Session N4: Hybrid Quantum Systems

Microresonator based Optomechanical Cavity: Calibration of Quantum Noises for LIGO

Recent improvements in the properties of micro-mechanical oscillators provide promising avenues towards -more sensitive interferometry measurements and hence observation of weaker forces. In an opto-mechanical system, radiation pressure noise is a quantum back-action effect on the mechanical oscillator, due to the intensity fluctuations of the driving laser field. We study the radiation pressure noise in a centimeter scale Fabry-Perot Cavity using a microresonator as the end mirror of the cavity. The micro-resonator is fabricated from a multilayer stack of $\rm{Al}_{0.92}\rm{Ga}_{0.08}\rm{As/GaAs}$ forming a dielectric mirror pad with a mass of about 250 nanograms. This work is in effort towards developing a new quantum noise evading scheme for the Laser Interferometer Gravitational Observatory (LIGO). Radiation pressure noise is expected to be one of the limiting noise sources in Advanced LIGO. Further, the microresonators that we have developed are promising candidates for testing other noise reduction schemes including quantum non-demolition schemes, speed meters, squeezing of radiation pressure noise, and variational-readout.   

Force detection with an optically levitated microsphere in vacuum

A microsphere levitated using purely optical forces in vacuum has a high quality factor and can be used as a micro-mechanical sensor for the precise measurements of small forces such as non-Newtonian gravity in the nanoscale regime and Casimir forces [1]. In this talk, I will discuss the progress on our experiment towards the cooling of the center-of-mass motion of a dielectric microsphere trapped in an optical cavity. I will also discuss the calibration of the force sensitivity using known modulated electric fields. \\[4pt] [1] Andrew A. Geraci, Scott B. Papp, and John Kitching, Phys. Rev. Lett. 102, 101101 (2010)   

Spin-mediated optomechanical cooling of a mechanical resonator

We report recent results on using an ultracold gas of atoms to sympathetically cool a mesoscopic mechanical resonator. The optomechanical response of a `membrane in the middle' system is tuned by controlling the dispersion of the ultracold gas. This coupling between the mechanical motion of the resonator and the quantum spins of the atomic gas realizes a hybrid optomechanical system that is effectively in the resolved sideband regime, thereby enhancing the cooling of the resonator. We also report progress towards using the ultracold gas for enhanced measurement and control of the mechanical system. \\[4pt] This work is supported by the DARPA QuASAR program through a grant from the ARO.   

Backaction driven transport of Bloch oscillating atoms in ring cavities

We predict that an atomic Bose-Einstein condensate strongly coupled to an intracavity optical lattice can undergo resonant tunneling and directed transport when a constant and uniform force is applied. The bias force induces Bloch oscillations, causing amplitude and phase modulation of the lattice which resonantly modifies the site-to-site tunneling. For the right choice of parameters a net atomic current is generated. The direction and amplitude of the transport velocity depend on the detuning between the pump laser and the cavity, and transport can be enhanced through imbalanced pumping of the two counter-propagating running wave cavity modes. Our results add to the cold atoms quantum simulation toolbox, with implications for quantum sensing and metrology.   

A nanophotonic atom trap toward collective atom-light interactions and the design of a novel protection layer for superconducting circuits toward a hybrid quantum system

A centimeter long silicon nitride nanophotonic waveguide with inverse-tapered ends has been developed to address and trap many cold neutral atoms ($^{87}$Rb) for studying collective atom-light interactions and a hybrid quantum system. Two-color evanescent trapping fields (750nm and 1064nm) of guided modes (TE0) can confine cold neutral atoms above the waveguide, and its inverse-tapered waveguide-end has been used for higher input coupling. For a hybrid quantum system which couples trapped cold neutral atoms to superconducting (SC) circuits through magnetic dipole coupling, we consider a novel SC protection layer because SC circuits are vulnerable to the scattered light from trapping fields. Therefore, we design several types of dielectric and lossy multi-wavelength Bragg layers to protect SC circuits from NIR scattered optical photons and from a broadband MIR blackbody radiation of the nanophotonic device, considering the maximal back-transmission of the SC circuits' electro-magnetic fields through the layer and the heat transfer to SC circuits through the protection layer from absorbed scattered photons.   

Quantum correlations of light from atoms trapped around an optical nanofiber

We interrogate an ensemble of rubidium atoms trapped around an optical nanofiber (ONF) through the quantum correlations of the light emitted into the ONF mode. Using trapping light at 750 and 1064 nm, we create an optically thick atomic ensemble. The conditional detection, $g^{(2)}(\tau)$, allows for the study of atomic dynamics and of different contributions to the correlation function from single and multiple atoms. The conditional dynamics can not only probe ensemble properties but also provide information about this unique 1D atomic system, opening the opportunity to study many-body physics with long-range interactions. One example is the recent proposal of self-organization into crystalline order of trapped atoms around an ONF [1]. As an atomic ensemble, the guided mode assures high optical depth and good optical coupling of the photons, which may enable protocols that employ states beyond the first Dicke state. \\[4pt] [1] T. Grie{\ss}er \textit{et al}., Phys. Rev. Lett. \textbf{111}, 055702 (2013); D. E. Chang, \textit{et al.} Phys. Rev. Lett. \textbf{110}, 113606 (2013).   

Engineering of micron-sized electron trap in a superconducting tuning-fork resonator

Electrons on helium is a unique two-dimensional electron gas system formed at the interface of a quantum liquid (superfluid helium) and vacuum. The motional and spin states of single-electron quantum dots defined on such systems have been proposed for hybrid quantum computing [1,2]. Here, We will present experiments in which an ensemble of electrons are trapped above a tuning fork superconducting resonator and describe their coupling with both the differential and common mode. Next, we will discuss the design of superconducting resonators with a micron-sized trapping area and a reduced number of trapped electrons, and the experimental progress towards a single trapped electron regime.\\[4pt] [1] S. Lyon, Phys. Rev. A. 74, 5 (2006)\\[0pt] [2] D.I. Schuster, et al. Phys. Rev. Lett. 105, 040503 (2010)   

Engineered atom-light interactions in 1D photonic crystals

Nano- and microscale optical systems offer efficient and scalable quantum interfaces through enhanced atom-field coupling in both resonators and continuous waveguides. Beyond these conventional topologies, new opportunities emerge from the integration of ultracold atomic systems with nanoscale photonic crystals. One-dimensional photonic crystal waveguides can be engineered for both stable trapping configurations and strong atom-photon interactions,\footnote{C.-L. Hung, \textit{et al.}, New J. Phys. \textbf{15}, 083026 (2013).} enabling novel cavity QED and quantum many-body systems,\footnote{D. E. Chang \textit{et al.}, New J. Phys. \textbf{14}(6), 063003 (2012).} as well as distributed quantum networks. We present the experimental realization of such a nanophotonic quantum interface based on a nanoscale photonic crystal waveguide, demonstrating a fractional waveguide coupling of $\Gamma_{1D}/\Gamma'$ of $0.32\pm 0.08$, where $\Gamma_{1D}$ ($\Gamma'$) is the atomic emission rate into the guided (all other) mode(s).\footnote{A. Goban et al., arXiv:1312.3446 (2013).} We also discuss progress towards intra-waveguide trapping of ultracold Cs.   

Efficient Qubit Readout Using Josephson Photomultipliers

A Josephson photomultplier (JPM) can absorb and detect weak microwave signals with high sensitivity (PRL 107, 217401 (2011)). When strongly coupled to a microwave cavity, the JPM can detect single microwave photons with large bandwidth and with near unit efficiency (PRB 86, 174506 (2012)). The switching of a JPM into its measured state acts on the adjacent cavity via the back action of photon subtraction (PRA 86, 032311 (2012)). While a destructive measurement of the microwave cavity, this switching can perform a binary non-demolition measurement of a quantum system coupled to the cavity. We present a protocol by which the presence and subsequent detection of a cavity photon by a JPM conveys information about the state of a qubit coupled to the cavity without destroying it, thus performing a quantum non-demolition measurement of the qubit's state. Multi-qubit generalizations of this protocol are discussed.   

Circuit QED with superconducting qubits --- a multi-pole approach

Circuit quantum electrodynamics --- superconducting Josephson junction ``transmon'' qubits coupled via microwave cavities --- is a promising route towards scalable quantum computing. Here we report on experiments coupling two transmon qubits through multiple strongly coupled planar superconducting cavities --- the multi-pole cavity QED architecture. The main feature of this architecture is that the on/off ratio is expected to scale exponentially in the number of cavities (poles). In this talk we will discuss our gate protocol --- the adiabatic multi-pole (AMP) gate --- and report on producing a high fidelity Bell state ($|gg\rangle+|ee\rangle$) measured from state and process tomography. We will also report on measurements of the off-resonant coupling rate. Finally, we will discuss future plans for scaling this architecture beyond two qubits and our progress towards implementing multi-pole QED with flux insensitive qubits in 3D microwave cavities.