Transfer of information between various types of harmonic oscillators is of key importance for aspects of quantum information storage and conversion. In particular, the regime where the coupling rate exceeds the individual loss rates is of special interest, as this regime promises an efficient transfer of excitations. Our group investigates linearly and non-linearly coupled harmonic resonators based on superconducting microwave resonators, nano-mechanical string resonators, and spin ensembles.
Electron spin resonance (ESR) is a key technique for quantum information technologies as it lays the foundation for coherent spin control and the determination of coherence times. Additionally, ESR is a key spectroscopy tool for multiple disciplines like chemistry, biology, and physics.
We presently focus on magnetic resonance techniques at millikelvin temperatures. One aspect is to achieve a large thermal spin polarization and hereby a highly sensitive readout. This, together with a suitable spin density, allows to investigate the coupling between the spin ensemble and the microwave resonator in the strong coupling regime. The investigation of the dynamics of such strongly coupled systems and the understanding how to coherently control the spin ensemble under these conditions are key questions for utilizing these systems for quantum information storage or spectroscopy and sensing applications. We combine conventional ESR with ultra-sensitive microwave detection schemes derived from superconducting circuit QED to further improve the sensitivity of ESR. Long-term research goals in this field are the use of quantum states for sensing spin properties and or magnetic field sensing.
The field of nano-electromechanics aims at studying quantum mechanics in the literal sense. One prerequisite is to cool a vibrational mode to its quantum ground state and then subsequently prepare this mode in a specific quantum state e.g. like a phonon Fock state, squeezed state or cat state. In this context, the integration of nanomechanical elements in superconducting microwave cavities was a big step forward enabling some of the goals stated above. We started with conventional circuit nano-electromechanics, where we demonstrated electromechanically induced transparency and slow light physics in nano-string superconducting microwave resonator hybrid devices. Typically, devices in this field rely on a capacitive coupling between the mechanical element and the microwave resonator where the capacitive participation ratio limits the coupling strength. To overcome this limit, we are presently implement an inductive coupling scheme based on a dc-SQUID with a vibrational element. In parallel, we start combining circuit nano-electromechanics with circuit QED to address the key challenge of preparing phonon Fock states in a nano-string resonator.