Session B35: Circuit QED, Optomechanics and Hybrid Systems

Experimental Study of Short-Time Brownian Motion

We report our progress on the study of short-time Brownian motion of optically-trapped microspheres. In earlier work, we observed the instantaneous velocity of microspheres in gas and in liquid, verifying a prediction by Albert Einstein from 1907. We now report a more accurate test of the energy equipartition theorem for a particle in liquid. We also observe boundary effects on Brownian motion in liquid by setting a wall near the trapped particle, which changes the dynamics of the motion. We find that the velocity autocorrelation of the particle decreases faster as the particle gets closer to the wall.   

Loading an Optical Trap with Diamond Nanocrystals Containing Nitrogen-Vacancy Centers from a Surface

We present a simple and effective method of loading particles into an optical trap. Our primary application of this method is loading photoluminescent material, such as diamond nanocrystals containing nitrogen-vacancy (NV) centers, for coupling the mechanical motion of the trapped crystal with the spin of the NV centers. Highly absorptive material at the trapping laser frequency, such as tartrazine dye, is used as media to attach nanodiamonds and burn into a cloud of air-borne particles as the material is swept near the trapping laser focus on a glass slide. Particles are then trapped with the laser used for burning or transferred to a second laser trap at a different wavelength. Evidence of successful loading diamond nanocrystals into the trap presented includes high sensitivity of the photoluminecscence (PL) to the excitation laser and the PL spectra of the optically trapped particles   

Investigations of a voltage-biased microwave cavity for quantum measurements of nanomechanical resonators

Quantum electromechanical systems incorporating superconducting qubits have received extensive interest in recent years due to their promising prospects for studying fundamental topics of quantum mechanics such as quantum measurement, entanglement and decoherence in new macroscopic limits, also for their potential as elements in technological applications in quantum information network and weak force detector, to name a few. In this presentation we will discuss ours efforts toward to devise an electromechanical circuit to strongly couple a nanomechanical resonator to a superconductor qubit, where a high voltage dc-bias is required, to study quantum behavior of a mechanical resonator. Preliminary results of our latest generation of devices integrating a superconductor qubit into a high-Q voltage biased microwave cavities are presented. Developments in the circuit design to couple a mechanical resonator to a qubit in the high-Q voltage bias CPW cavity is discussed as well prospects of achieving single-phonon measurement resolution.   

A broadband reflective filter for applying dc biases to high-Q superconducting microwave cavities

The integration of dc-bias circuitry into low-loss microwave cavities is an important technical issue for topics in many fields that include research with qubit- and cavity-coupled mechanical system, circuit QED and quantum dynamics of nonlinear systems. The applied potentials or currents serve a variety of functions such as maintaining the operating state of device or establishing tunable electrostatic interactions between devices (for example, in order to couple a nanomechanical resonator to a superconducting qubit to generate and detect quantum states of a mechanical resonator). Here we report a bias-circuit design that utilizes a broadband reflective filter to connect to a high-Q superconducting coplanar waveguide (CPW) cavity. Our design allows us to apply dc-voltages to the center trace of CPW, with negligible changes in loaded quality factors of the fundamental mode. Simulations and measurements of the filter demonstrate insertion loss greater than 20 dB in the range of 3 to 10 GHz. Transmission measurements of the voltage-biased CPW show that loaded quality factors exceeding $10^5$ can be achieved for dc-voltages as high as V$= \pm$ 20V for the cavity operated in the single photon regime.   

Superconducting Coplanar Waveguide Resonators For A Hybrid Rydberg Atom-Superconductor Interface

Superconducting qubits achieve fast gate times ($\sim$ 1 ns); however, coherence times are relatively short ($\sim$ 10 $\mu$s). In contrast, atomic qubits based on Rydberg atoms achieve long coherence times of order 1 s, but are limited by slow gate times ($\sim$ 1 $\mu$s). Combining these disparate technologies in a hybrid quantum processor would provide both a long-lived memory and the ability to run computations quickly. Here we describe the design, fabrication, and characterization of superconducting coplanar waveguide resonators optimized to achieve strong coupling between the resonator mode and a single trapped Cs Rydberg atom. We discuss the dependence of resonator quality factor and coupling strength on device geometry and describe the integration of superconducting thin-film processing with MEMS-style thick film fabrication in order to increase the spatial extent of the resonator's electric field.   

Microwave frequency electromagnetic coupling to a thin membrane as one end of a cylindrical cavity

We demonstrate coupling of an 11.1 GHz radio frequency (RF) TE011 cylindrical cavity mode to the mechanical motion of a silicon nitride (Si3N4) membrane. The membrane is driven into motion through radiation pressure forces arising from the transverse magnetic field present at the membrane boundary. We use a cylindrical aluminum cavity where one end consists of a 500-nm thick Si3N4 membrane that has been sputtered with 300 nm of niobium (Nb). Cavity frequency tuning is controlled via an aluminum plunger attached to a micrometer at the other end of the cavity. The membrane is driven into motion by modulating the amplitude of the RF signal at the membrane's resonant frequency in the KHz range. The membrane's displacement is measured by means of a Michelson interferometer. We compare results from experimental runs utilizing both square and circular membrane geometries. This experiment shows that the TE011 mode gives rise to radiation pressure on the ends of a cylindrical cavity and demonstrates the feasibility of future work using high Q superconducting RF cavities to realize a dynamical Casimir effect (DCE) due to the membrane's motion at GHz frequencies.   

Motional sideband asymmetry in a quantum electro-mechanical device

Quantum electro-mechanical systems offer a unique opportunity to probe quantum noise properties in macroscopic devices, properties which ultimately stem from Heisenberg's uncertainty relations. A simple example of this is expected to occur in a microwave parametric transducer, where mechanical motion generates motional sidebands corresponding to the up and down frequency-conversion of microwave photons. Due to quantum vacuum noise, the rates of these processes are expected to be unequal. We measure this fundamental imbalance in a microwave transducer coupled to a radio-frequency mechanical mode, cooled near the ground state of motion. We also discuss the subtle origin of this imbalance: with linear detection of the output light field, the imbalance is most naturally attributed to the quantum fluctuations of the electromagnetic field.   

Tailoring the flow of light and sound in an optomechanical array

Recent progress in the field of optomechanics may soon allow the realization of optomechanical arrays, i.e. periodic arrangements of optical and vibrational modes whose interaction can be tuned in-situ by a laser. The most promising implementation is based on a simple setting, a dielectric slab with a suitable pattern of holes. The flow of light and sound in such a device could be tailored by engineering the laser wavefront, e. g. creating effective potential landscapes, tuning the phonon hopping range, or creating artificial gauge fields. We show that photons and phonons on a honeycomb lattice will produce an optically tunable Dirac-type band structure. Transport in such a system can exhibit transmission through an optically created barrier, similar to Klein tunneling, but with interconversion between light and sound.   

Testing Time Reversal Symmetry in Artificial Atoms

Over the past several decades, a rich series of experiments has repeatedly verified the quantum nature of superconducting devices, leading some of these systems to be regarded as artificial atoms. In addition to their application in quantum information processing, these ``atoms'' provide a test bed for studying quantum mechanics in macroscopic limits. Regarding the last point, we present here a feasible protocol for directly testing time reversal symmetry in a superconducting artificial atom. Time reversal symmetry is a fundamental property of quantum mechanics and is expected to hold if the dynamics of the artificial atom strictly follow the Schr\"odinger equation. However, this property has yet to be tested in any macroscopic quantum system. The test we propose is based on the verification of the microreversibility principle, providing a viable approach to verify quantum work fluctuation theorems - an outstanding challenge in quantum statistical mechanics. For this, we outline a procedure that utilizes the microreversibility test in conjunction with numerical emulations of Gibbs ensembles to verify these theorems over a large temperature range.   

Theory of on-chip integrated microwave-to-optical quantum coherent converter

We present a proposal for implementing a direct quantum electrooptical converter based on a nonlinear optical microresonator. The hybrid system combines planar superconducting microwave circuits and integrated nonlinear ultrahigh-Q crystalline whispering gallery mode (WGM) optical microresonator. It features low footprint and scalability. The electrooptical dynamics of the device is similar to that of a cavity quantum optomechanical system and Pockels effect is used as the coupling interaction. We simulate the system and show that high-conversion performance can be achieved with current technology. On-chip, such integrated device holds promise for use in large-scale quantum applications as a first direct microwave-to-optical quantum repeater.   

Ultrahigh Q Bulk Acoustic Wave Cavities at the Quantum Limit

A Fabry-Perot cavity is an optical resonator, which can store photons for milliseconds and enhance interaction between light and matter. The acoustics analogue (phonon trapping), is the Bulk Acoustic Wave device (in thin film or crystal lattice). Measurements provide the ultimate material loss regimes, minimizing clamping losses and achieving record high Q.f products [1], allowing observation of various loss mechanisms such as Landau-Rumer, phonon-phonon dissipation and Rayleigh phonon scattering, as well as previously non-observed non-linear effects [2]. This presentation will summarize our recent work towards cooling such modes to the ground state and operating the device at the Quantum Limit [3]. This includes the first measurements of the Nyquist noise near at 4K [4], as well as details on using such devices to test fundamental physics [5]. [1] M Goryachev etal, Observation of rayleigh phonon scattering through excitation of extremely high overtones in low-loss cryogenic acoustic cavities for hybrid quantum systems, PRL, 111 085502 2013 [2] M. Goryachev etal Jump chaotic behaviour of ultra low loss bulk acoustic wave cavities, APL 105 063501 2014 [3] M Goryachev, M Tobar, Effects of geometry on quantum fluctuations of phonon-trapping acoustic cavities, NJP 16 083007 2014 [4] M Goryachev etal, Observation of the Fundamental Nyquist Noise Limit in an Ultra-High Q-Factor Cryogenic Bulk Acoustic Wave Cavity, APL 105 153505 2014 [5] M Goryachev, M Tobar, Gravitational wave detection with high frequency phonon trapping acoustic cavities, PRD 2014   

Coherent coupling between ferromagnetic magnon and superconducting qubit

Coherent coupling between paramagnetic spin ensembles and superconducting quantum circuits is now widely studied for quantum memories and microwave-to-optical quantum transducers. Since those applications require strong coupling and sufficiently long coherence time simultaneously, collective excitation (magnon) in yttrium iron garnet (YIG), a typical ferromagnetic insulator, is an alternative promising candidate. The material is known to have a high spin density and a narrow ferromagnetic-resonance (FMR) linewidth. Recently, we achieved strong coupling between a 3D microwave cavity and the uniformly precessing spin mode. In this talk, we step forward to the control and measurement of magnons using superconducting circuits. We demonstrate coherent coupling between a magnon excitation in a millimeter-sized ferromagnetic sphere and a superconducting qubit, in which the interaction is mediated by a microwave cavity. We observe the coupling strength exceeding the damping rates, revealing that the system is in the strong coupling regime. Furthermore, we study a tunable coupling scheme using a microwave drive and the time-domain control of magnons. Our approach provides a versatile tool for control and measurement of the magnon excitations in the quantum regime.   

Novel techniques for strong coupling between spin ensembles and cavity resonators

Spins in solids are a promising physical subsystem for the realization of hybrid quantum systems. We focus on experiments coupling spins to three dimensional cavities, a system where it is critical to achieve operation in the strong coupling regime. This has been achieved using two approaches: coupling to impurity ions in single-crystal Whispering Gallery photonic resonators, and by using a novel field focusing re-entrant cavity. The first approach has allowed us to investigate various impurities in sapphire, quartz, and YAG, as well as iron group ions in YSO. This method is characterised by relatively narrow photon linewidths, higher filling factors and lower impurity concentration. The second approach allowed strong coupling to P1 impurities in diamond and operation in the ultra-strong coupling regime with magnons in YIG. This method is designed to achieve spatial separation of the cavity magnetic and electric fields, relatively high filling factors with sub-mm crystals of any shape and a high concentration of magnetic ions, as well as arbitrary engineering of the cavity spectrum and field distribution.   

High-frequency and multi-mode operation of substrate-free micromembrane resonator

Micromemembrane mechanical resonator is fabricated from stoichiometric silicon nitride and its resonant motions are actuated with electrical field gradient pumping method. Using electrical field gradient force by electrode deposited near the suspended structure, micromembrane resonator can be actuated without electrical components on the movable component. We can drive and investigate multiple modes of micromembrane up to 32th mode with 78 MHz resonant frequency by optical measurement technique. This membrane can be applicable to optical system compatible with cavity without external driving technique.