Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session L28: Material Issues for Superconducting Qubits |
Hide Abstracts |
Sponsoring Units: DCMP Chair: John Martinez, University of California, Santa Barbara Room: Morial Convention Center 220 |
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L28.00001: Fabrication and Characterisation of Superconducting Coplanar Waveguide Resonators for Circuit QED Applications Martin G\"oppl, Rob Schoelkopf, Andreas Wallraff Superconducting thin film microwave cavities have gained great interest in recent years for studying qubit-photon or qubit-photon-qubit interactions in circuit quantum electrodynamics (QED) experiments $[1,2]$. Thin film cavities made by using standard optical lithography and microfabrication techniques have the potential for enabling quantum computing applications such as coupling several qubits via a quantum bus $[2]$. In order to specifically design circuit QED systems the cavity parameters such as resonance frequency and quality factor, determined by the input and output coupling strength need to be precisely controlled. We have fabricated niobium and aluminum resonators on Si, Si/SiO$_x$ and Al$_2$O$_3$ substrates. Resonators with quality factors up to several hundred thousands have been characterized. Furthermore, the effect of external parameters such as temperature and drive power have been investigated. The measured frequency dependent transmission of the cavities is found to be in good agreement with analytical circuit models.\\ $[1]$ A. Wallraff et al, Nature \textbf{431}, 162 (2004).\\ $[2]$ J. Majer et al, Nature \textbf{449}, 443 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L28.00002: Materials for superconducting qubits: Measurements of dielectric loss at low temperatures Aaron O'Connell, M. Ansmann, R.C. Bialczak, R. McDermott, M. Hofheinz, N. Katz, E. Lucero, M. Neeley, H. Wang, E.M. Weig, J.M. Martinis, A.N. Cleland The energy relaxation time ($T_{1})$ of the Josephson phase qubit is significantly impacted by the microwave loss in the dielectric materials used in fabrication. This loss mechanism is most likely due to the qubit coupling to a fermionic bath of two-level defect states embedded in the dielectrics. At high temperatures or high excitation voltages, these states can become saturated, so that the low loss tangents reported in the literature do not accurately represent the material performance at low temperatures and very small excitation energies. We have probed the microwave ($\sim $6 GHz) loss tangents of a number of common thin-film dielectrics at $\sim $100mK, using very low excitation voltages, in order to obtain values for the loss tangents relevant to quantum computation. We present these loss tangent data, and illustrate a technique to extract material values from measurements of the quality factors of coplanar waveguide resonators. [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L28.00003: Multi-level Spectroscopy of Microstates Coupled to a dc SQUID Phase Qubit Tauno Palomaki, S. K. Dutta, R. M. Lewis, A. J. Przybysz, Hanhee Paik, B. K. Cooper, H. Kwon, E. Tiesinga, J. R. Anderson, C. J. Lobb, F. C. Wellstood We report spectroscopic measurements at 25 mK of discrete two-level systems (TLS) coupled to a 16 $\mu $m$^{2}$ area Al/AlO$_{x}$/Al dc SQUID phase qubit. When the energy level spacing of the qubit equals that of the TLS, the coupling between the two systems lifts the degeneracy. By applying microwaves to excite transitions in the qubit, we map out the ``flat'' splittings in the 0$\to $1 transition spectrum of the qubit. We see 8 splittings, over a 1 GHz range, ranging in size from 14 to 240 MHz. We observe ``tilted'' splittings in the spectrum of 0$\to $2 transitions for the qubit, corresponding to the $\vert $1,e$>$ state interacting with the $\vert $2,g$>$ state, where e and g were the excited and ground state of the TLS, and the first index is the qubit and the second is the microstate. The spectra were compared to predictions from a model with the junction coupled to charged TLS and the agreement was reasonable. Finally, we show that away from TLS the coherence time of the qubit is limited by the bias leads. [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:18PM |
L28.00004: Effect of Two Level System Saturation on Charge Noise in Josephson Junction Qubits Magdalena Constantin, Clare Yu, John Martinis It is not widely appreciated that two-level systems in small qubits can easily be strongly saturated when the applied electromagnetic flux $J$ is much larger than the critical flux $J_c$. We show that charge noise $S_Q$ in Josephson qubits can be produced by fluctuating two-level systems with electric dipole moments in the substrate using the standard flat density of states. At high frequencies the frequency and temperature behavior of the charge noise depends on the ratio $J/J_c$. Our results are consistent with experimental conclusions that $S_Q\sim 1/f$ at low frequencies and $S_Q\sim f$ at high frequencies. [Preview Abstract] |
Tuesday, March 11, 2008 3:18PM - 3:30PM |
L28.00005: Crystalline Josephson phase qubits with improved performance Jeffrey Kline, Seongshik Oh, Haohua Wang, John Martinis, David Pappas One of the greatest challenges in the development of a practical solid-state quantum computer is to overcome decoherence due to coupling between the environment and the qubit. Superconducting quantum computers based on Josephson phase qubits are susceptible to decoherence due to charge noise in both the tunnel barrier and crossover insulators. We have demonstrated that the usage of crystalline tunnel barriers greatly reduce the density of spurious charge fluctuators in the tunnel barrier when compared to the ubiquitous amorphous barrier. However, a performance gain in coherence time was not realized due to decoherence in the crossover insulator. In the latest generation of devices, we have optimized both the tunnel barrier and crossover insulator materials. Low temperature measurements performed on a large area (50 \textit{$\mu $}m$^{2})$ device yielded a coherence time of 500 ns which is tied for the Josephson phase qubit world record. We expect increased performance in future generation devices where small area (13 \textit{$\mu $}m$^{2})$ junctions will be used. [Preview Abstract] |
Tuesday, March 11, 2008 3:30PM - 3:42PM |
L28.00006: 1/f Flux Noise in Josephson Phase Qubits Robert McDermott, Radek Bialczak, Markus Ansmann, Max Hofheinz, Nadav Katz, Erik Lucero, Matthew Neeley, Aaron O'Connell, Haohua Wang, Andrew Cleland, John Martinis We present the results of a novel measurement in a Josephson phase qubit that uses the resonant response of the qubit to directly measure the spectrum of low-frequency noise. This general method can be applied to any qubit system. By alternating the sense of the qubit bias, we show that the noise is predominantly flux-like, as opposed to a critical-current noise. The magnitude of the noise is compatible with previous measurements of excess low-frequency flux noise in SQUIDs cooled to millikelvin temperatures. We present the results of calculations of flux noise from paramagnetic defects in the native oxides of the superconductors, and show that the measured flux noise cannot be explained by the standard model of two-level state defects. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L28.00007: Measurements of Decoherence in rf SQUID Qubits Douglas Bennett, Luigi Longobardi, Vijay Patel, Dmitri Averin, James Lukens We report measurements of coherence times of an rf SQUID qubit using pulsed microwaves and rapid flux pulses. The modified rf SQUID has independent, in situ, controls for the relative positions of levels in different fluxoid wells and the barrier height between the wells. The decay of coherent oscillations is dominated by the lifetime of the excited state and low frequency flux noise. The low frequency flux noise is observed using microwave spectroscopy and resonant tunneling between fluxoid states in addition to the decay of coherent oscillations. These measurements are useful for evaluating the various insulating layers which are believed to be an important source of 1/f noise in many superconducting qubits. [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L28.00008: Josephson junction microscope for probing and quantum manipulation of low-frequency fluctuators Lin Tian, Raymond Simmonds The high-Q harmonic oscillator mode of a Josephson junction can be used as a novel probe of spurious two-level systems (TLSs) inside the amorphous oxide tunnel barrier of the junction. In particular, we show that spectroscopic transmission measurements of the junction resonator mode can reveal how the coupling magnitude between the junction and the TLSs varies with an external magnetic field applied in the plane of the tunnel barrier. The proposed experiments offer the possibility of clearly resolving the underlying coupling mechanism for these spurious TLSs, an important decoherence source limiting the quality of superconducting quantum devices. Meanwhile, quantum manipulation of the TLSs via the junction oscillator mode can also be achieved. L. Tian and R. W. Simmonds, Phys. Rev. Lett. 99, 137002 (2007). [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L28.00009: Low-frequency Flux Noise in SQUIDs and Superconducting Qubits Steven Sendelbach, David Hover, Achim Kittel, Michael Mueck, Robert McDermott Superconducting qubits are a leading candidate for scalable quantum information processing. In order to realize the full potential of these qubits, it is necessary to develop a more complete understanding of the microscopic physics that governs dissipation and dephasing of the quantum state. In the case of the Josephson phase and flux qubits, the dominant dephasing mechanism is an apparent low-frequency magnetic flux noise with a 1/f spectrum. The origin of this excess noise is not understood. We report the results of SQUID measurements that explore the dependence of the excess low-frequency flux noise on SQUID inductance, geometry, materials, and temperature. We discuss contributions to the measured noise from temperature fluctuations, trapped vortices in the superconducting films, and surface magnetic states in the native oxides of the superconductors. We discuss implications of our measurements for qubit dephasing. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L28.00010: Small shadow evaporated junctions for superconducting phase qubits F. Altomare, M.S. Allman, K. Cicak, M.A. Sillanpa\"{a}, J.D. Whittaker, R.W. Simmonds One of the biggest problems facing the fabrication of quantum computers based on superconducting qubits is the short coherence time of the quantum states. This is due to interaction of qubits with both the environment and defects (two level systems-TLS) in the Josephson junction (JJ). Because of the large JJ area, this problem is particularly obvious in phase qubits where it has been shown that TLS greatly affect the coherence time.\footnote{PRL 93, 077003 (2004)} One way to overcome this problem is to reduce the size of the JJ thus reducing the number of TLS.\footnote{PRL 97, 050502 (2006)} We will discuss the results of our approach to this solution, namely using shadow evaporated JJ ($<1\mu m^2$) and low-loss capacitor, and the results of our experiments on coupled qubits. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L28.00011: Investigating two-level systems in the Josephson tunnel barrier Joshua Strong, Fabio Altomare, Raymond Simmonds The presence of two-level system defects within the Josephson tunnel barrier has been made apparent in the spectroscopy of superconducting phase quantum bits. Here, we present a different circuit-- a tunable harmonic resonator based on the Josephson inductance-- which is better suited to the study of these two-level systems. A typical circuit is tunable over a gigahertz range and behaves linearly at sufficiently low drive amplitudes. We use this circuit to investigate the properties of individual defects, extracting parameters such as strength of coupling to the resonator, etc. The sea of two-level systems also has a bulk effect, causing a degradation in the quality factor of the resonator. This means that the Josephson tunnel barrier has a non-zero loss tangent in the microwave regime. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L28.00012: Probing dissipation from vortices with superconducting microwave resonators. C. Song, T.W. Heitmann, M.P. DeFeo, K. Yu, B.L.T. Plourde, R. McDermott One potential source of dissipation in superconducting qubits comes from vortices trapped in the thin films. We present a design for a system of microwave resonators for studying the loss contributed by trapped flux over the frequency range from 2 - 12 GHz. This consists of a multiplexed set of superconducting resonators with a wide range of lengths that are capacitively coupled to a common superconducting feed-line. By cooling the resonators in different magnetic fields, it is possible to probe the loss from vortices as a function of field and frequency, at least at the discrete frequencies of the resonators in our set. [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L28.00013: Decoherence in Superconducting Qubits from Surface Magnetic States David Hover, Steven Sendelbach, Achim Kittel, Michael Mueck, Robert McDermott Unpaired spins in amorphous surface oxides can act as a source of decoherence in superconducting and other solid-state qubits. A density of surface spins can give rise to low-frequency magnetic flux noise, which in turn leads to dephasing of the qubit state. In addition, magnetic surface states can couple to high-frequency resonant magnetic fields, and thereby contribute to energy relaxation of the qubit. We present the results of low-frequency measurements of the nonlinear and imaginary spin susceptibility of surface magnetic states in superconducting devices at millikelvin temperatures. In addition, we describe high-frequency magnetic resonance measurements that directly probe the surface spin density of states. We present calculations that connect the measurement results to qubit energy relaxation and dephasing times. [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L28.00014: Measurement of low frequency flux noise in superconducting flux qubits Wei Qiu, Bo Mao, Yang Yu, Shaoxiong Li, Siyuan Han The development of superconducting quantum interference devices (SQUIDs) as flux qubits for scalable quantum computation has been impeded significantly in the last several years by excessive low frequency flux noise which has become the dominant decoherence mechanism in several experiments. We measured the low frequency flux noise in SQUIDs with inductance ranging from about 30 pH to 1 nH. We found that for the Nb SQUIDs fabricated with the same process the measured rms low frequency flux noise has a linear dependence on the inductance of the SQUIDs. Implications of the result on material, design, fabrication of flux qubit will be discussed. [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L28.00015: Measurement of Spin Susceptibility of Thin Films and Nano-Scale Structures Julie Bert, Hendrik Bluhm, Nicholas Koshnick, Martin Huber, Kathryn Moler We report measurements of a spin-like paramagnetic susceptibility signal from high purity metallic and insulating thin films.~ The measurements were performed using a Superconducting Quantum Interference Device (SQUID) in a scanning microscope.~ By using the SQUID to scan areas of the sample both near and far from the metallic films, we found a paramagnetic susceptibility associated with both Au and AlOx films that was ten times larger than could be explained by the concentration of impurity spins expected for 6N gold.~ The 1/T temperature dependence and the paramagnetic sign indicate that the susceptibility signal is caused by localized spins that are at most weakly coupled to each other and to the conduction electrons.~ Moreover, the signal exhibits a measurable out of phase response which can be related to 1/f noise due to fluctuating spins [Koch, DiVincenzo, and Clarke, Phys. Rev. Lett. \textbf{98}, 267003 (2007)].~ These results demonstrate the utility of scanning SQUID based susceptibility measurements for characterizing spin related effects.~ Further applications of this technique may include probing 1/f noise origins in superconducting devices as well as imaging magnetic structures such as nanomagnets. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700