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Integrated Quantum Clock

Periodic Reporting for period 2 - iqClock (Integrated Quantum Clock)

Reporting period: 2020-04-01 to 2022-03-31

Optical clocks are amazingly stable frequency standards, which would be off by only one second over the age of the universe. This precision is orders of magnitude better than the one of commercial frequency standards, which operate at microwave frequencies, and is enabled by the interrogation of ultranarrow optical transitions in atoms that are cooled to near standstill. So far these clocks have been complex, fragile and maintenance intensive devices, built by researchers and only usable with their experience. Bringing those clocks from the laboratory into a robust, compact and easy to use form will have a large impact on telecommunication and navigation (e.g. network synchronization, increased traffic bandwidth, GPS spoofing and outage resilience, terrestrial navigation with cm precision), geology (e.g. underground exploration, monitoring of water tables, volcanoes or ice sheets), astronomy and space (e.g. low-frequency gravitational wave detection, radio telescope synchronization, deep space navigation) and other fields.

To make this a reality, we have founded the iqClock consortium, assembling leading experts from academia, strong industry partners, and relevant end users. We will seize on recent developments in clock concepts and technology to start-up a clock development pipeline. The objectives of iqClock are arranged in four scientific work packages (WPs) along the technology readiness level (TRL) scale and one outreach WP. WP3 is building an industrial prototype of a field-ready, compact and robust optical lattice clock. The University of Birmingham’s (UoB’s) Quantum Sensing Hub has guided our industry partners (Toptica (laser systems), Teledyne e2v (vacuum chamber and control electronics), NKT Photonics (multicolour optical fibres), Acktar (internal black coating of vacuum chamber for stray light suppression)) in the development and production of the components of this clock. The components have been tested at UoB and integrated into a clock, which will soon be benchmarked in a real use case, the synchronization of a network simulator built by British Telecom (BT) and integrating test equipment developed by Chronos.

Three smaller scientific WPs have laid the foundation to develop a new type of optical clock, which promises enhanced robustness, reduced complexity and potentially better performance than existing types. Existing optical clocks reference their frequency to atomic transitions by shining laser light onto atoms and detecting how strongly the atoms get excited. This scheme leads to complications that could be circumvented if the atoms would not be passively interrogated, but be enticed to actively emit light on the optical clock transition, forming a laser beam. Our goal is to build the first continuously operating laser of this type, a “superradiant” laser. In WP4 we have built the simplest version of such a device, operating on a kHz-linewidth transition. We have demonstrated pulsed, quasi-continuous emission and have reached the regime in which continuous superradiance should be possible. In WP5 and 6 we have developed two complementary experimental apparatuses that can push the technology to the ultimate level, continuous superradiant lasing on mHz-linewidth transitions. In WP6 we theoretically and experimentally explored the foundations of superradiant lasing. These WPs have been executed jointly by partners from the Universities in Copenhagen (UCPH), Torun (UMK), Amsterdam (UvA), Vienna (TUW) and Innsbruck (UIBK).
Toptica developed rack-mounted laser system, including frequency comb and ultrastable laser. NKT developed multi-color optical fibres. Acktar developed ultrablack coating suitable for clock vacuum chamber. Teledyne e2v build clock vacuum chamber and control system. Chronos and BT built clock test equipment. Optical lattice clock testbed assembled at UoB. Industry clock is being integrated and tested at UoB.

UCPH experimentally showed pulsed, quasi-continuous superradiant lasing on kHz-line. Numerical simulations and a new proposal [Phys. Rev. Lett. 125, 253602 (2020)] clarified beam characteristics needed for continuous superradiance, in particular that hot beam is sufficient. UCPH and UvA constructed new apparatus based on this approach. The operating parameters (beam flux, velocity, internal state preparation) are individually in regime in which continuous superradiance should be achievable if reached simultaneously for all parameters.

To realize the first continuous superradiant clocks operating on mHz-line, UvA and UMK have constructed two complementary experimental apparatuses. TUW guided machine design by theoretical exploration of operating conditions. The laser systems, electronics and vacuum chambers for both apparatuses have been finished and system integration is underway. These apparatuses will allow us to explore continuous mHz-transition superradiance during the second phase of the Quantum Flagship.

TUW and UIBK studied influence of real-life effects on superradiant lasing. Performed simulations ofour superradiant lasers. Open-source programming package for simulation of quantum optical systems made available online. Benchmarked theoretical model by reproducing pulsed Ca superradiance experiments. Developed efficient approach to treat laser with multilevel atoms and used to find optimal operation parameters. Novel implementations of superradiant lasing and alternative clock spectroscopy setups developed.
UvA demonstrated continuous high phase-space density strontium sources as needed for continuous mHz-line clocks. Allowed to continuously Bose-Einstein condense, creating the first atomic Bose-Einstein condensate in steady-state [arXiv:2012.07605 accepted by Nature].

WP2 Outreach
Highlights: iqClock YouTube video, research school on clocks.
Company partners developed components of integrated optical lattice clock, which have or can become products. Industry clock assembly and test in progress. This brings an optical lattice clock product closer to reality. Once cheap and reliable enough this clock can lead to applications outlined in Sec. 1.1.

Close to reaching continuous superradiance and reached quasi-continuous superradiance. A kHz-line superradiant laser is the foundation of a compact optical clock at the performance level of microwave hydrogen masers.

Designed and nearly finished building two complementary mHz-line superradiant clocks. A mHz-line superradiant laser can lead to more compact and robust optical clocks towards the end of the Quantum Flagship. Superradiant clocks can be more suitable for operation on moving platforms. Real-world applications may thereby come closer, e.g. in navigation.

Real-life effects limiting the best possible performance of superradiant clocks are theoretically better understood. New schemes for superradiant lasing have been proposed. We provide open-source tools to simulate quantum optical systems.
Continuous Sr sources enable new types of ultracold atom sensors. Continuous operation promises increased measurement bandwidth and can help to achieve higher accuracy.
iqClock lab
iqClock lab, Volkskrant
iqClock lab
Francesca Famà (left) and Camila Beli Silva (right) at work on kHz superradiant clock at UvA