The first main result of EQUEMI was the demonstration of a laser machining method allowing to produce spherical and free-form optical micromirrors and lenses on the tip of an optical fiber [K. Ott et al., Opt. Express 24, 261274 (2016)]. We have then used this method to produce millimeter-long FFP cavities for generating spin squeezing - a form of entanglement that is expected to improve the stability of atomic clocks and sensors - in an ensemble of more than 10000 trapped rubidium atoms. This allowed us to generate spin-squeezed states in a metrology-grade device, a trapped-atom clock on a chip that we have developed in collaboration with SYRTE, the French national time and frequency metrology laboratory. Due to the extremely well-controlled, low-noise conditions in this clock, we could observe the time evolution of this entangled quantum state on previously inaccessible timescales up to 1s, more than two orders of magnitude longer than previous experiments. We have also observed an unexpected measurement amplification mechanism, caused by spin exchange interaction. This effect remained undetected on the much shorter timescales of earlier experiments, but plays an important role on the timescale of practically relevant atomic clocks [M. Huang et al., arXiv:2007.01964]. These results open up perspectives for squeezing-enhanced atomic clocks.
Another main result is the construction of a new, very compact and simplified experiment producing cold trapped strontium atoms [M. Bertrand et al, to be published, also see our group website referenced below]. This new design has attracted substantial attention in its research field, and has already been adopted by several other research groups. In parallel with this experiment, we have also developed a new type of microcavity that is designed to perform well with this class of atoms. This cavity is a triangular ring resonator with a total length of 600 micrometers and a laser-machined focusing mirror. In contrast to Fabry-Perot cavities, this is a running-wave cavity, which has no "blind spots" with vanishing atom-field coupling. This will be a major advantage for our envisioned experiments where single atoms trapped in a tweezers array will be entangled using a resonant mode of this cavity. Finally, to enable this vision, we have also developed a miniature, monolithic device combining FFP microcavity and a high-resolution objective achieving single-site resolution in these tweezers [F, Ferri et al, Rev. Sci. Instr. 91, 033104 (2020)].
