheme achieves state-of-the-art phase noise in the relevant 10 Hz–100 kHz range, with reliable performance and long-term stability.