"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." --

We consider the open Jaynes-Cummings model driven on the two{photon resonance, in the regime of very strong coupling. The open nature of the system means that loss of photons through the cavity mirrors as well as through spontaneous emission is allowed for. A variety of interesting quantum{optical phenomena have already been found to occur in this system. Specifically, behaviour indicative of a multiphoton blockade has been observed, and the system undergoes a transition from strongly bunched to antibunched light production as the drive strength is increased. In this thesis, we examine the possibility of finding squeezed light in the cavity output field, through numerical simulations and an analytic model of the system. For this purpose, calculations exploring squeezing of the cavity internal mode in the system steady{state are performed, and the squeezing spectrum of the cavity output field is computed. In addition to the two{photon resonance, the Jaynes-Cummings model with the atom, cavity mode and external drive all resonant is considered. The quasienergies and quasienergy states for this system are known, and exhibit interesting behaviour with regards to squeezing as a certain critical point in the drive strength is approached. We explore the possibility of performing an adiabatic following of the system ground state in order to obtain squeezing of the cavity internal mode, both in the presence and absence of damping.

Electron spin qubits in microwave cavities represent a promising foundation for developing quantum computing hardware. Electron spin states have high coherence times compared to gate interaction time scales, and photon coupling allows for long-distance interaction between qubits. The strong spin-photon coupling regime can be realized via the dipole interaction, as Petta et al. [1] demonstrated by placing an electron in a double quantum dot (DQD) with a magnetic field gradient inside a microwave cavity. These studies later included two DQDs inside the cavity to demonstrate long-range spin-spin interactions [2]. Input-output theory describes the system's behavior when driven by an external field and determines the internal qubit states from the transmission amplitude of the output field [3]. The scaling strategy for this system that has been considered thus far by others has involved placing multiple qubits inside of a single cavity. As an alternative setup, we analyze results from a model of a double DQD qubit system consisting of two cavities, each containing one qubit, coupled via a capacitive coupler that allows for single photon exchange. In analogy with previous works, we utilize input-output theory to obtain transmission amplitudes and discuss the various regimes determined by tuning different parameters

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.

This book gathers the lecture notes of courses given at the 2011 summer school in theoretical physics in Les Houches, France, Session XCVI. What is a quantum machine? Can we say that lasers and transistors are quantum machines? After all, physicists advertise these devices as the two main spin-offs of the understanding of quantum mechanical phenomena. However, while quantum mechanics must be used to predict the wavelength of a laser and the operation voltage of a transistor, it does not intervene at the level of the signals processed by these systems. Signals involve macroscopic collective variables like voltages and currents in a circuit or the amplitude of the oscillating electric field in an electromagnetic cavity resonator. In a true quantum machine, the signal collective variables, which both inform the outside on the state of the machine and receive controlling instructions, must themselves be treated as quantum operators, just as the position of the electron in a hydrogen atom. Quantum superconducting circuits, quantum dots, and quantum nanomechanical resonators satisfy the definition of quantum machines. These mesoscopic systems exhibit a few collective dynamical variables, whose fluctuations are well in the quantum regime and whose measurement is essentially limited in precision by the Heisenberg uncertainty principle. Other engineered quantum systems based on natural, rather than artificial degrees of freedom can also qualify as quantum machines: trapped ions, single Rydberg atoms in superconducting cavities, and lattices of ultracold atoms. This book provides the basic knowledge needed to understand and investigate the physics of these novel systems.

The field of atomic, molecular, and optical (AMO) science underpins many technologies and continues to progress at an exciting pace for both scientific discoveries and technological innovations. AMO physics studies the fundamental building blocks of functioning matter to help advance the understanding of the universe. It is a foundational discipline within the physical sciences, relating to atoms and their constituents, to molecules, and to light at the quantum level. AMO physics combines fundamental research with practical application, coupling fundamental scientific discovery to rapidly evolving technological advances, innovation and commercialization. Due to the wide-reaching intellectual, societal, and economical impact of AMO, it is important to review recent advances and future opportunities in AMO physics. Manipulating Quantum Systems: An Assessment of Atomic, Molecular, and Optical Physics in the United States assesses opportunities in AMO science and technology over the coming decade. Key topics in this report include tools made of light; emerging phenomena from few- to many-body systems; the foundations of quantum information science and technologies; quantum dynamics in the time and frequency domains; precision and the nature of the universe, and the broader impact of AMO science.