In this thesis, we investigate dynamics of many-body atomic systems coupled to electromagnetic fields. We find that collective effects present in cavity-mediated laser cooling and high temperatures of bubble in sonoluminescence can be explained using a two-stage model which combines quantum-optical models and thermodynamical processes. We show how the collective processes are strongly dependent on mutual atomic coherences and how these coherences need to be recreated for the continous collective processes to take place. The model mechanism behind both cavity-mediated laser cooling and sonoluminescence heating is alternating periods of thermalisation with cooling or heating cycles. The thermalisation stage is characterised by relatively weak interactions between the atomic system and its environment, while allowing the system to thermalise and to create phonon and electronic coherences necessary for the next stage. The second stage, when cooling or heating occurs, marks strong interactions of the atomic system with the surrounding radiation field, which renders interactions between the particles negligible. During this stage, the atomic coherences created earlier fuel the cooling or heating process, allowing the system to reach a more beneficial stationary state. For cavity-mediated laser cooling of an atomic gas, we show that dispersing cooling pulses with periods of thermalisation in an asymmetric potential can result in very low temperatures of the atomic gas. By applying this to atomic interactions of sonoluminescence, we can describe different parts of the lifecycle of the cavitating bubble and how very high temperatures arise inside of it.
In this thesis, we investigate dynamics of many-body atomic systems coupled to electromagnetic fields. We find that collective effects present in cavity-mediated laser cooling and high temperatures of bubble in sonoluminescence can be explained using a two-stage model which combines quantum-optical models and thermodynamical processes. We show how the collective processes are strongly dependent on mutual atomic coherences and how these coherences need to be recreated for the continous collective processes to take place. The model mechanism behind both cavity-mediated laser cooling and sonoluminescence heating is alternating periods of thermalisation with cooling or heating cycles. The thermalisation stage is characterised by relatively weak interactions between the atomic system and its environment, while allowing the system to thermalise and to create phonon and electronic coherences necessary for the next stage. The second stage, when cooling or heating occurs, marks strong interactions of the atomic system with the surrounding radiation field, which renders interactions between the particles negligible. During this stage, the atomic coherences created earlier fuel the cooling or heating process, allowing the system to reach a more beneficial stationary state. For cavity-mediated laser cooling of an atomic gas, we show that dispersing cooling pulses with periods of thermalisation in an asymmetric potential can result in very low temperatures of the atomic gas. By applying this to atomic interactions of sonoluminescence, we can describe different parts of the lifecycle of the cavitating bubble and how very high temperatures arise inside of it.
This thesis summarizes the research work we did in the past years, which focuses on the theoretical study of quantum effects of ordered atomic structures interacting with the quantum electric field of an optical resonator. Within this work we investigated the quantum properties of the light at the cavity output, emitted by this kind of system, and the quantum state of the atoms confined by the light fields of the cavity mode. Following one possible partition, we have organized the thesis in two parts. The first part of the thesis is devoted to the quantum properties of the light at the output of optical cavities, in which atoms are confined. After a brief introduction on the basics of atom-photon interactions, we show theoretically, that two atomic dipoles in a resonator can behave as a quantum non-linear medium, whose response can be controlled through the interactomic distance of the atoms inside the resonator. We identify the parameter regime, for which the system operates as a parametric amplifier, based on the atoms' collective dipole cascade emission. Moreover we also determine the corresponding field squeezing spectrum at the cavity output. These dynamics could be observed as a result of self-organization processes of laser-cooled atoms in resonators. The second part of the thesis studies the quantum ground state of an ultracold atomic gas trapped inside an optical resonator by the mechanical effects of atom-photon interactions. Due to the strong coupling regime, the ultracold atomic gas feels a quantum potential, where the fluctuations at the single photon level may be relevant in determining the trapping conditions. This potential depends on the atomic distribution, which at the same time determines the intracavity field amplitude. Here, after a brief introduction on ultracold atoms in optical lattices, we study the low temperature physics of an ultracold atomic gas in the quantum potential formed inside a pumped cavity, by mapping the dynamics to an effective Bose-Hubbard model. We predict the existence of non-compressible states of the atomic gas, and determine a phase diagram as a function of the cavity parameters. Novel effects are encountered, such as the existence of overlapping stability regions corresponding to competing insulator-like states when the laser pumps the cavity, to provide one example out of many. At the end of the thesis a concluding chapter presents some general outlooks to this thesis work.
In this paper, we identify a quantum coherence with the ability to induce collective dynamics in an atomic gas, when a cooling laser is applied. When present, the mean phonon number per particle changes at a rate, which is proportional to the total number of particles in the system. We then propose to use this effect to cool atoms in a cyclic two-stage process to very low temperatures. During each displacement stage, the atoms move freely inside an asymmetric trapping potential, thereby accumulating away from its centre. During each cooling stage, a cooling laser is applied, which returns the atoms to the centre of the trap, while decreasing their mean phonon number. Alternating both stages is shown to transfer the atoms to a final temperature, which vanishes in the infinitely-many particle limit.
Ultracold atomic gases loaded into optical cavities constitute one of the most versatile platforms for the study of out-of-equilibrium dynamics and quantum simulation of many- body systems. In this thesis, we explore these two possibilities, motivated by current state-of-the-art atom-cavity experiments, uncovering novel phenomena and opening up new theoretical and experimental avenues into the intriguing world of quantum many- body physics. First, we study a gas of ultracold atoms coupled to three optical resonators. We show that the inherent Z2 symmetries, associated with each atom-cavity coupling, can be combined into a global SO(3) rotational symmetry which can be spontaneously broken. We determine the phase diagram of the system, which shows the emergence and breaking of the continuous symmetries and displays first- and second-order phase transitions. We argue that by coupling the atoms equally to n different modes, it will in general be possible to generate a global SO(n) symmetry. Second, we investigate the dynamics of a spinor BEC inside an optical cavity. We focus on a two-component Dicke model with complex light-matter couplings, accounting for photon losses. We compute the steady-state phase diagram and find dynamical in- stabilities in the form of limit cycles, heralded by the presence of exceptional points and level attraction. We show that the instabilities are induced by dissipative processes that generate nonreciprocal couplings between the two collective spins. Lastly, we explore the dynamics of a BEC inside an optical cavity in the presence of an optical lattice. We derive an effective master equation by adiabatically eliminating the cavity field. In the bad-cavity regime, we find an infinite-temperature steady state, but with relaxation dynamics that can be highly nontrivial. For small hopping, the interplay between dissipation and strong interactions leads to an algebraic relaxation of the system, whereas for large hopping, the approach to the steady state is exponential. In the good- cavity regime, we show that the system allows for optical pumping between momentum modes, enabling cavity cooling.
An ensemble of atoms coupled to the radiation field exhibits collective behaviours when the interatomic separation is comparable to the wavelength of photons emitted by the atoms. The collective coupling of the atoms with the electromagnetic field gives rise to coherent exchange interactions between the atoms. In addition, the spontaneous decay of photons from the ensemble becomes a collective process that can happen at a much faster (superradiant) or slower (subradiant) rate than that of a single atom. These processes affect the way that fluorescence light is scattered from the system when it is driven by a laser. However, little is known about the response of a dense atomic gas beyond the linear regime of very weak driving, due to the involved complexity of the problem. The first part of this thesis investigates how the presence of strong laser driving affects properties of the far-field fluorescence light emitted by the ensemble, such as the power spectrum and second order coherence. Depending on the strength of the driving laser and the particular arrangement of the atoms, the laser can modify considerably how the collective behaviours of the atomic ensemble are manifested in the fluorescence. The relative strengths of the coherent interaction and collective decay process are intrinsically linked to the way that the atoms couple to the radiation field; one cannot be changed without modifying the other. Altering the environment by, for example, placing the atoms near a surface or a waveguide, can change this relation significantly from that for atoms in free space. Therefore, modified environments represent a platform for the generation of otherwise inaccessible regimes of collective behaviour, with potential applications in quantum state engineering. The second part of this thesis provides a numerical investigation into how the collective properties of a dense atomic system can be manipulated by placing the atoms close to planar surfaces or an optical nanofiber. These structures modify the light-matter coupling by altering the electric field experienced by the atoms. It is shown that surfaces of metallic and dielectric materials can be used to either enhance or suppress the collective behaviours, depending on the particular system arrangement. In particular, the coupling of atoms with bound surface modes, such as the surface plasmon polaritons that are formed at an air-metal interface, is demonstrated to have a strong effect on the interactions. An optical nanofiber supports a set of fundamental guided modes, that propagate along the fiber. If the atoms are suitably polarised, then they will emit primarily into the guided modes travelling in one direction. By calculating and analysing the collective decay states of a chain of atoms placed near a nanofiber, it is demonstrated that the atom-fiber coupling leads to the formation of a superradiant mode that is preserved at large atomic separations. The presence of chiral emission is shown to introduce phase gradients to the collective modes.
Cavity-mediated cooling has the potential to become one of the most efficient techniques to cool molecular species down to very low temperatures. This thesis studies the use of rate equations to analyse the cooling process in such systems. In particular the master equation is used to find rate equations that can determine the rate of change of phonons in the system. The general idea behind cavity cooling is the continuous conversion of phonons into cavity photons. While there is no spontaneous emission and decay rate associated with the concept of phonons, photons are created after a change in the phonon number has occurred and can then leak out through the cavity mirrors easily. Hence the conversion of phonons into photons can result in the constant removal of phonon energy from the system. In this thesis we compare cavity mediated cooling with single particle laser cooling. It is shown that cavity cooling is essentially the same as ordinary laser cooling. This is done by calculating the stationary state phonon number mss and the cooling rate y as a function of the system parameters. For example, when the trap phonon frequency υ is either much larger or much smaller than the cavity decay rate k , the minimum stationary state phonon number scales as k²/16v² (strong confinement regime) and as k/4 (weak confinement regime), respectively. Replacing k with Ѓ yields the steady states associated with ordinary laser cooling.
This thesis investigates thermalisation, correlations and entanglement in Bose-Einstein condensates. Bose-Einstein condensates are ultra-cold collections of identical bosonic atoms which accumulate in a single quantum state, forming a mesoscopic quantum object. They are clean and controllable quantum many-body systems that permit an unprecedented degree of experimental flexibility compared to other physical systems. Further, a tractable microscopic theory exists which allows a direct and powerful comparison between theory and experiment, propelling the field of quantum atom optics forward at an incredible pace. Here we explore some of the fundamental frontiers of the field, examining how non-classical correlations and entanglement can be created and measured, as well as how non-classical effects can lead to the rapid heating of atom clouds. We first investigate correlations between two weakly coupled condensates, a system analogous to a superconducting Josephson junction. The ground state of this system contains non-classical number correlation arising from the repulsion between the atoms. Such states are of interest because they may lead to more precise measurement devices such as atomic gyroscopes. Unfortunately thermal fluctuations can destroy these correlations, and great care is needed to experimentally observe non-classical effects. We show that adiabatic evolution can drive the isolated quantum system out of thermal equilibrium and decrease thermal noise, in agreement with a recent experiment [Esteve et al. Nature 455, 1216 (2008)]. This technique may be valuable for observing and using quantum correlated states in the future. Next, we analyse the rapid heating that occurs when a condensate is placed in a moving periodic potential. The dynamical instability responsible for the heating was the subject of much uncertainty, which we suggest was due to the inability of the mean-field approximation to account for important spontaneous scattering processes. We show that a model including non-classical spontaneous scattering can describe dynamical instabilities correctly in each of the regimes where they have been observed, and in particular we compare our simulations to an experiment performed at the University of Otago deep inside the spontaneous scattering regime. Finally, we proposed a method to create and detect entangled atomic wave-packets. Entangled atoms are interesting from a fundamental perspective, and may prove useful in future quantum information and precision measurement technologies. Entanglement is generated by interactions, such as atomic collisions in Bose-Einstein condensates. We analyse the type of entanglement generated via atomic collisions and introduce an abstract scheme for detecting entanglement and demonstrating the Einstein-Podolsky-Rosen paradox with ultra-cold atoms. We further this result by proposing an experiment where entangled wave-packets are created and detected. The entanglement is generated by the pairwise scattering that causes the instabilities in moving periodic potentials mentioned above. By careful arrangement, the instability process can be controlled to to produce two well-defined atomic wave-packets. The presence of entanglement can be proven by applying a series of laser pulses to interfere the wave-packets and then measuring the output populations. Realising this experiment is feasible with current technology.