CooLQuanD:Cooling of Levitated Quantum-Spin-Hosting Diamonds

The world we live in appears to be governed by the rules of classical physics. At the atomic scale, however, it is actually quantum mechanics that describes the laws of nature. Research on quantum mechanical systems has provided us with remarkable findings that resulted in new technological advances that have proven to be useful in our macroscopic world, including magnetic resonance imaging, laser technology and an accurate global positioning system. Nevertheless, how nature behaves at the intersect between the atomic and the macroscopic world remains largely unanswered. A way to answer this question is provided by studies of microscopic objects that are actively brought into the quantum domain. Here, various experimental platforms are developed by researchers across the globe. A promising system is given by levitated microscopic particles in vacuum, an optomechanical system that enables previously unattainable isolation from the environment and precise control of the mechanical motion down to the single quantum level. In our project CooLQuanD: Cooling of Levitated Quantum-Spin-Hosting Diamonds, we will further enrich this optomechanical system, by combining it with intrinsic quantum spins delivered by so-called color-centers in diamond, into a powerful hybrid system, not yet established within Austria. Such color-centers host elementary spins, well-controllable quantum states, that behave quite similarly to atomic systems. The envisioned hybrid system is expected to enable coupling between the intrinsic single spins and the mechanical mode of the levitated diamond nanoparticle, with far-reaching implications both for fundamental tests of quantum mechanics and for quantum sensing and quantum technology. However, reaching the quantum regime with this hybrid system has remained an outstanding challenge within the field. The CooLQuanD project aims to take an important step towards this goal, by implementing techniques that control both the external and the internal temperature of mesoscopic levitated diamonds in high vacuum. The external temperature will be cooled via temperature exchange between the levitated diamond and a co-trapped and actively cooled silica nanosphere. The internal temperature will be lowered via so-called laser-refrigeration cooling, exploiting the internal energy level structure of the nanodiamonds color centers. Further, the intrinsic quantum spins will serve as a sensor of local temperature, which will facilitate the implementation of tailored sequences and the optimization of the proposed cooling methods. Our research is expected to unlock new quantum experiments with levitated spin-optomechanics and gives prospects for the realization of quantum-enhanced sensors, non-classical states of motion or for lab scale tests of quantum gravity.

The mystery of how nature behaves at the boundary between the quantum and classical worlds remains unresolved. A route to investigate this regime is to engineer well-isolated and precisely controllable mesoscopic systems and to probe quantum effects at these larger scales. Among the platforms explored worldwide, levitated particles in vacuum stand out for their exceptional environmental decoupling and the offered precise control over their mechanical motion down to the single-quantum level. In our ESPRIT project "CooLQuanD: Cooling of Levitated Quantum-Spin-Hosting Diamonds" we have enriched this platform by combining it with intrinsic quantum spins, forming a hybrid spin-mechanical system. These so-called color centers host elementary spins-well-controllable quantum states that behave similarly to atomic systems-and enable interfacing single-spin dynamics with the motion of a levitated diamond nanoparticle, with implications for fundamental tests of quantum mechanics as well as quantum sensing and quantum technologies. However, reaching the quantum regime with this hybrid system remains an outstanding challenge. CooLQuanD has taken important steps toward this goal by establishing key capabilities of this hybrid system in high vacuum. We have achieved external cooling of the mechanical motion of levitated mesoscopic diamonds to an effective temperature of a few kelvin using laser scattering and electric feedback, and have levitated the particles stably at record pressures below 510^7 mbar. We have implemented coherent spin control of NV centers within the particle trap, and we have realized in situ readout of the internal diamond temperature using the embedded spins as local sensors. Together, these outcomes establish the control toolbox needed for advanced spin-mechanical experiments. Our research opens avenues toward quantum-enhanced sensors, the preparation of nonclassical states of motion, and tabletop experiments probing quantum mechanics at mesoscopic scales.