Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session L48: Decoherence in Superconducting Qubits: Materials and TLSs |
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Sponsoring Units: GQI Chair: David Pappas, NIST-Boulder Room: 349 |
Wednesday, March 16, 2016 11:15AM - 11:27AM |
L48.00001: Characterizing and reducing microfabrication-induced loss in superconducting devices, Part I: Resonators Andrew Dunsworth, A. Megrant, Z. Chen, C. Quintana, B. Burkett, J. Kelly, R. Barends, A. Fowler, E. Jeffrey, T. White, D. Sank, J. Mutus, B. Campbell, Y. Chen, B. Chiaro, C. Neill, P.J.J. O'Malley, P. Roushan, A. Vainsencher, J. Wenner, J.M. Martinis Planar and 3D superconducting qubits have previously been shown to be limited by microfabrication induced loss. Using finite element simulations, we have identified a major source of this decoherence in superconducting qubits. Furthermore, we experimentally verified this dominant loss channel using a novel resonator based approach, which we call 'Hydra' resonators. We fully characterized and then substantially reduced this loss channel using these Hydra resonators. I will report on these measurements and their implications on improving the coherence of superconducting qubits. [Preview Abstract] |
Wednesday, March 16, 2016 11:27AM - 11:39AM |
L48.00002: Characterizing and reducing microfabrication-induced loss in superconducting devices, Part II: Xmon qubits Anthony Megrant, A. Dunsworth, C. Quintana, J. Kelly, R. Barends, B. Campbell, Y. Chen, Z. Chen, B. Chiaro, A. Fowler, E. Jeffrey, J. Mutus, C. Neill, P.J.J. O'Malley, P. Roushan, D. Sank, A. Vainsencher, J. Wenner, T. White, J.M. Martinis Microfabrication-induced loss has previously been shown to limit the coherence times of both planar and 3-D superconducting qubits. Energy loss in these qubits arises from interactions with two-level state defects which are located in thin lossy surface dielectrics. More recently, we have identified a major source of this loss and then substantially improved this decoherence channel using a novel resonator structure for characterization and improvement. I will report on recent measurements of Xmon qubits with substantially improved coherence times due to our new fabrication process. [Preview Abstract] |
Wednesday, March 16, 2016 11:39AM - 11:51AM |
L48.00003: Supercritical Fluid Assisted Cleaning of Patterned Aluminum Microstructures for Removal of Post-Processing Residue and Surface Contamination Marvin G. Warner, Christopher A. Barrett, Cynthia L. Warner, Christopher J.K. Richardson, Nathan Siwak We report the development of preliminary methods for the supercritical CO$_{2}$ assisted removal of post-processing residue and surface contamination from delicate aluminum structures, such as those found in superconducting quantum circuits based on Josephson Junctions. The supercritical CO$_{2}$ serves as an effective solvent system to assist in the delivery of various co-solvents, stripping agents, and surfactants to the surface of the patterned devices without introducing harsh chemical reagents that can lead to degradation and etching of the aluminum structures. This work was conducted to determine the feasibility and benefits of inserting a supercritical CO$_{2\, }$cleaning step into device patterning and cleaning processes. The results presented here will discuss our efforts to determine the optimal formulation, shortest exposure time, and exposure methods ($e.g.$, pulsed versus static) necessary to completely remove surface contamination while preserving the integrity of the underlying patterned aluminum structures. The methods presented here could make great strides in removing fabrication based residue and surface contamination, which has been shown to lead to decoherence in superconducting quantum circuits. [Preview Abstract] |
Wednesday, March 16, 2016 11:51AM - 12:03PM |
L48.00004: Low temperature internal friction of amorphous silicon Xiao Liu, Thomas Metcalf, Glenn Jernigan, Battogtokh Jugdersuren, Brian Kearney, James Culberston The ubiquitous low-energy excitations, known as two-level tunnelling systems (TLS), are one of the universal phenomena of amorphous solids. These excitations dominate the acoustic, dielectric, and thermal properties of structurally disordered solids. Using the double-paddle oscillator internal friction measurement technique, we have shown that TLS can be made to almost completely disappear in e-beam deposited amorphous silicon (a-Si) as the growth temperature increased to 400\textdegree C. However, there is a mysterious broad maximum in internal friction at 2-3K, which we suspect to come from metallic contamination of our oscillators and is not related to a-Si. Our new result of a-Si, deposited in a different UHV system and on oscillators with a different type of metallic electrodes, confirms our suspicion. This lowers the upper bound of possible TLS content in a-Si, in terms of tunnelling strength, to below 10$^{\mathrm{-6}}$. Our results offer an encouraging opportunity to use growth temperature to improve the structure order of amorphous thin films and to develop high quality amorphous dielectrics for applications, such as in modern quantum devices. [Preview Abstract] |
Wednesday, March 16, 2016 12:03PM - 12:15PM |
L48.00005: Elastic measurements of TLSs in amorphous silicon at mK temperatures Andrew Fefferman, Xiao Liu, Thomas Metcalf, Glenn Jernigan, Eddy Collin The low temperature properties of glass are distinct from those of crystals due to the presence of poorly understood low-energy excitations. These are usually thought to be atoms tunneling between nearby equilibria, forming tunneling two level systems (TLSs). Elastic measurements on amorphous silicon films deposited with e-beam evaporation showed that this material contains a variable density of TLSs that decreases as the growth temperature increases from 45 to 400 deg C [1]. We will present an analysis of the elastic properties of these films down to the low mK range in the framework of the standard tunneling model. [1] X. Liu, D. R. Queen, T. Metcalf, J. Karel and F. Hellman, Phys. Rev. Lett., 113, 025503 (2014) [Preview Abstract] |
Wednesday, March 16, 2016 12:15PM - 12:27PM |
L48.00006: Device Applications for the Tunneling Atom Maser Yaniv Rosen, Samaresh Guchhait, Alex Burin, Kevin Osborn Random two-level system defects in dielectrics absorb energy and limit the coherence of superconducting qubits and resonators used in quantum computing applications. So far attempts to reduce this loss have been confined to device design and material optimization. We present the ability to control the loss of a dielectric by directly manipulating the population of its two level system defects (TLSs). The defect populations can be controlled and the resonator can pass continuously through regimes of ordinary defect loss, to negligible material dissipation, and finally to coherent amplification. The theory behind the device is discussed and matched to experimental results. Using similar methods, we propose devices that control loss, exhibit reduced noise, and amplify signals. [Preview Abstract] |
Wednesday, March 16, 2016 12:27PM - 12:39PM |
L48.00007: A qubit designed with a dynamically controlled bath of two-level system defects Tim Kohler, Yaniv Rosen, Samaresh Guchhait, Kevin Osborn Although superconducting qubits have made tremendous gains, they still suffer decoherence from two level system (TLS) defects that are found at dielectric films including the native oxides on superconductors. Qubits often minimize TLS effects by choosing optimal device geometries. We have previously investigated methods to electrically sweep the energy of a bath of TLSs and control their excited-state population. Here we discuss a qubit design that incorporates a similar control over the TLSs, and which may prove to minimize TLS decoherence effects. In this device the qubit energy can remain constant while the TLS bath is dynamically controlled. We will discuss experimental progress towards realizing this qubit. [Preview Abstract] |
Wednesday, March 16, 2016 12:39PM - 12:51PM |
L48.00008: Surface Participation Effects in Titanium Nitride and Niobium Resonators Allison Dove, John Mark Kreikebaum, William Livingston, Remy Delva, Yanjie Qiu, Reinhard Lolowang, Vinay Ramasesh, Kevin O'Brien, Irfan Siddiqi Improving the coherence time of superconducting qubits requires a precise understanding of the location and density of surface defects. Superconducting microwave resonators are commonly used for quantum state readout and are a versatile testbed to systematically characterize materials properties as a function of device geometry and fabrication method. We report on sputter deposited titanium nitride and niobium on silicon coplanar waveguide resonators patterned using reactive ion etches to define the device geometry. We discuss the impact of different growth conditions (temperature and electrical bias) and processing techniques on the internal quality factor (Q) of these devices. In particular, to investigate the effect of surface participation, we use a Bosch process to etch many-micron-deep trenches in the silicon substrate and quantify the impact of etch depth and profile on the internal Q. [Preview Abstract] |
Wednesday, March 16, 2016 12:51PM - 1:03PM |
L48.00009: Fabrication and characterization of highly disordered TiN thin films by reactive evaporation for circuit-QED. Yen-Hsiang Lin, Raymond Mencia, BaoLong Nguyen, Vladimir Manucharyan Titanium nitride (TiN) has been identified as one of the potentially new materials for circuit-QED. In particular, disordered TiN films close to superconductor-insulator transition can be beneficial to greatly enhance kinetic inductance due to low superfluid density. Here we report TiN thin films prepared by e-beam evaporation within a nitrogen rich environment. By controlling nitrogen gas flow rate, the normal sheet resistance of TiN film can be tuned higher than 1kOhms while superconductivity still remains above 2K. Here, we present our characterization results and microwave measurement of quality factor Q and kinetic inductance L. [Preview Abstract] |
Wednesday, March 16, 2016 1:03PM - 1:15PM |
L48.00010: TiN superconducting coplanar waveguide resonators with single-photon quality factors of 1.5 million Greg Calusine, Danna Rosenberg, David Hover, Rabindra Das, Alexander Melville, Xhovalin Miloshi, Wayne Woods, Jonilyn Yoder, William Oliver The investigation of loss mechanisms in superconducting coplanar waveguide (CPW) resonator provides an efficient means to elucidate relevant loss mechanisms affecting superconducting qubit circuits. As compared to superconducting qubits, the reduced complexity of CPW fabrication coupled with the straightforward characterization of CPW properties facilitates the deconvolution of the impact of individual fabrication steps on the CPW performance. We assess this impact by characterizing the statistically significant differences in internal quality factors (Q$_i$) at the single-photon level resulting from different fabrication processes in aluminum and titanium nitride (TiN) superconducting thin film CPW resonators on silicon. We demonstrate repeatable Qi's at the single-photon level of approximately 1.5x10\textsuperscript{6} in TiN CPW resonators with 90 percent of devices showing Q$_i$'s above 1x10$^6$ and single Q$_i$'s as high as 3.8x10$^6$. This work is sponsored in part by the Laboratory for Physical Science, IARPA, and the Assistant Secretary of Defense for Research and Engineering under Air Force Contract FA8721-05-0002. Opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the United States Government. [Preview Abstract] |
Wednesday, March 16, 2016 1:15PM - 1:27PM |
L48.00011: Anomalous thickness dependence of quality factor in TiN film resonators grown on functionalized Si substrates Peng Xu, Tim Kohler, Evgeniya Lock, Yaniv Rosen, Aruna Ramanayaka, Samaresh Guchhait, Kevin Osborn Various properties affect the quality factor of superconducting resonators at millikelvin temperatures including the presence of nanoscale interfacial dielectric films and residual quasiparticles. Superconducting titanium nitride is polycrystalline such that growth phases may also affect the resonator quality. Here, we functionalize Si substrates in different hydrophobic and hydrophilic plasma environments, sputter titanium nitride on top and pattern the latter films into resonators. For each functionalization we study the quality factor dependence on the superconducting film thickness, where the thicknesses are changed only between 25 and 50 nm. As expected, most functionalizations reveal very little quality factor dependence on superconducting film thickness. However, other functionalizations dramatically, and even anomalously, increase or decrease the quality with thickness. For example, oxygen plasma functionalization causes the quality factor to increase by a factor of more than ten at single photon power with increased thickness. We report on the progress towards finding the intrinsic reason for strong quality factor dependences on surface functionalization. [Preview Abstract] |
Wednesday, March 16, 2016 1:27PM - 1:39PM |
L48.00012: Optimized Shielding and Fabrication Techniques for TiN and Al Microwave Resonators John Mark Kreikebaum, Eunseong Kim, William Livingston, Allison Dove, Gregory Calusine, David Hover, Danna Rosenberg, William Oliver, Irfan Siddiqi We present a systematic study of the effects of shielding and packaging on the internal quality factor (Q$_{\mathrm{i}})$ of Al and TiN microwave resonators designed for use in qubit readout. Surprisingly, Q$_{\mathrm{i}}$ $=$1.3x10$^{\mathrm{6\thinspace }}$TiN samples investigated at 100 mK exhibited no significant changes in linewidth when operated without magnetic shielding and in an open cryo-package. In contrast, Al resonators showed systematic improvement in Q$_{\mathrm{i}}$ with each successive shield. Measurements were performed in an adiabatic demagnetization refrigerator, where typical ambient fields of 0.2 mT are present at the sample stage. We discuss the effect of 100 mK and 500 mK Cu radiation shields and cryoperm magnetic shielding on resonator Q as a function of temperature and input power in samples prepared with a variety of surface treatments, fabrication recipes, and embedding circuits. [Preview Abstract] |
Wednesday, March 16, 2016 1:39PM - 1:51PM |
L48.00013: Characterizing the performance of waveguide technologies for microwave-frequency quantum communication Philipp Kurpiers, Tobias Frey, Andreas Wallraff In circuit quantum electrodynamics (QED) systems quantum communication over distances beyond chip-scale requires low-loss waveguides. We measure the loss per unit length and the phase stability of commercially available waveguide technologies down to Millikelvin temperatures and single photon levels. More specifically, we characterize the frequency dependent attenuation and dispersion properties of a range of semi-rigid microwave cables and waveguides. We study the properties of various, commonly used conducting and dielectric materials with high accuracy in resonant structures to extract the internal quality factor which is inversely proportional to the loss per unit length. Furthermore, we compare our data with corresponding loss models. The results of our characterization are relevant to applications in which quantum communication is needed between nodes of a small network, e.g. between quantum circuits realized on different chips within the same or in distinct cryogenic systems. [Preview Abstract] |
Wednesday, March 16, 2016 1:51PM - 2:03PM |
L48.00014: Spectrum of a Resonator Coupled to a Driven Superconducting Qubit in the Strong Dispersive Regime of Circuit Quantum Electrodynamics Yonuk Chong, Hyun-Gue Hong, Dong-Gwang Ha The resonator spectrum in the strong dispersive coupling regime of circuit-QED has been a useful nondestructive indicator of a stationary qubit state. Here we present experimental observation of the further modification of the resonator spectrum as the qubit undergoes the dynamic transition by a resonant driving field. The quartet resonance associated with the polarized qubit is observed for the resonant driving at one-photon as well as the multi-photon transition in a 3D transmon qubit. The evolution of the resonance as a function of the driving power and the detuning of the driving field is well understood by a simple model which is based on the analytic diagonalization of Hamiltonian and described in terms of dressed states, Lamb shift, and AC Stark shift. [Preview Abstract] |
Wednesday, March 16, 2016 2:03PM - 2:15PM |
L48.00015: Characterizing Ensembles of Superconducting Qubits Adam Sears, Jeff Birenbaum, David Hover, Danna Rosenberg, Steven Weber, Jonilyn L. Yoder, Jamie Kerman, Simon Gustavsson, Archana Kamal, Fei Yan, William Oliver We investigate ensembles of up to 48 superconducting qubits embedded within a superconducting cavity. Such arrays of qubits have been proposed for the experimental study of Ising Hamiltonians, and efficient methods to characterize and calibrate these types of systems are still under development. Here we leverage high qubit coherence ($> 70~\mu s$) to characterize individual devices as well as qubit-qubit interactions, utilizing the common resonator mode for a joint readout. This research was funded by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA) under Air Force Contract No. FA8721-05-C-0002. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of ODNI, IARPA, or the US Government. [Preview Abstract] |
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