Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session K40: Focus Session: Materials for Quantum Computing III |
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Sponsoring Units: DMP Chair: Seongshik Oh, National Institute of Standards and Technology-Boulder Room: Baltimore Convention Center 343 |
Tuesday, March 14, 2006 2:30PM - 3:06PM |
K40.00001: Decoherence in Josephson Qubits Invited Speaker: The Josephson junction can be thought of as an artificial “atom,” with energy levels determined by the circuit design parameters and bias. This system shows great promise for quantum computing, since it should be straightforward to scale to many-qubit circuits using standard integrated circuit technology. To date, however, device performance has been severely limited by coupling of the qubit to spurious materials defects. Here I discuss a recent breakthrough which has enabled striking improvements in phase qubit coherence and visibility, and which has deep implications for other Josephson qubits. I present a model of qubit decoherence induced by two-level defect states, and describe an intimate connection between intrinsic (low-temperature and low-power) dielectric loss and qubit performance: coupling to individual defects in the tunnel barrier of the Josephson junction results in a loss of visibility of coherent qubit oscillations, while coupling to a continuum of defects in the wiring dielectric leads to energy relaxation from the qubit $|$1$>$ to the $|$0$>$ state. Optimization of the phase qubit proceeds along two lines. The first approach involves a novel circuit architecture which promotes statistical avoidance of resonant defects, and leads to dramatic enhancements in qubit visibility and measurement fidelity. The second approach is to explore novel high-Q dielectrics for qubit circuits. Recent improvements in materials have led to a factor of 30 increase in qubit coherence time. I describe progress in the development of novel epitaxial dielectrics grown on epitaxial refractory metal underlayers for qubit applications. These results open the door to the realization of many-qubit algorithms in superconducting circuits. [Preview Abstract] |
Tuesday, March 14, 2006 3:06PM - 3:18PM |
K40.00002: Developing Phase Qubits Without Dielectric Materials R.W. Simmonds, M.S. Allman, K. Cicak, Jeffrey S. Kline, Seongshik Oh, K.D. Osborn , G. Prokopenko, M.A. Sillanpaa, A.J. Sirois, J.A. Strong, J.D. Whittaker, D.P. Pappas Recently amorphous insulating materials have been found to be detrimental to the energy retention of Josephson phase qubits. Most of this amorphous material has presently been used as an insulating layer between two wiring layers used in fabricating trilayer based Josephson phase qubits. In an effort to improve phase qubit performance we have developed two new fabrication techniques in order to produce phase qubits that do not require insulating layers. Here we will describe the fabrication processes and recent measurements on these systems. [Preview Abstract] |
Tuesday, March 14, 2006 3:18PM - 3:30PM |
K40.00003: Using Phase Qubits to Evaluate Dielectrics Materials J.A. Strong, M.S. Allman, K. Cicak, Jeffrey S. Kline, Seongshik Oh, K.D. Osborn, G. Prokopenko, M.A. Sillanpaa , A.J. Sirois , J.D. Whittaker, John M. Martinis, D.P. Pappas, R.W. Simmonds It has been known since the late 1970's that Amorphous insulating materials contain defects that can be modeled as a bath of interacting two-level fluctuators. Measurements of LC oscillator microwave circuits have identified these two-level systems as an inherent energy loss mechanism in Josephson phase qubits. By developing methods to reduce the volume of dielectric material in the fabrication of Josephson phase qubits, we have been able to conclusively show that the energy relaxation time can be improved in these systems. Furthermore, by using a different dielectric material with less intrinsic low temperature loss, it is possible to further improve this situation. In addition, we have fabricated new phase qubits which have strong coupling to substrate materials in an effort to evaluate loss from different substrates. We show that substrate materials with high crystalinity result in qubits with improved relaxation times. [Preview Abstract] |
Tuesday, March 14, 2006 3:30PM - 3:42PM |
K40.00004: Further Microwave Resonator Studies of Loss Mechanisms to Improve Qubits K.D. Osborn, M.S. Allman, K. Cicak, Jeffrey S. Kline, Seongshik Oh, G. Prokopenko, M.A. Sillanpaa, A.J. Sirois, J.A. Strong, J.D. Whittaker, John M. Martinis, D.P. Pappas, R.W. Simmonds Microwave resonators have been previously used to identify materials that limit the coherence time of Josephson phase qubits. For example, amorphous silicon dioxide was found to exhibit an unsaturated loss tangent of 0.005 in resonators at low temperatures. Minimized use of this dielectric has shown improved phase qubit performance as well as using lower loss materials like silicon-nitride. Furthermore, superconducting aluminum, the major superconductor used in many present types of qubits, forms a native surface oxide which may also contribute to dielectric loss. To prevent any native oxides on superconducting circuits, we have fabricated resonators with superconducting wiring that has been covered with a thin layer of gold. We will measure the power dependence of resonant peaks in these circuits in order to determine losses in these systems as well as investigate various substrate materials and compare these results to losses from silicon-oxide and silicon-nitride. [Preview Abstract] |
Tuesday, March 14, 2006 3:42PM - 3:54PM |
K40.00005: Ohmic and step noise from a single trapping center hybridized with a Fermi sea$^{\dag}$ Rogerio de Sousa, K. Birgitta Whaley, Frank K. Wilhelm, Jan von Delft We show that single electron tunneling devices such as the Cooper-pair box or double quantum dot can be sensitive to the zero-point fluctuation of a single trapping center hybridized with a Fermi sea. If the trap energy level is close to the Fermi sea and has line-width $\gamma > k_BT$, its noise spectrum has an Ohmic Johnson-Nyquist form, whereas for $\gamma < k_B T$ the noise has a Lorentzian form expected from the semiclassical limit. Trap levels above the Fermi level are shown to lead to steps in the noise spectrum that can be used to probe their energetics, allowing the identification of individual trapping centers coupled to the device. \newline $^\dag$R. de Sousa, K.B. Whaley, F.K. Wilhelm, and J. von Delft, Phys. Rev. Lett. in press; cond-mat/0504149. [Preview Abstract] |
Tuesday, March 14, 2006 3:54PM - 4:06PM |
K40.00006: Quasiparticle Poisoning in a Cooper-Pair Box B. S. Palmer, C. A. Sanchez, A. Naik, M. A. Manheimer, J. F. Schneiderman, P. M. Echternach, F. C. Wellstood We have used a single-electron transistor (SET) to measure the Coulomb staircase of a single Cooper-pair box (CPB) from a temperature of 30 mK to 300 mK. At the lowest temperature, the data shows that the CPB, which is fabricated from Al/AlO$_{\mbox{x}}$/Al tunnel junctions, is poisoned by nonequilibrium quasiparticles. As the temperature is increased from 30 to 150 mK, the width of the odd step in the staircase, which corresponds to a quasiparticle on the island of the box, decreases linearly with temperature. Above 180 mK, the width of the odd step increases, eventually producing a staircase with 1\textit{e} steps. The low-temperature poisoning is consistent with the assumptions of Aumentado \textit{et al.} that quasiparticles are spontaneously generated in the leads.\footnote{J. Aumentado, M. Keller, J. Martinis, \& M. Devoret, Phys. Rev. Lett. \textbf{92}, 066802 (2004).} For particular gate voltages it is energetically favorable to have a nonequilbrium quasiparticle occupy a state on the island; hence poisoning the pure 2\textit{e} staircase. The data above 180 mK is consistent with the quasiparticle states of the island being thermally populated. [Preview Abstract] |
Tuesday, March 14, 2006 4:06PM - 4:18PM |
K40.00007: Kinetics of the superconducting charge qubit in the presence of a quasiparticle R. Lutchyn, L. Glazman, A. Larkin We investigate the energy and phase relaxation of a superconducting qubit caused by a quasiparticle. In our model, the qubit is an isolated system consisting of a small island (Cooper-pair box) and a larger superconductor (reservoir) connected by a Josephson junction. If such system contains an odd number of electrons, then even at lowest temperatures a single quasiparticle is present in the qubit. The quasiparticle resides in the reservoir with an overwhelming probability, but its quick round-trips to the box lead to the relaxation of the qubit. We derive master equations governing the evolution of the qubit coherences and populations. We find that the kinetics of the qubit can be characterized by two time scales - quasiparticle escape time from reservoir to the box $\Gamma^{-1}_{in}$ and quasiparticle relaxation time $\tau$. The former is determined by the normal-state conductance $g_{_T}$ of the Josephson junction and one-electron level spacing $\delta_r$ in the reservoir ($\Gamma_{in}\!\sim\!g_{_T}\delta_r$), and the latter is due to electron-phonon interaction. The phase coherence is damped on the time scale of $\Gamma^{-1}_{in}$. The qubit energy relaxation depends on the ratio of the two characteristic times, $\tau$ and $\Gamma^{-1}_{in}$, and also on the ratio of temperature $T$ to the Josephson energy $E_{_J}$. In the limit $\Gamma_{in}\tau\! \gg\! 1$ and $T\!\ll\! E_{_J}$, the relaxation of the qubit populations occurs in two stages. In the first stage, $\!t \!\sim\!1/g_{_T}\delta_r\!$, the initial population of the excited state changes only by a small amount $\!\sim\!(T/E_{_J}\!) ^ {1/2} \!$. This quasi-stationary state relaxes to full equilibrium over a longer time scale $t\!\sim\!\tau\!(E_{_J}/T\!)^{1/2}\!$. [Preview Abstract] |
Tuesday, March 14, 2006 4:18PM - 4:30PM |
K40.00008: Charge motion near metallic single electron transistors on oxidized Si substrates K. R. Brown, L. Sun, B. E. Kane Many proposals for spin qubits in semiconductors rely on spin-charge conversion combined with charge measurement for determination of the final state. In pursuit of such a measurement for donor spins in Si we have performed systematic experiments using Al-AlO$_x$-Al single electron transistors (SETs) on doped, oxidized Si substrates. One of the first priorities has been to identify other sources of charge motion that could disguise or overwhelm the signals from donors. We have identified reproducible peaks in the ac susceptibility of our samples as a function of electric field, similar to the response that would be expected from donor electrons moving between two different states. Nevertheless, preliminary results indicate that these peaks are associated with defects above the interface and not with charge motion in the Si itself. We will discuss planned device refinements to eliminate these and other defects and to isolate donor electron signals. [Preview Abstract] |
Tuesday, March 14, 2006 4:30PM - 4:42PM |
K40.00009: Design of a metallic SET gated by lateral Schottky gates for measurements of electric-field-dependent ionization of donors Luyan Sun, K. R. Brown, B. E. Kane Spins associated with donors in silicon are ideally suited as qubits for a solid state quantum computer due to their long coherence times and potential scalability. One method for spin measurement incorporates electric-field-dependent ionization of two-electron systems to distinguish singlet and triplet spin states. Therefore the electric field at donor sites needs to be known accurately. The electric field can be applied via a heavily doped back gate, but sharp density profiles are difficult to obtain both with ion implantation (due to straggle) and with molecular beam epitaxy. To resolve this issue, we have fabricated lateral PtSi Schottky gate devices. Schottky gates should make the transition from conducting layer to intrinsic Si far more abrupt. They can also tune the Fermi level in the substrate, so that we can populate donors in the substrate by applying an appropriate bias and shining an LED. By studying the Coulomb blockade peak spacing of an Al/AlO$_{x}$/Al SET while sweeping a nearby Schottky gate, we can identify the flat band condition. We will present preliminary data for the filling and emptying of Si/SiO$_{2}$ interface states and for the determination of electric field below the SET island from such devices. [Preview Abstract] |
Tuesday, March 14, 2006 4:42PM - 4:54PM |
K40.00010: Experimental demonstration of an oscillator stabilized Josephson flux qubit R. H. Koch, G. A. Keefe, F. P. Milliken, J. R. Rozen, C. C. Tsuei, J. R. Kirtley, D. P. DiVincenzo We experimentally demonstrate the use of a superconducting transmission line, shorted at both ends, to stabilize the operation of a tunable flux qubit. Our qubit consists of three Josephson junctions and three loops coupled to a fixed-length superconducting transmission line. The bare qubit has two control parameters, the flux and the control flux. This allows the qubit to have a tunable difference frequency between the ground and first excited states and at the same time to be biased at a degenerate point with respect to the flux parameter. This condition can be met for a wide range of junction critical currents. This flexibility of our structure is a very desirable property for a scalable qubit. To stabilize the operation of our qubit and increase its coherence time, we couple the bare qubit to the lowest mode of a superconducting transmission line, which we model as a harmonic oscillator. Using harmonic oscillator stabilization and pulsed dc operation, we have observed Larmor oscillations with a single shot visibility of 90 percent and a coherence time of 100 ns. In another qubit the visibility was 60 percent and there was no measurable visibility reduction after 35 ns. [Preview Abstract] |
Tuesday, March 14, 2006 4:54PM - 5:06PM |
K40.00011: Design and Implementation of Devices for Flux Qubit Entanglement Experiments Paul Reichardt, Travis Hime, Britton Plourde, Timothy Robertson, Cheng-En Wu, Alexey Ustinov, John Clarke We report measurements on two superconducting flux qubits coupled to a readout Superconducting QUantum Interference Device (SQUID). The chosen device parameters allow for the implementation of a fast, controllable qubit coupling scheme based on variations in the current bias of the readout SQUID in the zero-voltage state. The devices have Al-AlOx-Al tunnel junctions and were fabricated with e-beam lithography on a single substrate. Two on-chip flux bias lines allowed independent flux control of any two of the three elements. By applying microwave radiation, we observed resonant excitation of each qubit and thereby individually mapped out energy dispersions for both qubits. These dispersions displayed the expected hyperbolic dependence with tunnel splittings of 9.0 $\pm $ 0.2 GHz, which agreed well with the calculated and measured device parameters. Single qubit coherence properties including relaxation times, Rabi oscillations, Ramsey fringes, and echoes were also measured. [Preview Abstract] |
Tuesday, March 14, 2006 5:06PM - 5:18PM |
K40.00012: Variable Coupling of Two Flux Qubits T. Hime, P.A. Reichardt, B.L.T. Plourde, T.L. Robertson, C.-E. Wu, A.V. Ustinov, John Clarke We report observations of variable coupling of two flux qubits. The qubits are coupled inductively to each other and to a readout Superconducting QUantum Interference Device (SQUID). By applying microwave radiation to the device, we observed resonant absorption in each of the qubits when the level splitting in the qubit matched the energy of the microwave photons. Using the two on-chip flux bias lines we adjusted the bias of each qubit so that the energy levels of the two qubits were equal; we then observed a splitting of the resulting absorption peak characteristic of coupling between the qubits. We varied the coupling between the qubits by changing the current bias in the SQUID in the zero voltage state, thereby changing its dynamic inductance and thus modifying the effective mutual inductance between the qubits. We compare the resulting changes in splitting with our predictions. This controllable coupling should be extendable to many qubits. [Preview Abstract] |
Tuesday, March 14, 2006 5:18PM - 5:30PM |
K40.00013: Actively Tuned and Spatially Trapped Polaritons Ryan Balili, David Snoke, Loren Pfeiffer, Kenneth West The resulting eigenstate of the strong coupling of light and excitons in a two-dimensional semiconductor microcavity produces the quasi-particles called polaritons. Owing to their light mass and bosonic character, these particles are predicted to Bose condense at much higher temperatures and lower densities than their atomic counterparts. However, standard methods of producing strongly coupled semiconductor microcavities are very inefficient. Only tiny regions of the microcavity wafer end up in the strong coupling regime due to the wedge of the layer thicknesses formed during the growth process. Here we present a method to actively control the exciton coupling with cavity photon modes and at the same time create an in-plane spatial trap for polaritons, which is necessary for two-dimensional BEC. The exciton energy of quantum well excitons in a semiconductor microcavity is actively tuned using applied stress. Starting with the quantum well exciton energy higher than the cavity photon mode, stress is used to reduce the exciton energy and bring it into resonance with the photon mode. At the point of zero detuning, line narrowing and strong increase of the photoluminescence are seen. By the same means, an in-plane harmonic potential for the polaritons is created, which allows trapping, potentially making possible BEC of polaritons analogous to trapped atoms. Drift of the polaritons into this trap is also demonstrated. [Preview Abstract] |
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