Author(s): Luna Rios, Jose Fernando | Advisor(s): Bouwmeester, Dirk | Abstract: There are several unknown aspects about the decoherence mechanisms that cause thetransition of a system from the quantum to the classical regime. In this work we presentoptomechanical systems, in which light couples to mechanical motion, as a suitable platformfor the study of decoherence in macroscopic systems.We start by discussing some of the features of optomechanical systems, focusing onthe membrane-in-the-middle configuration. We then explain how optomechanical systemscan be extended to include multiple modes, which enables different state transfermechanisms, and we show an experimental demonstration of two such transfer schemes.We also explain how multimode systems can exhibit an enhanced optomechanical couplingrate for individual and collective mechanical modes.Later we introduce periodic structures and how they can be applied at different scalesin order to improve the performance of our optomechanical devices. We first analyze thevibrational modes of a thin membrane within the framework of linear elasticity andproceed to show how a phononic crystal reduces the dissipation of mechanical energy inthe membrane. Next, we investigate an interference model for how a photonic crystalcan enhance the reflectivity of a membrane for out-of-plane propagating radiation. Wemove on to outline the fabrication process for our optomechanical membrane devices.We conclude with a theoretical proposal for a measurement of decoherence in a macroscopicsuperposition state. This proposal relies on the state transfer techniques discussedearlier and should be possible to implement with current technologies.
Author(s): Luna Rios, Jose Fernando | Advisor(s): Bouwmeester, Dirk | Abstract: There are several unknown aspects about the decoherence mechanisms that cause thetransition of a system from the quantum to the classical regime. In this work we presentoptomechanical systems, in which light couples to mechanical motion, as a suitable platformfor the study of decoherence in macroscopic systems.We start by discussing some of the features of optomechanical systems, focusing onthe membrane-in-the-middle configuration. We then explain how optomechanical systemscan be extended to include multiple modes, which enables different state transfermechanisms, and we show an experimental demonstration of two such transfer schemes.We also explain how multimode systems can exhibit an enhanced optomechanical couplingrate for individual and collective mechanical modes.Later we introduce periodic structures and how they can be applied at different scalesin order to improve the performance of our optomechanical devices. We first analyze thevibrational modes of a thin membrane within the framework of linear elasticity andproceed to show how a phononic crystal reduces the dissipation of mechanical energy inthe membrane. Next, we investigate an interference model for how a photonic crystalcan enhance the reflectivity of a membrane for out-of-plane propagating radiation. Wemove on to outline the fabrication process for our optomechanical membrane devices.We conclude with a theoretical proposal for a measurement of decoherence in a macroscopicsuperposition state. This proposal relies on the state transfer techniques discussedearlier and should be possible to implement with current technologies.
In the field of optomechanics, one typically studies the interaction of a single mechanical resonator and a single optical cavity mode. In this thesis, we investigate collective quantum effects in optomechanical systems with two or more optical and mechanical modes. First, we study the generation and destruction of entanglement between two mechanical resonators effected by optical cavity fields. We also describe how the optomechanical interaction can be employed to mediate strong coherent coupling and entanglement between a micromechanical membrane and a single atom. Secondly, we show that a configuration with two optical and a single mechanical mode can lead to enhanced nonlinear interactions, which can be exploited for quantum non-demolition measurements of the phonon and photon numbers. Finally, we explore the quantum many-body dynamics of optomechanical arrays. In particular, we research into the transition from a disordered state due to quantum noise to a state with phasecoherent mechanical oscillations. As an intermediate step towards this study, we also investigate the nonlinear classical dynamics of single- and multi-mode optomechanical setups and compare theoretical predictions to experimental data.
Quantum entanglement is an essential feature of quantum physics and an important resource for applications in quantum information processing and quantum communication, including prominent examples such as quantum teleportation and quantum cryptography, as well as for fundamental tests of quantum theory. This thesis explores the generation and verification of continuous wave optomechanical entanglement between a light mode and a multimode mechanical oscillator. The optomechanical system consists of a thin silicon nitride membrane placed within a high finesse Fabry-Pérot cavity. It is successively improved and verified to operate in the strong cooperativity regime, a necessary condition for the generation of entanglement. A pulsed-continuous verification protocol is applied to correlation measurements with strong cooperativity. The results demonstrate that the naive approach of filtering a single high-Q mode to obtain a witness for entanglement is not viable. The multimode nature of the mechanics has to be, and is, explicitly considered in the evaluations as well as spectral features of the detection scheme, thereby paving the way towards multimode optomechanical entanglement.
In this thesis, I studied two different approaches which are promising for creating macroscopic quantum superpositions. The first approach, which is the main focus of this thesis, is based on optomechanical systems. The basic challenge to realize quantum effects in optomechanical systems, is considered to be the weak optomechanical coupling rate compared to the decoherence rates of the system caused by the cavity and mechanical damping. To enhance the optomechanical coupling it has been suggested to pump the cavity with coherent laser light \cite{Vitali07}. By this method the strong coupling regime in which the optomechanical coupling rate exceeds all decoherence rates has been demonstrated very recently \cite{Verhagen12,Groblacher09}. It is also known that the optomechanical systems may exhibit nonlinear behaviours such as bistability. After introducing the basic ideas of optomechanical systems in chapter 1, in chapter 2 we studied the connection between optomechanical entanglement and bistability. In chapter 3, we studied the drawback of using a laser to enhance the interaction. We studied the effect of the laser phase noise, as an additional decoherence channel, on the optomechanical entanglement. We have shown that the system is very sensitive to this new source of decoherence and the sensitivity is maximum in the bistable threshold. We discussed that our results are independent of how we model the laser phase noise. In chapter 4, we studied single photon optomechanics. In this regime, we proposed a new scheme to enhance the optomechanical coupling based on post selection. Our scheme is potentially promising to create a superposition of the massive oscillator in different motional states. We also discussed the potential application of our scheme to test some decoherence models. The second approach that we consider in this thesis is based on amplifying single photons with high gain to create micro-macro superpositions . In chapter 5, we discussed the De Martini scheme. We then present our scheme for creating optical micro-macro superposition and discussed its experimental realization under realistic conditions when losses are taken into account.
In this dissertation, I theoretically investigate the open system dynamics of cavity optomechanical systems in the quantum limit, where a mechanical resonator couples to a cavity mode via radiation pressure force. Recent developments in nano-fabrication and cooling techniques make it possible to optically control and manipulate the motion of micro- and nano-mechanical resonator. Several experiments have demonstrated the possibility of preparing macroscopic mechanical resonators into their quantum ground state, which makes optomechanical systems an ideal candidate for studying the quantum behavior of macroscopic objects. One important direction in quantum optomechanics is the study of the decoherence of optomechanical system due to the interaction with their external environments. This dissertation explores the decoherence effects by considering the influence of the radiation pressure coupling on the interaction between optomechanical system and its environments. Because the effects of the nonlinear interaction vary with the optomechanical coupling strength, three different interaction regimes are fully studied here: the weak coupling, the intermediate coupling and the strong coupling.
In this thesis, I report on the realization of a hybrid optomechanical system in which ultracold atoms are coupled to a micromechanical membrane. The atoms are trapped in the intensity maxima of an optical standing wave formed by retroreflection of a laser beam from the membrane surface. Vibrations of the membrane displace the standing wave, thus coupling to the center-of-mass motion of the atomic ensemble. Conversely, atoms imprint their motion onto the laser light, thereby modulating the radiation pressure force on the membrane. In this way, the laser light mediates a long-distance, coherent coupling between the two systems. \n \nWhen the trap frequency of the atoms is matched to the membrane frequency, we observe resonant energy transfer. In addition, by applying simultaneous laser cooling to the atoms, we can dissipate energy from the coupled system leading to sympathetic cooling of the membrane mode. The experimental data follows the theoretical estimations that predict the coupling to scale with the number of trapped atoms. Furthermore, by including the finite temperature of the atoms and their spatially inhomogeneous trapping potential in the theoretical model of the optomechanical coupling, we can accurately describe the width and shape of the resonance. \n \nIn an improved experimental setup, the membrane is enclosed in a cavity while the atoms are trapped in the standing wave lattice outside the cavity. The presence of the cavity results in a considerable enhancement of the coupling strength in proportion to the cavity finesse. So far we have observed sympathetic cooling of the membrane mode by a factor of 32 starting from room temperature. Theoretical estimates show that in such a setup ground-state cooling of the membrane mode should be possible, allowing one to access the quantum coherent coupling regime.
) mechanical modes in a phononic-bandgap shielded membrane resonator. An optical mode of a compact Fabry-Perot resonator detects these modes' motion with a measurement rate (96 kHz) that exceeds the mechanical decoherence rates already at moderate cryogenic temperatures (10 K). Reaching this quantum regime entails, inter alia, quantum measurement backaction exceeding thermal forces and thus strong optomechanical quantum correlations. In particular, we observe ponderomotive squeezing of the output light mediated by a multitude of mechanical resonator modes, with quantum noise suppression up to -2.4 dB (-3.6 dB if corrected for detection losses) and bandwidths ≲90 kHz. The multimode nature of the membrane and Fabry-Perot resonators will allow multimode entanglement involving electromagnetic, mechanical, and spin degrees of freedom.