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Exploring Gravity with Ultracold Cadmium and Strontium Optical Clocks and Bragg Interferometers

Periodic Reporting for period 2 - TICTOCGRAV (Exploring Gravity with Ultracold Cadmium and Strontium Optical Clocks and Bragg Interferometers)

Reporting period: 2019-12-01 to 2021-05-31

The main aim of TICTOCGRAV is to develop novel inertial sensors and clocks based on alkali-earth and alkali-earth-like atoms with the eventual aim of being implemented in definitive tests of fundamental physics, focusing at the interplay between quantum mechanics (QM) and general relativity (GR).
While both GR and QM have been tested to an unprecedented level, after 100 years there still remain open fundamental theoretical questions and all attempts so far to unify the two theories have not been successful. GR is therefore still a "classical theory" and there is no clear understanding on how to construct a full quantum space-time structure. Nevertheless, it is not expected that gravity will not play a role in quantum systems, nor is it expected that quantum effects will vanish completely at the large scales where GR effects dominate. But at which energy/length scale does the classical to quantum transition lie exactly? Is it possible at the fundamental level to discover a clear mechanism of decoherence? Is gravitation itself a source of decoherence in macroscopic objects? All these open questions directly arise from our limited knowledge of gravitational interaction and the limited number of experimental tests performed with quantum objects.

A possible way to search for potential extensions to present gravitational theory is then to either look at the modification of gravity with large-scale quantum objects or to observe tiny gravitational effects on microscopic quantum objects. The groundbreaking nature of TICTOCGRAV is to address this issue with a low-energy table-top experiment, capable of performing simultaneous measurements of time and gravity with the highest precision. The experimental setup will be a novel dual-species atom interferometer based on optical clock transitions of ultra-cold cadmium (Cd) and strontium (Sr) atoms.

The two main targets of TICTOCGRAV are:
- Test of the Weak Equivalence Principle (WEP) and spin-gravity coupling
- First observation of quantum interference of clocks and test of gravity-induced decoherence effect due to time dilation
In the first 30 months of activity we have been developing the theoretical and experimental tools towards high-precision measurement of gravity and gravity gradients via atom interferometry of Sr and Cd atoms. The concepts and tools developed relate to both trapped and freely-falling configurations.
A complete characterization of a novel gravimeter and a gradiometer based on the optical clock transition of strontium has given precise directions for the design of an upgraded version of a cold strontium atom interferometer. At the same time, new concepts to enhance the precision of gravity and gravity gradients measurements in atom interferometers have been studied.
In parallel, new designs for atomic packages and high-power UV lasers have been explored to efficiently trap and cool cadmium atoms at ultra-cold temperatures. By using these approaches, preliminary measurements of cadmium optical transitions properties have been conducted.
Matter-wave atom interferometers are currently well-established tools for extremely sensitive and accurate measurements of gravity and gravity gradients and inertial sensing more generally. Nevertheless, there is plenty of possibility for advancing atom interferometry into new regimes and scenarios. One new technique is to utilise ultra-narrow optical clock transitions to impart momentum to the matter wave, in contrast to multi-photon transitions typically used previously. This approach is especially favoured by proposed and forthcoming Sr interferometers designed to detect gravitational waves and to search for dark matter. The project has already demonstrated and analysed the first gravimeter based upon such a single-photon clock-transition atom interferometer. Part of the expected results from TICTOCGRAV therefore derives from applying these well-known and emerging techniques to novel situations, for example the planned Cd-Sr dual atom interferometer which is expected to yield new results about Einstein's Equivalence Principle and potentially the nature of quantum decoherence itself.
The other main avenue for advancement beyond the current state of the art, comes from refining and improving atom interferometry as applied to specific atoms. This is especially true of Cd for which atom interferometers have yet to be demonstrated, meaning the expected demonstration of matter-wave interference with Cd will likely be a first. Conversely, although atom interferometers with Sr have previously been demonstrated, they are not widespread and have mainly been demonstrated on a relatively small-scale. We expect to operate a metre-scale fountain-based interferometer and also trapped configurations with Sr, which will advance the currently achievable sensitivities and bring this technology in line with other interferometers utilising e.g. rubidium.