Session A4: Focus Session: Hybrid Systems and Quantum Information Science in Atomic, Molecular, and Optical Physics

A Light-Matter Interface with NV Centers

NV centers in diamond offer much promise as solid state qubits for scalable quantum communication and information processing. However, for bulk diamond systems the low collection efficiency and large phonon sideband emission represent substantial limitations for applications in quantum information science. In this talk we will describe the realization of nanoscale photonic cavities containing NV centers with desired optical properties. The experimental realization of spontaneous emission control and strong coupling regime of cavity QED will be discussed.   

Probing the motion of a mechanical resonator via coherent coupling to a single spin qubit

Mechanical systems can be influenced by a wide variety of extremely small forces, ranging from gravitational to optical, electrical, and magnetic. When mechanical resonators are scaled down to nanometer-scale dimensions, these forces can be harnessed to enable coupling to individual quantum systems. In this talk we will present results showing that the coherent evolution of a single electronic spin associated with a Nitrogen Vacancy (NV) center in diamond can be coupled to the motion of a magnetized mechanical resonator. Specifically we use coherent manipulation of the spin to sense the driven and Brownian motion of the resonator under ambient conditions at a picometer length scale. We will discuss potential applications of this technique including the decetion of the zero-point fluctuations of a mechanical resonator, the realization of strong spin-phonon coupling at a single quantum level, and the implementation of quantum spin transducers.   

A nanoscale quantum interface for single atoms

Neutral atoms are ideal quantum systems: they have long ground-state coherence times and strong optical cycling transitions that enable state detection and preparation. Building quantum networks of atoms interacting through photons is challenging, however, as many schemes for atom-photon interaction are inefficient or hard to scale. We propose a scheme to trap neutral atoms near silver nanowires, which are tightly confining waveguides for surface plasmons. The nanowire tip is used to generate a near-field optical trapping potential, and to enhance and efficiently collect spontaneous emission from the atom. We present experimental results on using the atom to sense the optical field at submicron distances from the wire and our current efforts towards loading the nanotrap.   

Estimating and characterizing electromechanical coupling of superfluid helium to a microwave 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 state of single-electron quantum dots defined on such systems has been proposed as a good candidate for hybrid quantum computing and as a gateway to measuring the electron spin [1,2]. Incoherent fluctuations of the thickness or density of the helium film are potential sources of motional dephasing, hence require further experimental characterization. In addition, if these ripplons or phonons could be coherently coupled to an electromagnetic cavity one could realize a quantum electro-mechanical system. Here, I will present estimates and preliminary experimental characterization of the electromechanical couplings as well as progress with electrons on helium. \\[4pt] [1] S. Lyon, Phys. Rev. A. 74, 5 (2006) \\[0pt] [2] D.I. Schuster, et al. Phys. Rev. Lett. 105, 040503 (2010)   

Tunable High Q Superconducting Microwave Resonator for Hybrid System with $^{87}$Rb atoms

We have developed a frequency tuning system for a ``lumped-element'' thin-film superconducting Al microwave resonator [1] on sapphire intended for coupling to hyperfine ground states of cold trapped $^{87}$Rb atoms, which are separated by about $f_{Rb}=6.83$ GHz. At \emph{T}=12 mK and on resonance at 6.81 GHz, the loaded quality factor was 120,000. By moving a carefully machined Al pin towards the inductor of the resonator using a piezo stage, we were able to tune the resonance frequency over a range of 35 MHz and within a few kHz of $f_{Rb}$. While measuring the power dependent response of the resonator at each tuned frequency, we observed anomalous decreases in the quality factor at several frequencies. These drops were more pronounced at lower power. We discuss our results, which suggest these resonances are attributable to discrete two-level systems.\\[4pt] [1] Z. Kim \emph{et al}., AIP ADVANCES \textbf{1}, 042107 (2011).   

Coupling quantum microwave circuits to quantum optics via cavity electro-optic modulators

Experimental circuit quantum electrodynamics has made great strides in recent years, but it remains an open question how the quantum information stored in the microwave circuits can be transferred for long distances. Just as in classical information, the most promising solution is to convert the microwave fields to optical frequencies, where ultra-low-loss photonic devices such as optical fibers can be used. Here I propose the use of cavity electro-optic modulators for coherent coupling between microwave and optical fields. The electro-optic effect is the change in optical refractive index in certain materials, such as lithium niobate, under an applied voltage. Leveraging the fact that cavity electro-optics has the same physics as cavity optomechanics, a cavity electro-optic modulator can realize various joint quantum operations on the microwave and optical fields, including coherent frequency conversion, laser cooling of microwave resonance, hybrid entanglement, hybrid parametric amplification and oscillation, and optical quantum-nondemolition measurements of microwave quadrature and energy.   

Hybrid Quantum Systems with Circuit Quantum Electrodynamics

Quantum Information Processing presents daunting challenges, with competing requirements of fast manipulation, long storage, and long distance transport of fragile quantum states. In aggregate, many of the challenges quantum computation have been met with nanosecond manipulations (quantum circuits), coherence times measured in seconds (atomic ions/nuclear spins), and entanglement transported over kilometers (linear optics), yet thus far no system has achieved all of the necessary components simultaneously. One promising direction is to leverage the best aspects of each system in a hybrid system, much as is done in a conventional computer, where transistors provide fast processing, magnetic memory provides massive long term storage, and information is transmitted via microwaves or fiber optics. A review of the constituent quantum systems, and the types of couplings between them will be presented. The coupling of a superconducting cavity/qubit system to electrons floating on helium will be discussed as an example of how to construct a hybrid system. Recent results on trapping and detection of electrons on helium using a superconducting cavity will be presented.   

Nonlinear optics quantum computing with circuit QED

One approach to quantum information processing is to use photons as quantum bits and rely on linear optical elements for most operations. However, some optical non-linearity is necessary to enable universal quantum computing. We consider a circuit-QED approach to linear optics quantum computing in the microwave regime, including a deterministic two-photon phase gate. Our model is a hybrid quantum system comprising an LC resonator coupled to a flux or phase superconducting qubit, which will be used to implement a non-linear two photon phase shift operation. Using this model, we show how fast, low-noise two-qubit gates between photons are possible, and discuss limitations of these ideas based on current technology.   

Towards Hybrid Quantum Information Processing with Electrons on Helium

Electrons above the surface of superfluid helium form a two-dimensional electron gas in which single-electron quantum dots can be defined using electrostatic gates submerged under the helium film. The quantized motion and spin state of such a trapped electron on helium can be coupled to a high finesse superconducting cavity in a hybrid circuit QED architecture [1]. The cavity is used for nondestructive readout and as a quantum bus mediating interactions between distant electrons or an electron and a superconducting qubit. Coupling between motional states and individual photons in the cavity is estimated at a Rabi frequency of $g/2\pi\sim 20$ MHz with coherence times exceeding 20 $\mu$s for charge and 1 s for spin [1, 2]. Here I will discuss recent experiments in which we successfully trap and detect a two-dimensional electron gas on helium in a dc-biased superconducting cavity. Experimental progress towards the single-electron regime will also be presented. \newline [1] D.I. Schuster, A.Fragner, M.I. Dykman, S. Lyon and R.J. Schoelkopf, Phys. Rev. Lett. 105, 040503 (2010) \newline [2] S. A. Lyon, Phys. Rev. A 74, 052338 (2006)   

An ab-initio microscopic theory of anomalous heating in planar ion traps

Anomalous heating of trapped ions limits the scalability of the planar trap architecture for quantum computation. Measurements of the electric field noise present in ion traps indicate that the noise-induced heating scales as the inverse fourth power of the distance from the trap electrodes to the ion and its spectral density scales with the inverse of frequency [1]. These measurements also suggest that some thermally activated random process is at work. In this work, we present an ab-initio theory of this noise due to oscillating dipoles on the trap electrode surface [2]. The dipoles are formed when atoms are adsorbed on the trap surface, whose interaction with the surface is described with density functional theory (DFT). We present calculations for the spectral noise density and its distance, frequency and temperature dependencies. We consider both independent and correlated dipoles.\\[4pt] [1] Q. A. Turchette et. al., Phys. Rev. A. 61, 63418 (2000).\\[0pt] [2] A. Safavi-Naini, P. Rabl, P. F. Weck, H. R. Sadeghpour, Phys. Rev. A. 84, 023412 (2011).   

Quantum logic for molecular quantum information processing

Very recently, molecular ions have been trapped and cooled to the mK regime in well defined internal states [1] opening a new window for precision spectroscopy of molecular species and quantum information with cold molecular ions. A first requirement for both applications is the ability to control and measure the state of molecular ions. I will present our proposal [2] of a fast, non-destructive and temperature independent spectroscopy method suitable to study electronic, vibrational, rotational and Zeeman transitions in complex ions that implements quantum logic schemes~[3] between an atomic ion and the molecular ion of interest, using optical forces on the atom, and optical forces or magnetic field gradients on the molecule. This method sets a starting point for a hybrid quantum computation scheme with molecular and atomic ions, covering the measurement and entangling steps. Finally I will discuss the remarkable decoherence properties of two Zeeman states of the $^{16}$O$_2^+$ molecular ion that make it a promising system for QIPC purposes~[4].\\[4pt] [1] X. Tong {\em et al.}, Phys. Rev. Lett. {\bf 105}, 143001 (2010).\\[0pt] [2] J. Mur-Petit {\em et al.}, arXiv:1106.3320\\[0pt] [3] P. O. Schmidt {\em et al.}, Science {\bf 309}, 749 (2005).\\[0pt] [4] J. Mur-Petit {\em et al.}, in preparation.   

Possibility of ``magic'' trapping of three-level system for Rydberg blockade implementation

The Rydberg blockade mechanism has shown noteworthy promise for scalable quantum computation with neutral atoms. Both qubit states and gate-mediating Rydberg state belong to the same optically-trapped atom. The trapping fields, while being essential, induce detrimental decoherence. Here we theoretically demonstrate that this Stark-induced decoherence may be completely removed using powerful concepts of ``magic'' optical traps. We analyze ``magic'' trapping of a prototype three-level system: a Rydberg state along with two qubit states, which are hyperfine states attached to a $J=1/2$ ground state. Our numerical results show that the group IIIB metals such as Al are suitable candidates. Such trapping may or may not be possible for the alkalis, as ``magic" conditions depend sensitively on the the trap-Rydberg interaction. Calculations of these effects are ongoing, and the results will be presented.   

Cooling in the single-photon regime of optomechanics

Optomechanics experiments are rapidly approaching the regime where the radiation pressure of a single photon displaces the mechanical oscillator by more than its zero-point uncertainty. We show that in this limit the power spectrum has multiple sidebands and that the cavity response has several resonances in the resolved-sideband limit [Phys.~Rev.~Lett.~\textbf{107}, 063602 (2011)]. We then discuss how red-sideband cooling is modified in this nonlinear regime. Using Fermi's Golden rule we calculate the transition rates induced by the optical drive. In the resolved-sideband limit we find multiple cooling resonances for strong single-photon coupling. They lead to non-thermal steady states and are accompanied by multiple mechanical sidebands in the optical output spectrum. Our study provides the tools to detect and take advantage of this novel regime of optomechanics.