Interfacing between matter qubits and light is a crucial provision for numerous quantum technological applications. However, a generic qubit may not directly interact with a relevant optical field mode, and hence, one could necessitate adjusting frequencies to match resonance conditions between parties. In this work, we show how a parametric coupling of the qubit with a mechanical oscillator, in conjunction with the trilinear radiation pressure coupling of the same object with light, can induce maximal qubit-light entanglement at an optimal time. Furthermore, we show how our method enables conditional (dynamical) nonclassical state preparation of the optical field via qubit measurement in the weak (moderate-to-strong) optomechanical coupling regime. Our scheme benefits from not requiring any cooling of the mechanical component and not needing an adjusting of the detunings and transition frequencies to have resonance between any pairs of quantum systems.
Interfacing between matter qubits and light is a crucial provision for numerous quantum technological applications. However, a generic qubit may not directly interact with a relevant optical field mode, and hence, one could necessitate adjusting frequencies to match resonance conditions between parties. In this work, we show how a parametric coupling of the qubit with a mechanical oscillator, in conjunction with the trilinear radiation pressure coupling of the same object with light, can induce maximal qubit-light entanglement at an optimal time. Furthermore, we show how our method enables conditional (dynamical) nonclassical state preparation of the optical field via qubit measurement in the weak (moderate-to-strong) optomechanical coupling regime. Our scheme benefits from not requiring any cooling of the mechanical component and not needing an adjusting of the detunings and transition frequencies to have resonance between any pairs of quantum systems.
Quantum entanglement in mechanical systems is not only a key signature of macroscopic quantum effects but has wide applications in quantum technologies. Here we propose an effective approach for creating strong steady-state entanglement between two directly coupled mechanical oscillators (or a mechanical oscillator and a microwave resonator) in a modulated optomechanical system. The entanglement is achieved by combining the processes of a cavity cooling and the two-mode parametric interaction, which can surpass the bound on the maximal stationary entanglement from the two-mode parametric interaction. In principle, our proposal allows one to cool the system from an initial thermal state to an entangled state with high purity by a monochromatic driving laser. Also, the obtained entangled state can be used to implement the continuous-variable teleportation with high fidelity. Moreover, our proposal is robust against the thermal fluctuations of the mechanical modes under the condition of strong optical pumping.
Non-relativistic quantum mechanics have proven to be a significant framework to understand the non-classical behaviour of light and the microcosmos. Perhaps, one of the first technological revolutions within quantum theory came with the invention of the transistor, whereby a purely quantum mechanical description was required. Currently, another outstanding revolution is taking place in a crossroad where information science meets quantum mechanics (this being the quantum information field). Such an area of work contemplates both the fascinating theoretical aspect of quantum correlations, as well as implementations towards quantum tasks performed by a universal quantum computer; tasks that cannot be realised (or they are hard to implement) within the classical domain. This Thesis is devoted to study the dynamics of quantum entanglement in spin-optomechanics systems. In particular, we explore the quantum stabilization of quantum entanglement, a quantum concentration scheme in opto-mechanics and an interfacing of matter and light towards quantum networking applications. Additionally, we also investigate theoretical aspects of quantum correlations within thermal environments, as well as the topical area of quantum sudden transitions. In Chapter 1, we provide a brief summary of quantum information and of the quantum optics framework to cover elementary concepts and techniques used subsequently in this work. Subsequently, in Chapter 2 we present the stabilization of quantum entanglement in a non-linear qubit-oscillator system. The inclusion of a modest nonlinearity gives three results, i) the loss of periodicity of the system, ii) the occurrence of quadrature squeezing appearing for a short time, and iii) the quantum entanglement reaches higher values in contrast to the case without non-linearity. In Chapter 3, a technique to concentrate/distill a two-mode vacuum state in optomechanics via unsharp measurements is presented. Here, one of the optical modes is injected into a cavity at first, and thereafter, it is nonlinearly coupled to a mechanical oscillator. Afterwards, the position of the oscillator is measured using pulsed optomechanics and homodyne detection. The results show that this measurement can conditionally increase the initial entanglement. Next, in Chapter 4, stimulated by optomechanical transducers and quantum networking, a light-matter system is constructed where a qubit is coupled to a cavity mode mediated through a mechanical oscillator. The qubit-oscillator conditionally displaced Hamiltonian and the oscillator-cavity radiation-pressure interaction generate a maximal qubit-cavity entanglement. Additionally, we consider the case in which the cavity mode is coupled to a waveguide, numerical calculations show a promising qubit-fibre entanglement under a weak matter-light coupling. For the quantum network case, we coupled a generic qubit in the first node to a second qubit-cavity distant Jaynes-Cummings system coupled through an optical fibre, where qubit-qubit correlations can be achieved in the quantum open systems scenario. In Chapter 5, we study the evolution of an open quantum system within the Born-Markov microscopic master equation (MME). Essentially, two distant two-level atoms are trapped in fibre-coupled cavities. Under the approximation of one-excitation allowed in the atom-cavity-fibre basis, we can obtain quantum correlations induced by thermal fluctuations from the environments. Lastly, in Chapter 6, we bring together previously elements explored in this Thesis. The system is a hybrid atomic-mechanical system formed from two remote qubits interacting with individual harmonic oscillators. This system, as in Chapter 4, explores interesting applications in quantum networking schemes. The two qubits are initially prepared in a Bell-diagonal state, and consequently the two-qubit correlations exhibit few interesting effects such as freezing, sudden changes and revivals in the evolution of the quantum entropic discord. To conclude, I summarize my findings in Chapter 7.
Optomechanical generation of entangled optical beams is usually hindered by thermal noise. We present a theoretical study of low frequency entanglement generation between two optical harmonics emitted from a cavity optomechanical system operating in the resolved-sideband regime. The system comprises three nearly equidistant optical modes in a high-finesse cavity, with the central mode coherently driven. This configuration enables radiation-pressure interactions that generate strong quantum correlations between the two sideband modes. Remarkably, these correlations persist even at large numbers of thermal quanta if one properly engineers the optical cooling rate of the mechanical mode. Our findings demonstrate the feasibility of robust entanglement under ambient conditions, opening new avenues for hybrid quantum technologies based on mechanical interfaces and continuous-variable quantum information processing.
In a nutshell, a cavity optomechanical system consists of a mechanical oscillator inside an optical cavity. Cavity optomechanics studies the interplay between the light-matter radiation-pressure interaction and the change in the cavity frequency induced by mechanical motion. By leveraging the tools of quantum optics and quantum measurement, cavity quantum optomechanics enables the engineering of nonclassical quantum states of bulk matter. In this thesis, I present protocols for engineering such nonclassical quantum states in Part II. First, I introduce a scheme to ‘grow’ a quantum kitten state of a mechanical oscillator into a macroscopic Schrödinger cat state using a series of nonlinear pulsed optomechanical interactions. Next, I detail our proposed schemes to generate and verify entanglement between two mechanical oscillators. Moving on from pulsed optomechanics, I describe the theory behind an experiment that achieved multi-phonon subtraction from a mechanical oscillator in a continuously driven Brillouin optomechanical system. In the final chapter of Part II, I provide a theoretical preview of an experiment that investigates the behaviour of the mechanical oscillator when no light is detected at the output. In Part III, I introduce a new framework for nonlinear cavity quantum optomechanics that incorporates the intrinsic nonlinearity of the radiation-pressure interaction, the nonlinearity from the cavity enhancement, and higher-order mechanical position terms in the expansion of the cavity frequency. I then propose a scheme for measuring mechanical position using general-dyne detection, applicable to pulsed and continuous measurements. This predicts a new regime of operation, the “stochastic Gaussian” regime. Finally, I discuss a protocol for deterministic mechanical squeezing that utilizes the cavity response.
The quantum nonlinear regime of optomechanics is reached when nonlinear effects of the radiation pressure interaction are observed at the single-photon level. This requires couplings larger than the mechanical frequency and cavity-damping rate, and is difficult to achieve experimentally. Here we show how to exponentially enhance the single-photon optomechanical coupling strength using only additional linear resources. Our method is based on using a large-amplitude, strongly detuned mechanical parametric drive to amplify mechanical zero-point fluctuations and hence enhance the radiation pressure interaction. It has the further benefit of allowing time-dependent control, enabling pulsed schemes. For a two-cavity optomechanical set-up, we show that our scheme generates photon blockade for experimentally accessible parameters, and even makes the production of photonic states with negative Wigner functions possible. We discuss how our method is an example of a more general strategy for enhancing boson-mediated two-particle interactions and nonlinearities.
Cavity optomechanics represents a flexible platform for the implementation of quantum technologies, useful in particular for the realization of quantum interfaces, quantum sensors and quantum information processing. However, the dispersive, radiation-pressure interaction between the mechanical and the electromagnetic modes is typically very weak, harnessing up to now the demonstration of interesting nonlinear dynamics and quantum control at the single photon level. It has already been shown both theoretically and experimentally that if the interaction is mediated by a Josephson circuit, one can have an effective dynamics corresponding to a huge enhancement of the single-photon optomechanical coupling. Here we analyze in detail this phenomenon in the general case when the cavity mode and the mechanical mode interact via an off-resonant qubit. Using a Schrieffer-Wolff approximation treatment, we determine the regime where this tripartite hybrid system behaves as an effective cavity optomechanical system in the strong coupling regime.