Session S3: Focus Session: Opto-Mechanical Systems

Quantum cavity opto-mechanics with cold atoms: measuring and controlling a mechanical oscillator with light

In cavity opto-mechanical systems, the motion of a mechanical element is sensed by its influence on the field within an electromagnetic resonator. While their experimental realizations are quite diverse, with mechanical elements ranging from picogram-scale nanofabricated metallic filament to the kilogram-scale mirrors of the LIGO detector and optical systems ranging from microfabricated stripline resonators to kilometers-long optical cavities, such systems are converging on the common goal of realizing quantum limited operation. In this talk, I will discuss the use of ensembles of ultracold trapped atoms, with atom numbers ranging presently from 10$^3$ to 10$^5$, as mechanical elements within a high-finesse optical cavity. With this system, my colleagues and I realize cavity opto-mechanics in the quantum regime, with opto-mechanical coupling parameters that may be readily tuned and extended into a distinct granular, or strong-coupling, regime. We have also begun exploring cavity optical interactions with internal quantum variables of these atoms (their spin), and the possibilities arising from interfacing their motional and spin degrees of freedom.   

Nanomechanical motion measured with an imprecision below the standard quantum limit

Observing quantum behavior of mechanical motion is challenging because it is difficult both to prepare pure quantum states of motion and to detect those states with high enough precision. We present displacement measurements of a nanomechanical oscillator with an imprecision below that at the standard quantum limit [1]. To achieve this, we couple the motion of the oscillator to the microwave field in a high-Q superconducting resonant circuit. The oscillator's displacement imprints a phase modulation on the microwave signal. We attain the low imprecision by reading out the modulation with a Josephson Parametric Amplifier, realizing a microwave interferometer that operates near the shot-noise limit. The apparent motion of the mechanical oscillator due the interferometer's noise is now substantially less than its zero-point motion, making future detection of quantum states feasible. In addition, the phase sensitivity of the demonstrated interferometer is 30 times higher than previous microwave interferometers, providing a critical piece of technology for many experiments investigating quantum information encoded in microwave fields. \\[4pt] [1] J. D. Teufel, T. Donner, M. A. Castellanos-Beltran, J. W. Harlow, K. W. Lehnert, Nature Nanotechnology, doi:10.1038/nnano.2009.343, (2009).   

Opto-mechanical transducers for long-distance quantum communication applications

We describe a new scheme for the implementation of a quantum interface between stationary qubits and a photonic channel. In our approach the coupling is mediated by an opto-mechanical device where the motion of a nano-mechanical resonator acts as a transducer for magnetic, electric and optical interactions. This scheme does not rely on coherent optical transitions and is therefore applicable for a wide range of spin and charge based qubits. We analyze a quantum network based on opto-mechanical quantum transducers and derive a simple model to describe the effective coupling of the qubits to a common optical fiber. We analyze the implementation of quantum state transfer protocols and study the influence of the intrinsic noise which is added by the opto-mechanical device. Our analysis shows that experimental conditions to achieve high state transfer fidelities are equivalent to conditions necessary for opto-mechanical ground state cooling and therefore are within experimental reach.   

Atomic probe Wigner tomography of a nanomechanical system

We propose a scheme to measure the quantum state of a nanomechanical oscillator cooled near its ground state of vibrational motion. This is an extension of the nonlinear atomic homodyning technique scheme first developed to measure the intracavity field in a micromaser. It involves the use of a detector-atom that is simultaneously coupled to the cantilever via a magnetic interaction and to (classical) optical fields via a Raman transition. We show that the probability for the atom to be found in the excited state is a direct measure of the Wigner characteristic function of the nanomechanical oscillator. We also investigate the back-action effect of this destructive measurement on the state of the cantilever.   

Optical trapping and cooling of microspheres: towards the quantum limit

Recently, there has been rapid progress in the field of cooling opto-mechanical systems. Here we propose to use optical tweezers to trap and cool glass microspheres to the quantum ground state, and use the microsphere as a micro-detector operating at the quantum limit. We have successfully trapped glass microspheres in air and in vacuum, and have studied the Brownian motion of single microspheres in air at different pressures with ultrahigh resolution. At short time scales, we observed ballistic Brownian motion. We have measured the instantaneous velocity of Brownian motion successfully for the first time, and verified the energy equipartition theorem of the Brownian motion directly. We are currently working on cooling the center-of-mass motion of glass microspheres in optical tweezers.   

Classical dynamics of the optomechanical modes of a Bose-Einstein Condensate in a ring cavity

We consider a Bose-Einstein condensate (BEC) interacting with two counterpropagating optical modes in a ring cavity. In contrast to a recent experiment involving a BEC in a Fabry-Perot cavity interacting with a standing wave mode [F. Brennecke et. al, Science 322, 235 (2008).] both symmetric and antisymmetric collective density side modes of the BEC are excited by the optomechanical effects of the light fields. We present a classical analysis of the system. Its steady state exhibits a rich multistable behavior including isolated domains of solutions (isolas). We also study a number of aspects of the dynamics of the system.   

Chip-Based Optical Interactions with Rubidium Vapor

Chip-based optical waveguides that are evanescently coupled to strongly resonant vapors offer significant potential for realizing low-photon and single-photon nonlinear interactions in a system consistent with an integrated optics approach. We demonstrate evanescent coupling of rubidium (Rb) vapor with chip-based optically guiding silicon-nitride nanowires in a small, robust and portable set-up. We perform spectroscopy of Rb D2 resonances, with optical depths of 2 observed for the guided mode. We observe noticeable broadening and shifting of the D2 lines with respect to their free-space counterparts. This is consistent with the homogeneous broadening due to transit-time effects, and inhomogeneous broadening and shifting due to Van der Waals interactions between the atoms and the surface of the waveguide. We also demonstrate excitation of ring resonators coupled to these waveguides.