Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session K45: Hybrid Quantum Systems IIFocus

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Sponsoring Units: GQI DAMOP Chair: Mark Friesen, University of Wisconsin  Madison Room: 348 
Wednesday, March 16, 2016 8:00AM  8:12AM 
K45.00001: Hamiltonian simulation for improved state transfer and readout in cavity QED F\'elix Beaudoin, Alexandre Blais, William A. Coish Quantum state transfer into a memory, state shuttling over long distances via a quantum bus, and highfidelity readout are important tasks for quantum technology. Generating the Hamiltonians that realize these tasks is challenging in the presence of realistic couplings to an environment. Here, we use average Hamiltonian theory to design the desired Hamiltonians in cavity QED. In particular, we present a protocol for state transfer between a qubit and a cavity. This approach makes use of a controllable qubitcavity coupling strength to achieve a high fidelity even in the presence of inhomogeneous broadening that is stronger than the qubitcavity coupling strength. In addition, we design a timeaveraged interaction that allows for an improved quantum nondemolition readout. These ideas can be applied directly to propel novel systems coupling single spins to a microwave cavity into the strong coupling regime [Viennot et al, Science 349, 408 (2015)]. The approach can also be employed to improve quantum operations with spin ensembles. [Preview Abstract] 
Wednesday, March 16, 2016 8:12AM  8:24AM 
K45.00002: Quantum efficiency of a double quantum dot microwave photon detector Clement Wong, Maxim Vavilov Motivated by recent interest in implementing circuit quantum electrodynamics with semiconducting quantum dots, we study charge transfer through a double quantum dot (DQD) capacitively coupled to a superconducting cavity subject to a microwave field. We analyze the DQD current response using inputoutput theory and determine the optimal parameter regime for complete absorption of radiation and efficient conversion of microwave photons to electric current. For experimentally available DQD systems, we show that the cavitycoupled DQD operates as a photontocharge converter with quantum efficiencies up to $80$\% [Preview Abstract] 
Wednesday, March 16, 2016 8:24AM  8:36AM 
K45.00003: OpticalFiberIlluminated Response of a Superconducting Microwave Resonator Below 1 K Kristen Voigt, J. B. Hertzberg, S. K. Dutta, J. E. Hoffman, J. A. Grover, J. Lee, P. Solano, R. P. Budoyo, C. Ballard, J. R. Anderson, C. J. Lobb, S. L. Rolston, F. C. Wellstood As a step towards building a hybrid quantum system that couples superconducting elements to neutral atoms trapped on a tapered optical nanofiber, we have studied how the presence of the fiber dielectric and light scattered from a fiber affect the response of a translatable thinfilm lumpedelement superconducting Al microwave resonator that is cooled to 15 mK. The resonator has a resonance frequency of about 6 GHz, a quality factor Q 2 x 10$^{5}$, and is mounted inside a 3D Al superconducting cavity. An optical fiber is tapered to a 60 um diameter and passes through two small holes in the 3D cavity such that it sits near the resonator. The 3D cavity is mounted on an xz piezotranslation stage that allows us to change the relative position of the thinfilm resonator and fiber. When the resonator is brought closer to the fiber, the resonance frequency decreases slightly due to the presence of the fiber dielectric. When 200 uW of 780 nm light is sent through the fiber, about 100 pW/mm is Rayleighscattered from the fiber. This causes a positiondependent illumination of the resonator, affecting its resonance frequency and Q. We compare our results to a model of the resonator response that includes the generation, diffusion, and recombination of quasiparticles in the resonator and find that the frequency response allows us to track the position of the fiber to within 10 um. [Preview Abstract] 
Wednesday, March 16, 2016 8:36AM  9:12AM 
K45.00004: Connecting trapped ions and quantum dots with photons Invited Speaker: Michael Koehl Coupling individual quantum systems lies at the heart of building scalable quantum networks. Here, we report the first direct photonic coupling between a semiconductor quantum dot and a trapped ion and we demonstrate that single photons generated by a quantum dot controllably change the internal state of an Yb$^+$ ion. We ameliorate the effect of the sixtyfold mismatch of the radiative linewidths with coherent photon generation and a highfinesse fiberbased optical cavity enhancing the coupling between the single photon and the ion. The transfer of information presented here via the classical correlations between the $\sigma_z$ projection of the quantumdot spin and the internal state of the ion provides a promising step towards quantum statetransfer in a hybrid photonic network. [Preview Abstract] 
Wednesday, March 16, 2016 9:12AM  9:24AM 
K45.00005: Encoding a Qubit into a Cavity Mode in CircuitQED using Phase Estimation Barbara Terhal, Daniel Weigand Gottesman, Kitaev and Preskill have formulated a way of encoding a qubit into an oscillator such that the qubit is protected against small shifts (translations) in phase space. The idea underlying this encoding is that error processes of low rate can be expanded into small shift errors. The qubit space is defined as an eigenspace of two mutually commuting displacement operators which act as large shifts/translations in phase space. We propose and analyze the approximate creation of these qubit states by coupling the oscillator to a sequence of ancilla qubits realizing the protocol of approximate phase estimation for a displacement operator. We analyze the performance of repeated and adaptive phase estimation as the experimentally most viable schemes given a realistic upper limit on the number of photons in the oscillator. We propose a physical implementation of the protocol using the dispersive coupling between an ancilla transmon qubit and a cavity mode in circuitQED. We estimate that in a current experimental setup one can prepare a good code state from a squeezed vacuum state using 8 rounds of adaptive phase estimation lasting in total about 4 microsec., with at least 80% (heralded) chance of success. See http://arxiv.org/abs/1506.05033 [Preview Abstract] 
Wednesday, March 16, 2016 9:24AM  9:36AM 
K45.00006: Longdistance entanglement of spin qubits via quantum Hall edge states Guang Yang, ChenHsuan Hsu, Peter Stano, Jelena Klinovaja, Daniel Loss The implementation of a functional quantum computer involves entangling and coherent manipulation of a large number of qubits. For qubits based on electron spins confined in quantum dots, which are among the most investigated solidstate qubits at present, architectural challenges are often encountered in the design of quantum circuits attempting to assemble the qubits within the very limited space available. Here, we provide a solution to such challenges based on an approach to realizing entanglement of spin qubits over long distances. We show that longrange RudermanKittelKasuyaYosida interaction of confined electron spins can be established by quantum Hall edge states, leading to an exchange coupling of spin qubits. The coupling is anisotropic and can be either Isingtype or XYtype, depending on the spin polarization of the edge state. Such a property, combined with the dependence of the electronspin susceptibility on the chirality of the edge state, can be utilized to gain valuable insights into the topological nature of various quantum Hall states. [Preview Abstract] 
Wednesday, March 16, 2016 9:36AM  9:48AM 
K45.00007: Entangling distant resonant exchange qubits via circuit quantum electrodynamics Vanita Srinivasa, Jacob M. Taylor, Charles Tahan Enabling modularity within a quantum information processing device relies on robust entanglement of coherent qubits at macroscopic distances. To address this challenge, we investigate theoretically a hybrid quantum system consisting of spatially separated resonant exchange qubits, defined in threeelectron semiconductor triple quantum dots, that are coupled via a superconducting transmission line resonator. By analyzing three specific approaches drawn from circuit quantum electrodynamics and HartmannHahn double resonance techniques for implementing resonatormediated twoqubit entangling gates in both dispersive and resonant regimes, we show that methods for entangling superconducting qubits map directly to resonant exchange qubits. We also calculate the rate of relaxation via phonons for resonant exchange qubits in silicon triple dots and show that such an implementation is particularly wellsuited to achieving the strong coupling regime. Our approach combines the robustness of encoded spin qubits in silicon with the rapid and robust longrange entanglement provided by circuit QED systems. [Preview Abstract] 
Wednesday, March 16, 2016 9:48AM  10:00AM 
K45.00008: Long distance coupling of resonant exchange qubits Maximilian Russ, Guido Burkard We investigate the effectiveness of a microwave cavity as a mediator of interactions between two resonant exchange (RX) qubits\footnote{J. Medford et al., Phys. Rev. Lett. 111, 050501 (2013)}\footnote{J. M. Taylor et al., Phys. Rev. Lett. 111, 050502 (2013)} in semiconductor quantum dots (QDs) over long distances\footnote{M. Russ and G. Burkard, arXiv: 1508.07122 (2015)}, limited only by the extension of the cavity. Our interaction model includes the orthonormalized Wannier orbitals constructed from FockDarwin states under the assumption of a harmonic QD confinement potential. We calculate the qubitcavity coupling strength $g_r$ in a Jaynes Cummings Hamiltonian, and find that dipole transitions between two states with an asymmetric charge configuration constitute the relevant RX qubitcavity coupling mechanism. The effective coupling between two RX qubits in a shared cavity yields a universal twoqubit iSWAPgate with gate times on the order of nanoseconds over distances on the order of up to a millimeter. Funded by ARO through grant No. W911NF1510149. [Preview Abstract] 
Wednesday, March 16, 2016 10:00AM  10:12AM 
K45.00009: Injection locking of a semiconductor doublequantumdot micromaser Y.Y. Liu, J. Stehlik, M. J. Gullans, J. M. Taylor, J. R. Petta Narrow linewidth lasers and masers are desirable for applications such as frequency standards and lownoise amplifiers. Recently we have demonstrated a doublequantumdot (DQD) micromaser, which generates photons through single electron tunneling events.\footnote{ Y.Y. Liu, J. Stehlik, C. Eichler, M. J. Gullans, J. M. Taylor, and J. R. Petta, Science \textbf{347}, 285 (2015).}$^{\mathrm{\thinspace }}$Charge noise couples to the DQD energy levels and results in a maser linewidth that is 100 times larger than the SchawlowTownes prediction. We demonstrate linewidth narrowing by more than a factor of 10 using injection locking. The injection locking range is measured as a function of input power and shown to be in excellent agreement with the Adler equation. The position and amplitude of distortion sidebands that appear outside of the injection locking range are quantitatively examined. Our results show that this unconventional maser, which is impacted by strong charge noise and electronphonon coupling, is well described by standard laser models.\footnote{ Y.Y. Liu, J. Stehlik, M. J. Gullans, J. M. Taylor, and J. R. Petta, Phys. Rev. A (in press).} [Preview Abstract] 
Wednesday, March 16, 2016 10:12AM  10:24AM 
K45.00010: Realtime tuning of a double quantum dot using a Josephson parametric amplifier J. Stehlik, Y.Y. Liu, C.~M. Quintana, C. Eichler, T. R. Hartke, J. R. Petta Josephson parametric amplifiers (JPAs)\footnote{B.~Yurke, J.~Opt.~Soc.~Am.~B, \textbf{4} 1551 (1987).} have enabled advances in readout of quantum systems. Here we demonstrate JPAassisted readout of a cavitycoupled double quantum dot (DQD).\footnote{ J.~Stehlik \textit{et al.}, Phys.~Rev.~Appl., \textbf{4} 014018 (2015).} Utilizing a JPA we improve the signaltonoise ratio (SNR) by a factor of 2000 compared to the situation with the parametric amplifier turned off. At an interdot charge transition we achieve a SNR of 76 (19 dB) with an integration time $\tau = 400$ ns, which is limited by the linewidth of our cavity. By measuring the SNR as a function of $\tau$ we extract an equivalent charge sensitivity of $8 \times 10^{5} \:\:e/\sqrt{\rm Hz}$. We develop a dualgatevoltage rastering scheme that allows us to acquire a DQD charge stability diagram in just 20 ms. Such rapid data acquisition rates enable device tuning in live ``videomode," where the results of parameter changes are immediately displayed. Live tuning allows the DQD confinement potential to be rapidly tuned, a capability that will become increasingly important as semiconductor spin qubits are scaled to a larger number of dots. [Preview Abstract] 
Wednesday, March 16, 2016 10:24AM  10:36AM 
K45.00011: Investigating the level broadening of a semiconductor charge qubit in microwave emission measurements A. Stockklauser, N. Hedrich, V. F. Maisi, J. Basset, K. Cujia, C. Reichl, W. Wegscheider, T. Ihn, K. Ensslin, A. Wallraff We investigate a hybrid circuit quantum electrodynamics architecture in which a double quantum dot charge qubit is coupled to a nearby microwave cavity. The discussed experiments explore the emission of microwave radiation from a voltagebiased GaAs double dot similar to Ref. [1]. A superconducting coplanar waveguide resonator serves as a tool to study aspects of the quantum dot level structure that are difficult to access in transport measurements. We explore resonances in microwave emission that arise from inelastic interdot transitions resonant with the cavity [2]. In particular, the line width of the emission resonances is investigated and linked to the level broadening of the double dot charge qubit. We study the dependence of the emission line width on the tunnel rates to the leads and identify this as the dominant contribution to the broadening of the qubit levels. For the explored bias conditions qubit decoherence is low in comparison. We extract the tunnel rates to the leads from the linewidth of the emission signal and compare it with the tunnel rates extracted from current measurements. \newline [1] Y.Y.~Liu {\em et al.}, Phys.~Rev.~Lett.~{\bf 113}, 036801 (2014).\newline [2] A.~Stockklauser {\em et al.}, Phys.~Rev.~Lett.~{\bf 115}, 046802 (2015).\newline [Preview Abstract] 
Wednesday, March 16, 2016 10:36AM  10:48AM 
K45.00012: Observation of magnon number states in a superconducting qubit spectrum Dany LachanceQuirion, Yutaka Tabuchi, Seiichiro Ishino, Atsushi Noguchi, Toyofumi Ishikawa, Rekishu Yamazaki, Koji Usami, Yasunobu Nakamura A quantum transducer interfacing qubits in the microwave domain to optical light requires a quantum system interacting with photons of both frequency domains. Coherent interaction between collective excitations (magnons) in the ferrimagnetic insulator yttrium iron garnet (YIG) and a superconducting qubit through virtual microwave photons has recently been demonstrated [1]. In this talk, we present results on the observation of magnon number states in a superconducting qubit spectrum when creating a coherent state in a magnetostatic mode of a YIG sphere interacting dispersively with the qubit. The dispersive interaction strength of 1.2~MHz measured in the straddling regime is in good agreement with numerical simulations. Furthermore the probability distribution of magnon number states, recovered from the qubit spectrum, is compared with the Poisson distribution expected for a coherent state. Resolving magnon number states constitutes a first step toward encoding quantum information into a quantum state of a magnetostatic mode [2].\\[4pt] [1] Y. Tabuchi, S. Ishino, A. Noguchi, T. Ishikawa, R. Yamazaki, K. Usami, and Y. Nakamura, Science 349, 405 (2015).\\[2pt] [2] Z. Leghtas, G. Kirchmair, B. Vlastakis, M. Devoret, R. Schoelkopf, and M. Mirrahimi, Phys. Rev. A 87, 042315 (2013). [Preview Abstract] 
Wednesday, March 16, 2016 10:48AM  11:00AM 
K45.00013: Emergent Curved space induced by adiabatic approximation Ran Cheng, Xiaochuan Wu, Di Xiao Berry curvature, serving as the imaginary part of quantum geometric tensor (QGT), gives rise to an effective Lorentz force to the dynamics of the adiabatic parameter. However, it is not clear whether the real part of QGT, the quantum metric, has any dynamical consequence as the Berry gauge force. We show in a general way that during an adiabatic process, the particle in a hybrid quantum system governed by an equation of motion second order in time is subject to an induced gravitational force. The adiabatic dynamics can be described by a geodesic equation as if the spacetime is curved by the quantum metric. As an example, we demonstrate the above result in a simple toy model. [Preview Abstract] 
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