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Entanglement and Quantum Engineering with optical Microcavities

Periodic Reporting for period 4 - EQUEMI (Entanglement and Quantum Engineering with optical Microcavities)

Reporting period: 2020-04-01 to 2021-09-30

An exciting and fast-growing research field has emerged at the interface of fundamental physics and technology,
where the nonclassical features of quantum mechanics are employed to engineer powerful, radically new technologies. Among them are quantum simulations and quantum metrology. This project leverages the unique properties of optical fiber Fabry-Perot (FFP) microcavities pioneered by the PI's group to advance these fields. One objective is to take quantum-enhanced measurement from its current proof-of-principle state to a true metrological level by applying cavity-based spin squeezing in a compact atomic clock, aiming to improve the clock stability beyond one part in 10^-13 in one second. A second objective is to create and manipulate entangled states of many atomic qubits using the light field in a high-finesse microcavity, with applications to both quantum metrology and quantum simulation. Ultracold alkaline-earth atoms such as strontium have particularly high potential in this context - these are the atoms used in today’s most precise atomic clocks. However, cooling these atoms currently requires far more complex setups than for the well-established alkaline atoms. Therefore, the project aims to simplify the production of ultracold strontium in a new experiment, and to develop a new type of microcavity that is suitable for entanglement generation using these atoms. A miniature quantum gas microscope working inside a microcavity will be also be developed, creating a rich new situation at the interface of quantum information, metrology, and cutting-edge quantum gas research. Finally, the project further improves FFP microcavity technology, which will open new horizons for light-matter interfaces in quantum technologies. The new microcavities developed here are also expected to generate spin-offs in fields outside fundamental research, such as optical metrology and trace gas sensing.
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)].
All the results described above go beyond the state of the art in their respective domains. The novel CO2 laser dot machining technique allows fabrication of a wide variety of user-defined shapes with ultralow roughness in fused silica, where earlier single-shot methods were limited to gaussian-shaped depressions. The new design of the strontium experiment provides not also size reduction, but also greater simplicity and ease of use, making research with laser-cooled alkaline-earth atoms significantly more accessible than before. The micro-ring cavity achieves a cooperativity (the figure of merit in the field of atom-light coupling in the quantum regime) that was previously achievable only in Fabry-Perot cavities, while removing the limitation associated with the intracavity standing wave. This technical progress will enable a new class of quantum simulation experiments with atoms trapped in and coupled by this cavity. The first such experiment is now under way, enabled by the progress made in this project.
Optical fiber microcavities on an atom chip. Developed and built as part of the EQUEMI project.