This thesis combines quantum electrical engineering with electron spin resonance, with an emphasis on unraveling emerging collective spin phenomena. The presented experiments, with first demonstrations of the cavity protection effect, spectral hole burning and bistability in microwave photonics, cover new ground in the field of hybrid quantum systems. The thesis starts at a basic level, explaining the nature of collective effects in great detail. It develops the concept of Dicke states spin-by-spin, and introduces it to circuit quantum electrodynamics (QED), applying it to a strongly coupled hybrid quantum system studied in a broad regime of several different scenarios. It also provides experimental demonstrations including strong coupling, Rabi oscillations, nonlinear dynamics, the cavity protection effect, spectral hole burning, amplitude bistability and spin echo spectroscopy.

"We present a generic cavity quantum electrodynamics (QED) set-up that can generate different types of strong multipartite entanglement. The proposed system is already accessible at the experimental level in different cavity QED architectures as it only involves a spin ensemble weakly coupled to a two-photon driven cavity. The first entangling scheme we propose, the dark-mode assisted protocol (DMAP), consists in transferring the squeezing from the cavity to the spin ensemble. The resulting spin squeezed state (with a Kitawaga spin squeezing parameter on the order of 10−2) has strong pairwise entanglement that is especially useful for quantum metrology. In a relatively short timescale (inversely proportional to the collective coupling strength), our scheme can almost reach the quantum Heisenberg limit (with a Wineland spin squeezing parameter close to 2/N), even for a few spins. Unlike one-axis twisting and two-axis countertwisting, the amount of spin squeezing generated by our scheme is not constrained by the number of spins. Our scheme is faster and doesn't suffer the oscillatory dynamics of one-axis twisting. We will also demonstrate how the DMAP can be analyzed as an adiabatic evolution even though achieving adiabaticity might seem impossible in our gapless system. In the second part of the thesis, we build on our previous work which showed that a two-photon drive can be used to simulate ultra-strong coupling in a standard cavity QED system where the cavity is weakly coupled to a single qubit. We extend this approach to a cavity weakly interacting with a spin ensemble and simulate the Dicke model, which is known to exhibit interesting quantum features such as strong entanglement and quantum phase transitions. The other entangling schemes we propose, make use of the simulated Dicke model: the realization of a Molmer-Sorenson gate, closely related to one-axis twisting, and the generation of large multipartite entangled cat-states. We demonstrate how our scheme can be used to prepare a remote entanglement protocol, a key ingredient in quantum information, where the weakly interacting Hamiltonian is used to grow a Greenberger-Horne-Zeilinger state which will then be transformed into an entangled-cat state by the simulated Dicke Hamiltonian. Finally we illustrate how our system can become an interesting platform to study superradiant phase transition and as well as the effects of squeezed input noise." --

Hybrid quantum circuits interfacing rare earth spin ensembles with microwave resonators are a promising approach for application as coherent quantum memory and frequency converter. In this thesis, hybrid circuits based on Er and Nd ions doped into Y?SiO? and YAlO? crystals are investigated by optical and on-chip microwave spectroscopy. Coherent strong coupling between the microwave resonator and spin ensemble as well as a multimode memory for weak coherent microwave pulses are demonstrated.

This book presents state-of-the-art research on quantum hybridization, manipulation, and measurement in the context of hybrid quantum systems. It covers a broad range of experimental and theoretical topics relevant to quantum hybridization, manipulation, and measurement technologies, including a magnetic field sensor based on spin qubits in diamond NV centers, coherently coupled superconductor qubits, novel coherent couplings between electron and nuclear spin, photons and phonons, and coherent coupling of atoms and photons. Each topic is concisely described by an expert at the forefront of the field, helping readers quickly catch up on the latest advances in fundamental sciences and technologies of hybrid quantum systems, while also providing an essential overview.

The counter-intuitive aspects of quantum physics have been long illustrated by thought experiments, from Einstein's photon box to Schrödinger's cat. These experiments have now become real, with single particles - electrons, atoms, or photons - directly unveiling the strange features of the quantum. State superpositions, entanglement and complementarity define a novel quantum logic which can be harnessed for information processing, raising great hopes for applications. This book describes a class of such thought experiments made real. Juggling with atoms and photons confined in cavities, ions or cold atoms in traps, is here an incentive to shed a new light on the basic concepts of quantum physics. Measurement processes and decoherence at the quantum-classical boundary are highlighted. This volume, which combines theory and experiments, will be of interest to students in quantum physics, teachers seeking illustrations for their lectures and new problem sets, researchers in quantum optics and quantum information.