Periodic Reporting for period 3 - PHOQS (Phononic Quantum Sensors)
Reporting period: 2024-07-01 to 2025-12-31
This project aims at developing a novel mechanical sensors and transducers under full quantum control. Such sensors can be applied to detect extremely small forces, such as those exerted by a single spin. This opens unprecedented possibilities for 3-dimensional imaging of objects at the nanoscale, such as viruses or proteins, complementing existing techniques. At the same time, it allows us to prepare objects visible to the bare eye in exotic quantum states, such as squeezed or superposition states. These experiments can thereby advance the elusive classical-quantum frontier, and yield new insights into limitations of motional decoherence due to material defects, geometry, or yet unobserved, new physics.
We have pioneered soft-clamped mechanical resonators at the low temperatures (T<50 mK) inside a dilution refrigerator. We could probe them both using optical [Page et al., Communications Physics 2021] and microwave [Seis et al., Nature Communications 2022] radiation. We could confirm that the quality factor improves with lower temperatures tovalues above 10^9 even for metallized membranes. We have also implemented optomechanical systems at mK-temperatures, with a high-Finesse optical cavity that accommodates a soft-clamped membrane. Furthermore, we have experimentally investigated parametric driving of soft-clamped membrane resonators. In collaboration with ETH Zurich, we could demonstrate strong parametric coupling between different membrane modes [Hälg et al., Physical Review Letters 2022]. In our own work, we have implemented strong thermomechanical noise squeezing through the parametric drive, stabilized by feedback. Finally, we have set up a room-temperature experimental platform in which a single spin system – hosted by a nitrogen vacancy defect in diamond – can be scanned over a sample. Optically detecting the NV’s electron spin resonance then allows us mapping magnetic fields and gradients. We are currently applying this setup to cobalt nanomagnets.
Already at the current early stage, the project work has pushed the state of the art in mechanical coherence, with coherence times in excess of 100 ms for a resonator consisting of some 10^14 atoms. We expect that during the project’s runtime, we will be able to generate squeezing of the mechanical position fluctuations far beyond what has been demonstrated today, and deep in the quantum regime. Another key result will be to observe the strong coupling of mechanical motion with a spin system. In particular, we expect to detect the orientation of a single electron spin using the mechanical force sensor. Eventually, by harnessing quantum control over the spin, we aim at preparing a highly non-classical mechanical superposition state, which has never been demonstrated at such a large mass scale.