Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session T29: Focus Session: Superconducting Qubits - Coherence and Materials I |
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Sponsoring Units: GQI Chair: Andrew Houck, Princeton University Room: C148 |
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T29.00001: Feedback suppression of the low-frequency noise in qubits by the low-frequency quantum measurements Qiang Deng, Dmitri Averin The problem of the low-frequency noise has dominated the development of the solid-state qubits. Up to now, the main approach to solve this was to employ the qubit structures with the basis states having the same values of the main qubit coordinate (e.g., electric charge or magnetic flux) that are, as a result, not decohered by the noise. The goal of this work is to suggest an alternative approach based on direct suppression of the low-frequency noise in a qubit through the measurement/feedback loop. Continuous quantum measurement required for this loop should also be ``low-frequency'' so that it does not affect the quantum dynamics of the qubit. We calculate the minimal noise induced in the qubit by such a feedback loop when the measurement is the quantum-limited. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T29.00002: Tunable quantum beam splitters for quantum manipulation of a hybrid tripartite qubit system G.Z. Sun, J. Chen, P.H. Wu, X.D. Wen, Y. Yu, B. Mao, S.Y. Han We demonstrated coherent control of quantum states in a tripartite system consisting of a superconducting qubit and two microscopic two-level states (TLS). An initially prepared qubit state was swept through qubit-TLS avoided crossings in the energy-level spectrum. The avoided crossings act as tunable quantum beam splitters of wave function. In an analogy to optics, the transmission coefficient of the beam splitters can be varied from zero to unity or any value in between by adjusting the rate of energy sweep. When performed within the decoherence time, consecutive crossings through the beam splitters lead to coherent quantum oscillations between the quantum states of the tripartite qubit-TLS system. This Landau-Zener-Stueckelberg interference controlled by the sweeping rate provides an alternative means to manipulate multiple qubits and demonstrates macroscopic quantum coherence. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T29.00003: Surface and Interface Defects in Linear and Nonlinear Superconducting Resonators Steven Weber, Kater Murch, Allison Dove, Gustaf Olson, Zack Yoscovits, R. Vijay, Eli Levenson-Falk, James Eckstein, Irfan Siddiqi We report on progress to identify and mitigate noise mechanisms in both linear superconducting resonators and devices embedded with Josephson junctions. Defects, either microscopic fluctuators or remnant residue layers associated with nanofabrication, can exist on metal surfaces, at the metal-dielectric interface, within the dielectric, or within the Josephson junctions themselves. We have investigated the quality factor and phase noise of lumped element and distributed element resonators at low temperature and photon number- the operating regime of superconducting qubits. In particular, we compare the performance of poly-crystalline and epitaxial films, silicon and sapphire substrates, and weak link and tunnel type Josephson junctions. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T29.00004: Quantum model for superconducting resonator loss via a two-level system Mishkatul Bhattacharya, K. Osborn, A. Mizel Clarifying the mechanisms of dissipation in superconducting resonators is crucial for advancing superconducting quantum computation. The models currently employed to study dielectric loss due to two level charge fluctuators have been based largely on a classical treatment of the problem. In contrast, we carry out a quantum mechanical investigation using a dissipative Jaynes-Cummings model in which the resonator is coupled to a two-level system that is in turn coupled to a bath. We present an analysis of the dynamics of energy decay in the system, comparing its predictions to those of well-known classical models, which agree with our results in the limit of high oscillator excitation. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T29.00005: Possible interactions between two-level system defects in SiNx films Sergiy Gladchenko, Moe Khalil, C.J. Lobb, F.C. Wellstood, Kevin D. Osborn Low-temperature properties of PECVD SiNx dielectric films are measured within the capacitor of superconducting LC resonators. Experiments are made at temperatures from 30 to 300 mK, and at storage energies from 1 to 10$^{6}$ photons in a resonant cavity. While the power and temperature dependence of the loss agrees with two-level system (TLS) theory above 60 mK, below this temperature we observe significant deviations. In this regime we observe a reduction in loss upon lowering dielectric temperature, in direct contrast with the independent TLS model of defects within our film. This new phenomena may indicate interactions between two-level systems. We can also spectroscopically resolve the loss from dominant defects in our capacitors, which have a volume of $\sim $2000 $\mu $m$^{3}$. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T29.00006: A study of glassy behavior in amorphous dielectrics using GHz frequency superconducting resonators Moe Khalil, M.J.A. Stoutimore, Aaron Holder, Charles Musgrave, C.J. Lobb, F.C. Wellstood, K.D. Osborn It has been shown that the dielectric constant of certain glassy materials can be changed with a dc electric field bias. Here we extend those studies to higher frequencies where both the real and imaginary part of the dielectric constant can be studied. We have designed a dc electric-field tunable LC resonator built from superconducting thin-film aluminum to test this effect in SiN$_{x}$ at GHz frequencies. We will report progress on measuring these devices down to single photon storage energies at temperatures of approximately 30 mK, where the dynamics are dominated by the tunneling of two-level defects. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T29.00007: Microwave loss of novel epitaxial superconductor-insulator-superconductor (SIS) trilayers U. Patel, K.H. Cho, L. Maurer, S. Sendelbach, D. Hover, C.B. Eom, R. McDermott The performance of superconducting phase qubits is currently limited by spurious coupling of the qubit to two-level state (TLS) defects in the amorphous dielectric materials of the circuits. Thus, it is highly desirable to develop defect free epitaxial dielectric materials for improved junction barriers and capacitor dielectrics. We have characterized the dielectric loss of several candidate SIS trilayers including Re/MgO/Al and Re/LaAlO$_{3}$/Al grown on c-sapphire substrates. We describe our multiplexed microstrip resonator device layout and present data on the intrinsic quality factors of the MBE-grown dielectrics. [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T29.00008: Growth of epitaxial superconductor/dielectric heterostructures using a sputtering PLD hybrid system with in-situ RHEED Kwang Hwan Cho, Jacob Podkaminer, Chad Folkman, Chang-Beom Eom One of limiting factors in superconducting qubits is decoherence caused by microscopic defects in dielectric layer such as nanocrystalline regions and grain boundaries in a shunted capacitor. We have grown epitaxial Re thin films on a c-plane sapphire substrate using RF magnetron sputtering, then transferred \textit{ex-situ} to a pulsed laser deposition (PLD) system where dielectrics thin film layer is deposited. One drawback of this fabrication approach is the necessity to expose the sample to air when the sample is transferred to different deposition chambers. In order to avoid these drawbacks, we have employed a hybrid PLD-sputtering deposition that will allow us to grow the oxide dielectric/Re heterostructures in an \textit{in-situ} environment without breaking vacuum. The system is also equipped with a reflection high energy electron diffraction (RHEED) which will allow us to perform \textit{in-situ} characterization of the structure and growth dynamics. We will discuss our strategy of epitaxial growth of various single crystal dielectrics on superconducting thin films in this system and their structural and electrical properties of the heterostructures [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T29.00009: Barrier defect analysis using Josephson junction resonators M.J.A. Stoutimore, Bahman Sarabi, Moe Khalil, C.J. Lobb, K.D. Osborn We have designed Josephson junction (JJ) resonators by adding an Al/AlO$_{x}$/Al Josephson junction in parallel with a coplanar sapphire capacitor and an inductor so that the total loss will be dominated by the junction barrier. JJ resonators couple to individual defects in the junction barrier, causing splittings in the spectroscopy of the resonator when it is excited near single-photon energies, similar to phase and other qubits. Measurements are performed in a dilution refrigerator at 30mK with a drive frequency of approximately 7GHz. By applying a dc flux bias, we can tune the resonance frequency by as much as 1GHz. Analysis of the frequency of splittings as a function of junction area and barrier growth process provides a method for determining the source of the defects. We will use these devices to study~amorphous aluminum oxide barriers and~will report~our progress towards studying novel barrier dielectrics. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T29.00010: Losses in Josephson junction resonators Martin Weides, Jiansong Gao, Jeffrey Kline, Michael Vissers, David Wisbey, David Pappas Josephson junctions for superconducting circuits such as SQUIDs and qubits are conventionally based on Al-AlO$_x$-Al multilayer technology, which was shown to have a low quality factor and two-level-fluctuators in the dielectric AlO$_x$ as limiting decoherence source. By replacing the amorphous Al-rich tunnel oxide with nearly stoichiometric Al$_2$O$_3$ we aim to increase the qubit coherence times by reducing the number of dangling bonds in the Josephson tunnel junction. In this talk a test platform for loss determination in high-Q tunnel oxides based on junction resonators will be presented. We will show alternative tunnel junctions based on high temperature grown tunnel oxides. [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T29.00011: Growth and Properties of Epitaxial Dielectrics/Superconducting Thin Film Heterostructures Chang-Beom Eom, Kwang-Hwan Cho, Jacob Podkaminer, Chad Folkman Our objective is the growth of epitaxial dielectrics on crystalline superconducting underlayers to improve the performance of superconducting Qbits. A major challenge is heteroepitaxial growth of single crystal dielectric layers with high crystalline quality and atomically sharp interfaces between the dielectric and superconducting electrodes. First, we have grown high quality epitaxial rhenium (Re) thin films on c-plane sapphire substrates by DC magnetron sputtering. The full width at half maximum (FWHM) of Re 0002 rocking curve is less than 0.5 degrees. The RMS surface roughness determined by AFM is less than 1 nm. We have also grown epitaxially various dielectric thin films on top of the single crystal Re bottom electrode by pulsed laser deposition with in situ high pressure reflection high energy electron diffraction (RHEED). In this talk, we will discuss our strategy of epitaxial growth of various single crystal dielectrics on superconducting thin films and their structural and electrical properties of the heterostructures. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T29.00012: Microwave Response of Superconducting Resonant Circuits based on 3D Aluminum Nanobridge Josephson Junctions Eli Levenson-Falk, R. Vijay, Kater Murch, Irfan Siddiqi Metallic weak links are attractive candidates for low loss superconducting circuits as they offer a route to realize Josephson junctions without the need for an amorphous tunnel barrier-a potential source of both low and high frequency noise. We discuss microwave measurements of high quality factor resonators incorporating both single nanobridges and nanobridge-based SQUIDs. Our results indicate low loss and strong nonlinearity, suggesting the future utility of these devices in qubit and amplifier circuits. Our data are in quantitative agreement with numerically computed nanobridge current-phase relations and dc transport measurements. We show preliminary results on nanobridge-based qubits and parametric amplifiers. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T29.00013: Josephson junctions formed from superconducting nanowires B. Xiao, H.Y. Chen, I. Nsanzineza, C. Song, B.L.T. Plourde We are investigating the possibility of forming Josephson junctions from thin-film superconducting nanowires. The Josephson coupling through such a constriction can provide the necessary nonlinearity, for example, for forming a qubit, while avoiding the influence of defects in the amorphous tunnel barriers used in conventional Josephson junctions that can contribute to qubit decoherence. We have developed a fabrication process based on high-resolution electron-beam lithography with a negative-tone resist combined with ion-beam etching to pattern nanowires from 10 nm-thick, sputter-deposited, amorphous MoGe thin films. We have studied nanowires with widths between 20 - 100 nm and lengths between 50 - 200 nm. A Nb wiring layer provides electrical connections to the nanowires. Low-temperature transport measurements allow us to study the nanowire critical current and the influence of microwave irradiation on the current-voltage characteristics. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T29.00014: Via fabrication process for epitaxial superconducting qubits Jeffrey Kline, Fabio da Silva, Michael Vissers, David Wisbey, Martin Weides, David Pappas Reducing the density of spurious two level systems (TLS) in the dielectric layers of superconducting qubits has been shown to improve performance. We aim to reduce TLS density in the Josephson junction tunnel barrier through the use of epitaxial materials. The investigation of some new material systems using a trilayer process wherein the base electrode, tunnel barrier, and top electrode are grown and subsequently patterned is problematic due to sidewall damage during the mesa etch. We apply the via fabrication process wherein the base electrode and wiring insulator layers are grown and patterned prior to tunnel barrier growth. The via process is compatible with a different set of electrode materials than the trilayer process and allows us to investigate the suitability of these materials for qubit applications. We present room temperature and low temperature data for Re/Al$_{2}$O$_{3}$/Re Josephson junctions fabricated using the via process. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T29.00015: Introduction of a DC Bias into a High-Q Superconducting Microwave Cavity Fei Chen, Juliang Li, M.P. Blencowe, A.J. Rimberg, Adam Sirois, Raymond Simmonds The circuit quantum electrodynamics (QED) architecture has been demonstrated to allow study of cavity QED physics in a high-Q on-chip microwave cavity[1]. Here we develop a technique to apply a DC current or voltage bias to nanostructures embedded in the microwave cavity without significantly degrading the Q at high frequencies. Experimental results show good agreement with theoretical predictions. New highly non-linear fully quantum mechanical devices can be developed by embedding Josephson junction devices such as single electron transistors (SETs) in the high-Q microwave cavity. The interplay between the SET and the microwave cavity offers an interesting system for studying nonlinear quantum dynamics and the quantum-to-classical transition. Recent experimental results will be discussed. \\[4pt] [1] A. Wallraff et al, Nature, 431, 162 (2004). [Preview Abstract] |
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