Periodic Reporting for period 4 - TICTOCGRAV (Exploring Gravity with Ultracold Cadmium and Strontium Optical Clocks and Bragg Interferometers)
Reporting period: 2022-12-01 to 2024-05-31
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 on 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 remain open fundamental theoretical questions and all attempts 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
While both GR and QM have been tested to an unprecedented level, after 100 years there remain open fundamental theoretical questions and all attempts 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
The work done during the project has regarded the study and development of new methods and technology toward novel atom interferometers based on ultracold cadmium and strontium atoms, with direct application to fundamental physics tests, such as the test of the equivalence principle with quantum objects, and towards the observation of quantum time dilation.
The work regarded not only pure experimental activities but also detailed theoretical studies. All the results obtained within this specific action and connected to it, have been reported in scientific articles published in peer-reviewed journals and presented as posters or oral presentations at international conferences.
The first realization and detailed analysis of novel, single-photon clock atom interferometers as a precise gravimeter and gradiometer was done. Seminal novel concepts for high-power laser sources operating at the wavelength of interest in the infrared, visible, and UV were introduced.
At the same time, the study of the potential use of non-classical sources (spin-squeezed) in atom interferometry has also been pursued, novel scheme to produce and use squeezed momentum states to surpass the projection noise limit in Bragg interferometers.
Interestingly, fruitful collaborations with other ERC projects, sharing a common technology platform, have also been established. During the action, 14 scientific works have been published in peer-reviewed journals and conference proceedings with high impact. Other publications, directly connected to the action, are also expected in the next months. Moreover, more than 15 contributions to conferences have been presented as oral or poster presentations at international conferences.
The work regarded not only pure experimental activities but also detailed theoretical studies. All the results obtained within this specific action and connected to it, have been reported in scientific articles published in peer-reviewed journals and presented as posters or oral presentations at international conferences.
The first realization and detailed analysis of novel, single-photon clock atom interferometers as a precise gravimeter and gradiometer was done. Seminal novel concepts for high-power laser sources operating at the wavelength of interest in the infrared, visible, and UV were introduced.
At the same time, the study of the potential use of non-classical sources (spin-squeezed) in atom interferometry has also been pursued, novel scheme to produce and use squeezed momentum states to surpass the projection noise limit in Bragg interferometers.
Interestingly, fruitful collaborations with other ERC projects, sharing a common technology platform, have also been established. During the action, 14 scientific works have been published in peer-reviewed journals and conference proceedings with high impact. Other publications, directly connected to the action, are also expected in the next months. Moreover, more than 15 contributions to conferences have been presented as oral or poster presentations at international conferences.
Matter-wave atom interferometers are well-established tools for extremely sensitive and accurate measurements of gravity, gravity gradients, and inertial sensing. Nevertheless, there is plenty of possibility for advancing atom interferometry into new regimes and scenarios. One new technique is to utilize ultra-narrow optical clock transitions to impart momentum to the matter wave, in contrast to previous multi-photon transitions. This approach is especially favored by proposed and forthcoming Sr interferometers designed to detect gravitational waves and search for dark matter. The project has already demonstrated and analyzed 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.
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.