PhD Stefan Putz

Circuit Cavity QED with Macroscopic Solid‐State Spin Ensembles


Hybrid quantum systems are one of the most promising implementations of future quantum computation, communication and simulation devices. These technologies will have a major impact on society and industry and herald the start of a new age. However, in the early stages of this development, crucial technological limitations have to be addressed and solved. As one part of this giant puzzle, this thesis examines the basic phenomena occurring when interfacing macroscopic spin ensembles with a single mode cavity. Understanding these underlying principles is key in the possible implementation of quantum memories based on solid-state spin ensembles.

The hybrid solid-state quantum system studied in this thesis consists of a superconducting microwave cavity strongly coupled to an ensemble of electron spins hosted by nitrogen-vacancy centers in diamond. One of the main results of the experiments carried out demonstrates how the total decoherence rate scales in these systems. As is shown the collective enhanced coupling strength allows the suppression of spin dephasing induced by inhomogeneous spin broadening. This effect is known as "cavity protection effect"; and the total decoherence rate scales with the collective cavity spin interaction strength. The hybridization acts beneficially on the system coherence time. However, these times can be drastically improved by spectral hole burning, beyond the natural limit attainable by the "cavity protection effect". The observed long-lived coherence truly lives up to the promise of hybrid systems to perform better than its individual subcomponents.

This demonstrates that dark states can be used to coherently exchange energy between the cavity and spin ensemble. These engineered dark states are used to induce coherent Rabi oscillations and a first step is made towards the implementation of a solid-state microwave frequency comb. Additionally this system is a versatile tool for studying strong non-linear dynamics. Such an effect is amplitude bistability, which has not been observed in a microwave solid-state spin ensemble coupled to a single mode cavity so far. This engineered hybrid system approach opens up the possibility for a new route to cavity QED experiments beyond the standard Dicke and Tavis-Cummings model, besides the realization of truly long-lived quantum memories, solid-state microwave frequency combs and optical-to-microwave quantum transducers.