Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session P43: Focus Session: Materials for Quantum Information Processing III |
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Sponsoring Units: DMP Chair: Ray Simmonds, National Institute of Standards and Technology, Boulder Room: Colorado Convention Center 506 |
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P43.00001: Complete stabilization and improvement of the characteristics of tunnel junctions by thermal annealing Ilari Maasilta, Panu Koppinen, Lasse Vaisto We have observed that submicron sized Al--AlO$_x$--Al tunnel junctions can be stabilized completely by annealing them in vacuum at temperatures between $350^{\circ}$C and $450^{\circ}$C. In addition, low temperature characterization of the samples after the annealing show a marked improvement of the tunneling characteristics, by disappearance of unwanted resonances in the current. Charging energy, tunneling resistance, barrier thickness and height all increase after the treatment. The superconducting gap is not affected, but supercurrent is reduced in accordance with the increase of the tunneling resistance. A useful application of the annealing is in increasing the sensitivity of Josephson junction threshold current detectors, currently used for example in superconducting quantum bit readouts. It is also expected that all other barrier dependent characteristics will also improve (e.g. critical current noise). [Preview Abstract] |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P43.00002: Gating a two dimensional electron gas in silicon using a metallic single electron transistor Luyan Sun, K.R. Brown, B.E. Kane A wealth of physical phenomena has been observed in two dimensional electron systems such as the silicon metal-oxide-semiconductor field effect transistor (MOSFET). Due to impurities and interface states, a silicon MOSFET channel is usually imperfect. A single electron transistor (SET) close to the channel could provide a useful probe of these imperfections and of the channel behavior. We have incorporated an Al/AlO$_{x}$/Al SET as the top gate of a conventional MOSFET. The SET is fabricated with standard electron-beam lithography and double-angle thermal evaporation. A thermally grown SiO$_{2}$ barrier layer about 20 nm thick isolates the SET from the lightly p-doped MOSFET channel beneath. The drain and source of the MOSFET are heavily n-doped and conduct at cryogenic temperatures. A nearby surface metal gate is used to modulate the width of the channel right beneath the SET island. Near the pinch off regime we expect to see a correlation between fluctuations in the current through the SET and fluctuations in the current of the MOSFET channel. We will present preliminary data from these devices. [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P43.00003: Fluctuating Potentials In Micrometer Scale Atomic Ion Traps J. Britton, S. Seidelin, J. Chiaverini, R. Reichle, J.J. Bollinger, D. Leibfried, J.H. Wesenberg, R.B. Blakestad, R.J. Epstein, N. Shiga, J.M. Amini, K.R. Brown, J.P. Home, D.B. Hume, W.M. Itano, J.D. Jost, C. Langer, R. Ozeri, D.J. Wineland Electromagnetic confinement of atomic ion qubits coupled with laser cooling has permitted observation of 10 minute coherence times [1, 2]. Recent work to miniaturize electromagnetic traps promises qubit densities attractive for large scale quantum computing [3]. However, motional heating resulting from poorly understood fluctuating trapping potentials is observed to increase as approximately dimensions$^{-4}$ [4]. We discuss efforts to suppress this heating and present experimental results for several microtrap fabrication techniques [5, 6]. [1] P. T. H. Fisk et al., IEEE Trans. Instrum. Meas. 44, 113 (1995). [2] J. J. Bollinger et al., IEEE Trans. Instrum. Measurement 40, 126 (1991). [3] A. Steane, quant-ph/0412165. [4] L. Deslauriers et al., Phys. Rev. Lett. 97, 103007 (2006). [5] S. Seidelin et al., Phys. Rev. Lett. 96, 253003 (2006). [6] J. Britton et al., quant-ph/0605170. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:27PM |
P43.00004: Oxide-Semiconductor Materials for Quantum Computation Invited Speaker: In this talk I will describe efforts to create a quantum information processor using ferroelectrically coupled electron spins in silicon. The constituent material systems are Ge quantum dots, whose size must be compatible with storage of single electrons, and and whose spacing must allow for significant spin exchange to occur. Epitaxial ferroelectric oxides must be capable of rectifying light to allow for optical gating of spin interactions. ~Progress toward these goals, pursued within the Center for Oxide-Semiconductor Materials for Quantum Computation (COSMQC), will be described. This work is supported by DARPA QuIST through ARO contract number DAAD-19-01-1-0650. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P43.00005: Ultra-efficient electron transfer above micro-channels of superfluid helium F.R. Bradbury, Guillaume Sabouret, Shyam Shankar, S.A. Lyon The spin of an electron, bound by its image charge to the surface of superfluid helium, is a promising two level system for quantum information processing. The ability to efficiently move these qubits is one of the key promises of this technology since it would allow for the large scale integration required for quantum computation. We have fabricated 60 parallel channels, 10 $\mu $m wide by 3 $\mu $m deep, which fill with helium through capillary action. The channels are reactive ion etched into a thin polymer layer. Electrons are photoemitted into the vacuum above the device, attracted to it electrostatically, and moved laterally within the channels by underlying electrostatic gates. The electrons are measured capacitively as a sequence of voltages (clocking sequence) is applied to the gates. Results show that initially there are $\sim $3 electrons per channel and the signal decays exponentially (due to transfer failures) with number of clocking cycles. At frequencies as high as 800 kHz the charge transfer efficiency is 0.99999992 $\pm $ 6\textbullet 10$^{-8}$. This ability to reliably transport electrons makes them a very attractive spin qubit. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P43.00006: Epitaxial growth of V and MgO films for Josephson junction qubits Jeffrey Kline, Seongshik Oh, David Pappas The growth of ultrathin epitaxial aluminum oxide tunnel barriers on rhenium has been proven to reduce the number of spurious resonators in Josephson phase qubits when compared to qubits fabricated with amorphous tunnel barriers. Other epitaxial tunnel barrier materials such as MgO may also improve device performance. The superconductor V is latticed matched to MgO and was studied in this work. Vanadium films were deposited on MgO(001) substrates by UHV magnetron sputtering in argon gas. Magnesium oxide tunnel barriers were deposited by reactive sublimation of magnesium in a controlled oxygen background. To achieve epitaxy, the substrate was held at elevated temperature during the deposition. Surface science characterization tools such as Auger electron spectroscopy, scanning tunneling microscopy, and reflection high energy electron diffraction were used for in-situ analysis of the films. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P43.00007: Investigating energy loss in substrates of gigahertz LC resontaors J.D. Whittaker, K.D. Osborn, A.J. Sirois, R.W. Simmonds Dissipation in superconducting qubits is a significant obstacle to the realization of a superconducting quantum computer. One source of dissipation is through coupling to two-level system defects in the substrate on which qubits are fabricated. To study this effect, loss measurements on LC resonators fabricated on bulk silicon were compared to those fabricated on thin silicon nitride membranes, where much of the substrate material has been removed. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P43.00008: Shadow Evaporated Josephson Junctions for superconducting qubits Fabio Altomare, Jos\'{e} Aumentado, Kevin Osborn, Joshua Strong, Raymond Simmonds Superconducting circuits are a promising system for the implementation of quantum computing. At present two-level system defects in junctions create a formidable obstacle for superconducting qubits. As shown previously, juctions of reduced size have fewer defects. Making high quality utra-small Josephson junctions is crucial for futher progress. In this talk we will discuss how we define and assess the quality of our shadow evaporated Josephson junctions. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P43.00009: Role of tunnel barrier crystallinity in the coherence properties of superconducting phase qubits Seongshik Oh, Jeffrey Kline, Mika Sillanpaa, Adam Sirois, Katarina Cicak, Kevin Osborn, Raymond Simmonds, David Pappas The standard amorphous AlO$_{x}$ tunnel barriers in superconducting qubits contain many nanoscopic fluctuators. These nanoscopic fluctuators destroy the coherent quantum information stored in the qubit. Recently, we eliminated 80{\%} of these fluctuators using single-crystal Al$_{2}$O$_{3}$ tunnel barriers. This clearly shows that the tunnel barrier crystallinity is important for coherent superconducting qubits. Along this line, we started investigating another well-known crystalline tunnel barrier, MgO. Unlike the aluminum oxide tunnel barrier, which requires $\sim $800 \r{ }C for crystallization, the MgO tunnel barrier grows crystalline even at room temperature. We will compare the Josephson junctions and the superconducting phase qubits made out of amorphous AlO$_{x}$, single-crytal Al$_{2}$O$_{3}$ and single-crystal MgO tunnel barriers, and discuss the effect of barrier crystallinity and electrode/tunnel-barrier interface quality on the performance of the coherent quantum-devices. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P43.00010: Generating Spin Echoes with Pi-Pulses: More is Different Dale Li, Anatoly Dementyev, Yanqun Dong, Rona Ramos, Sean Barrett NMR spin echo measurements in several solids linked through the form of the homonuclear dipolar coupling are shown to defy conventional expectations of the delta-function pulse approximation. Multiple pi-pulse echo trains may either freeze-out or accelerate the decay of the signal, depending upon pi-pulse phase. Average Hamiltonian theory, combined with exact quantum calculations, reveal an intrinsic cause for these coherent phenomena: the dipolar coupling has a many-body effect during any real, finite pulse. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P43.00011: Visualizing and Manipulating the Density Matrix to Understand Multiple Pulse NMR Yanqun Dong, Dale Li, Rona Ramos, Sean Barrett NMR spin echo experiments using Carr-Purcell-Meiboom-Gill (CPMG) sequence produce an abnormally long-lived echo train in many solid samples. Average Hamiltonian theory and exact quantum simulations reveal that the dipolar couplings during the finite pulses play an important role in this phenomenon. In this talk we will use density matrix tomography to visualize the effect of dipolar couplings during finite pulses. Using this visualization and other simulations, we explore how the complicated flow through many coherence transfer pathways can lead to a measurable signal. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P43.00012: New Spin Echo Phenomena Arising from Strong Pi-Pulses Rona Ramos, Yanqun Dong, Dale Li, Sean Barrett The application of average Hamiltonian theory to multiple pulse NMR experiments using finite pulses will be discussed. Through this analysis, we are able to explain an observed pulse phase sensitivity and develop new pulse sequences that exploit and enhance these finite pulse effects. Experiments in both the strong and weak coupling limits will be shown. [Preview Abstract] |
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