Single-photon optomechanics using superconducting quantum interference

• Applicant: Dr. Daniel Bothner
• Subject Area: Experimental Condensed Matter Physics ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 511315638

In the research field of cavity optomechanics, mechanical oscillators of various shapes, sizes and masses are integrated into electromagnetic resonators and then detected and controlled with quantum-limited precision by means of appropriately injected light fields. Most optomechnical systems to date, however, have in common that the single-photon coupling rate between the mechanical and the electromagnetic degree of freedom is orders of magnitude smaller than the dominant decay rate of the system. As a consequence, it has so far been impossible to observe or utilize the intrinsic quantum nonlinearity of the optomechanical Hamiltonian or the highly interesting higher order coupling terms, such as for instance the quadratic term, to their full extent.With this project, the coupling rates in an optomechanical system in the microwave regime shall be increased for the first time to a level, at which they reveal their full quantum nature and where they can be experimentally investigated and utilized for mechanical quantum state preparation. This dramatic enhancement of the single-photon coupling rate will be approached by using flux-mediated optomechanics (FMOM) or SQUID optomechanics (SQUID = superconducting quantum interference device), a platform that recently has been realized for the first time. In FMOM systems, a mechanical oscillator is integrated into a SQUID-LC-circuit in a way that its displacement is converted to a change of magnetic flux threading the SQUID loop by means of an externally applied magnetic transduction field. The single-photon coupling rate is then proportional to and can be scaled with the external transduction field. Previous realizations of FMOM were all implemented using Aluminum as superconducting material for the circuits. In the present proposal Niobium will be used for the first time, a superconductor with an at least one order of magnitude larger magnetic field tolerance. By means of a targeted optimization of the SQUID circuits, the implementation of non-trivial SQUID geometries, as well as a minimization of magnetic flux noise, it will be possible to achieve sufficiently large coupling rates for the optomechanical single-photon regime; both for the standard linear coupling term and for the usually very small quadratic term. As final highlight of the project, the single-photon regime shall be used to demonstrate the on-demand preparation of a mechanical quantum state.Reaching the optomechanical single-photon regime as envisioned with this project could usher in a new era for optomechanics, with uncountable possibilities for the experimental investigation of the optomechanical quantum nonlinearity, for novel optomechanical quantum technologies and for the investigation of gravity-induced effects on massive quantum states.

• DFG Programme: Research Grants