Andrew Kortyna, Judith Olson, Gabriel Ycas, Jim Duncan, Evan Salim, Hugo Leon, ColdQuanta

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Microwave atomic clocks are one of the near-term potential solutions to produce commercially available GPS-like timing and holdover services. Performance benchmarks for these systems of interest generally correspond to fractional frequency instabilities near or below a part in 1e-11 at one second and average to or below a part in 1e-12. Performance at or below 1e-12 then must be held for many weeks or months for many holdover interested applications. Additionally, accuracy at the 1e-12 level or NIST-traceable frequency measurements with highly reliable retrace at or below the 1e-12 level are needed to ensure useable performance in the field. In addition to these performance constraints, for these application-specific clocks to be effective, they must be lower in size, weight, power, and cost (SWaP-C) than existing available solutions while also operating successfully in a wider range of deployment scenarios. Performance stability and accuracy must be maintained in environments experiencing potentially wide fluctuations in temperature and pressure. Vibration and shock must also not limit the clock performance. Further benefits come from rapid system startup for unexpected demand and reduced power draw while in standby mode. Ultimately, while system volumes below 30 L would improve over state-of-the-art, pathways to volume below a liter and approaching cm-scale dimensions are of particular interest. Existing approaches to satisfy the needs of GPS-like backup or holdover systems are generally based on either power-hungry and large thermal atomic beams or heated vapor of gases, both of which limit field deployability. In these cases, environmentally coupled drift mechanisms, particularly thermal couplings, either severely limit the fieldable operations or require large, extensively tuned active thermal compensation schemes. One simple way to decouple the clock atoms from their thermal environment is through vacuum barriers or use of cold atoms. In the case of the work presented here, a cold atom approach was selected. Many of the successful spectroscopic techniques currently used in novel commercial systems are applicable to cold atom ensembles as well, which are generally much better shielded from environmental impacts through vacuum isolation. Additionally, the cold atom ensemble’s thermal properties are entirely determined by laser operation rather than a heated atom source, further decoupling the clock from environmental perturbations. Furthermore, cold atom ensembles result in far more uniform and coherent interactions with probe lasers, enabling enhanced signal-to-noise over their thermal ensemble-based clock counterparts. Lastly, interrogation of the microwave clock transitions with lasers instead of microwave cavities enables significant volume reduction in the physics package, some of which has already been realized in commercial atomic clock systems. The goal of the work presented here is to miniaturize a cold atom microwave clock ensemble with rapid startup, while basing the physics package on manufacturable designs for future scalability. This clock relies on a magneto-optical trap (MOT) to cool the ensemble to micro-Kelvin levels and localize the atoms within the designated interrogation region. To this end, we have miniaturized all the components required for a cold-atom, coherent population trapping (CPT) frequency reference and assembled them into a single package. This includes fabrication and miniaturization of multiple critical technology elements including an ultra-high vacuum cell, ion pump, atom source, in-vacuum optical delivery mechanisms, free-space laser optical delivery paths, clock laser module, and a single integrated control and signal conditioning system. The entire system is packaged into a 25-liter rack mountable chassis, and contains all components needed for independent, autonomously operated cold-atom CPT frequency standard operation from cold start. Further reductions in the system SWaP are extremely feasible. The device is run in a drip mode that is tolerant to orientation and acceleration. Electromagnetically induced transparency is shown to have contrasts approaching unity, corresponding to strong Ramsey fringes with fringe widths on the order of tens of Hertz for Ramsey times of 10’s of milliseconds. Data collected on both a benchtop version and in the miniaturized version show stability measurements comparable to commercially available timing standards, but with reduced system size and weight. Power draw reduction was not a main tenant of this effort, but will be addressed in future development efforts. Frequency stability better than 10-11 were observed at one second, and averaging as one over the square root of the averaging time are achieved for specific operating conditions. The miniaturized vacuum cell has also performed well in a battery of environmental and vibration tests indicative of successful operation in field deployable scenarios with significant thermal and pressure fluctuations while withstanding shock and vibration at levels seen on aircraft and in terrestrial uses. Future plans include optimizing long-term performance, undergoing ruggedization tests, improving component manufacturability, and achieving space qualification. Further miniaturization efforts are also considered.