Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session M36: Superconducting Qubits: Fabrication & Materials |
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Sponsoring Units: GQI Chair: Andreas Wallraff, ETH Zurich Room: 703 |
Wednesday, March 5, 2014 11:15AM - 11:27AM |
M36.00001: Tunable TiN or NbTiN resonators and couplers using nonlinear kinetic inductance for superconducting qubits Michael Vissers, Jiansong Gao, Clint Bockstiegel, Martin Sandberg, David Pappas Nitride superconductors such as TiN and NbTiN have a nonlinear kinetic inductance when driven at high current. Using this current-tunable reactance, we have designed superconducting devices that are tunable with a DC current without using Josephson junctions. We show that when the DC current is directly coupled to a lumped element resonator, the resonant frequency can be tuned by \textgreater 4{\%} without inducing loss. In other circuits, we can use a DC current to independently tune the coupling of a long microwave transmission line to a standard superconducting resonator from zero to maximum coupling. In addition to characterizing the non-linear current response of these materials, these tunable devices could be used as a tunable coupler in transmon qubits, by adjusting the strength of the cavity's Purcell effect to the qubit as needed. They also have potential to be used as tunable filters or parametric amplifiers in superconducting circuits. [Preview Abstract] |
Wednesday, March 5, 2014 11:27AM - 11:39AM |
M36.00002: Superconducting qubits using titanium nitride Matthias Steffen, Josephine Chang, David Pappas, Mike R. Vissers, Martin Sandberg, Jiansong Gao, Lynne Gignac, John Bruley, Chris Breslin, Marko Hopstaken, Chris Lirakis Recent results in the community strongly implicate surface loss as a dominant source of decoherence (primarily energy relaxation) for superconducting transmon qubits. Resonators and qubits made of titanium nitride (TiN) showed significant device improvement compared with lift-off aluminum, with quality factors of up to approximately 1 million. We present more detailed characterization results of the TiN films including the substrate-metal interface. [Preview Abstract] |
Wednesday, March 5, 2014 11:39AM - 11:51AM |
M36.00003: Superconducting metamaterial transmission line Francisco Rouxinol, Haozhi Wang, B.L.T Plourde Left-handed metamaterials are artificial composite structures with unusual properties. Such systems have a wide range of potential applications in photonics. We are developing transmission lines composed of superconducting metamaterials using thin-film lumped circuit elements. Such structures allow for the possibility of generating novel transmission spectra with a high density of modes in some frequency ranges and stop-bands in others. We discuss possible couplings of these lines to superconducting qubits in circuit QED architectures. [Preview Abstract] |
Wednesday, March 5, 2014 11:51AM - 12:03PM |
M36.00004: Fabrication of capacitively-shunted superconducting qubits Jonilyn L. Yoder, Theodore J. Gudmundsen, Vladimir Bolkhovsky, Paul B. Welander, Simon Gustavsson, David Hover, Andrew J. Kerman, Adam P. Sears, William D. Oliver Improvements in superconducting qubit coherence times and reproducibility have been demonstrated using capacitive shunting. In this study, we present methods for the preparation of both capacitively-shunted charge qubits (transmons) and capacitively-shunted flux qubits. Hybrid fabrication techniques were employed to combine high-quality-factor aluminum capacitive shunts with shadow-evaporated Josephson junctions, and the Josephson junctions were prepared using suspended-bridge germanium masks. We also will describe process testing results that were acquired to assess wafer-to-wafer reproducibility of our fabrication protocols. This research was funded in part by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA); and by the Assistant Secretary of Defense for Research and Engineering under Air Force Contract number FA8721-05-C-0002. All statements of fact, opinion or conclusions contained herein are those of the authors and should not be construed as representing the official views or policies of IARPA, the ODNI, or the U.S. Government. [Preview Abstract] |
Wednesday, March 5, 2014 12:03PM - 12:15PM |
M36.00005: Radially symmetric transmon with long lifetime Martin Sandberg, Michael Vissers, Jiansong Gao, David Pappas We present a radially symmetric design for a large pad transmon qubit. The symmetry reduces the dipole radiation by orders of magnitude relative to axial large pad qubits that are widely used for 3D-circuit QED experiments. The reduction in radiation allows for the use of large area structures that are needed to reduce the effects of interface losses. This enables long qubit lifetimes without the use of a high-Q cavity resonator. Energy relaxation and coherence times of up to 35 microseconds have been measured. The qubit can be implemented in a microstrip geometry. This gives the advantage of removing discontinuous ground planes that can cause stray resonances.~ In addition, this geometry is well suited for implementing and exploring circuits with direct qubit-qubit coupling. [Preview Abstract] |
Wednesday, March 5, 2014 12:15PM - 12:27PM |
M36.00006: Josephson Phase Qubit with a Distributed Reactance Anthony Przybysz, David Ferguson, Ofer Naaman, Joel Strand, James Medford, Aaron Pesetski We present our characterization of a novel phase qubit design in which the capacitance across the Josephson junction and the inductance of the SQUID are provided by a microstrip resonator instead of lumped circuit elements. The spectroscopic data from this device clearly shows a behavior with respect to applied flux that cannot be accurately described by a simple SQUID model. We present measurements of the devices coherence times and anharmonicity, and compare the spectrum to a theoretical model that treats the phase drop across the length of the resonator as a continuous field. [Preview Abstract] |
Wednesday, March 5, 2014 12:27PM - 12:39PM |
M36.00007: Fabrication of superconducting single crystal aluminum resonators on silicon and sapphire Christopher Richardson, Justin Hackley, Jenn Robinson, Zach Keane, Benjamin Palmer Superconducting Al on sapphire are the mainstay materials used for the current development of superconducting qubit devices. We have grown single crystal Al films using MBE on both sapphire and silicon wafers with different surface preparations. Structural analysis indicates high quality films on both substrates with the twinned single crystal aluminum films abruptly relaxing misfit strain at the substrate interface. Different fabrication recipes for etching quarter-wave resonators and their impact on resonator performance will also be discussed. We have observed internal quality factors at low photon numbers above 600k for resonators on both substrates. Most resonators exhibit a lower than expected power dependence. [Preview Abstract] |
Wednesday, March 5, 2014 12:39PM - 12:51PM |
M36.00008: Measurement of superconducting single-crystal Al resonators on Si and sapphire substrates J.E. Robinson, J. Hackley, Z.K. Keane, C.J.K. Richardson, B.S. Palmer Al made via molecular beam epitaxy offers improvements to existing superconducting qubit architectures due to decreased loss at the interface between the Al and the substrate [1]. We have studied this loss by measuring the quality factors of a variety of superconducting quarter-wave resonators fabricated under different conditions from single-crystal aluminum on both silicon and sapphire substrates. The resonators, which have resonant frequencies between 4.5 and 6 GHz, were measured at a temperature of 25 mK and from an average stored photon number n$\sim$1 up to 10$^{6}$. At low photon numbers, we consistently observe Q\textgreater 200k on both substrates. We will discuss potential limitations on the measured loss and steps taken to mitigate them. \\[4pt] [1] A. Megrant, et al., App. Phys. Lett., {\bf 100}, 113510 (2012). [Preview Abstract] |
Wednesday, March 5, 2014 12:51PM - 1:03PM |
M36.00009: Microscopic Sources of Paramagnetic Noise on $\alpha$-Al2O3 Substrates for Superconducting Qubits Jonathan DuBois, Donghwa Lee, Vince Lordi Superconducting qubits (SQs) represent a promising route to achieving a scalable quantum computer. However, the coupling between electro-dynamic qubits and (as yet largely unidentified) ambient parasitic noise sources has so far limited the functionality of current SQs by limiting coherence times of the quantum states below a practical threshold for measurement and manipulation. Further improvement can be enabled by a detailed understanding of the various noise sources afflicting SQs. In this work, first principles density functional theory (DFT) calculations are employed to identify the microscopic origins of magnetic noise sources in SQs on an $\alpha$-Al2O3 substrate. The results indicate that it is unlikely that the existence of intrinsic point defects and defect complexes in the substrate are responsible for low frequency noise in these systems. Rather, a comprehensive analysis of extrinsic defects shows that surface aluminum ions interacting with ambient molecules will form a bath of magnetic moments that can couple to the SQ paramagnetically. The microscopic origin of this magnetic noise source is discussed and strategies for ameliorating the effects of these magnetic defects are proposed. [Preview Abstract] |
Wednesday, March 5, 2014 1:03PM - 1:15PM |
M36.00010: Characterization of the temperature dependence of dielectric loss at microwave frequencies in Al$_2$O$_3$ and TiO$_2$ films grown by atomic layer deposition Martin Otto, Chunqing Deng, Jean-Luc Orgiazzi, Adrian Lupascu Low temperature dielectric loss is one of the primary sources of decoherence in superconducting quantum bits and resonators. We performed detailed dielectric loss measurements of Al$_2$O$_3$ and TiO$_2$ thin films grown by atomic layer deposition in the 3-8 GHz frequency range at temperatures ranging from 36mK to 1K. The intrinsic Q-factor is extracted by measuring superconducting Niobium lumped element resonators which contain the dielectric material of thickness ranging from 30-100 nm. We find the temperature dependence of the loss tangent and resonance frequency agree with the tunnelling two-level system model. We also find a systematic dependence of the saturation voltage on temperature and film thickness. We compare the results obtained for Al$_2$O$_3$ films grown by atomic layer deposition with those grown by plasma oxidation. For these two different growth methods, we find similar values of the loss tangent despite different impurity content. [Preview Abstract] |
Wednesday, March 5, 2014 1:15PM - 1:27PM |
M36.00011: Engineering Filters for Reducing Spontaneous Emission in cQED Nicholas Bronn, Nicholas Masluk, Srikanth Srinivasan, Jerry Chow, David Abraham, Mary Rothwell, George Keefe, Jay Gambetta, Matthias Steffen, Chris Lirakis Inserting a notch filter between a qubit and the external environment at the qubit frequency can significantly suppress spontaneous emission mediated by the cavity (``Purcell effect''). In order to realize this filtering in multi-qubit architectures, where space comes at a premium, we will present a filter with minimal space requirements. [Preview Abstract] |
Wednesday, March 5, 2014 1:27PM - 1:39PM |
M36.00012: Fabrication and Characterization of Aluminum Airbridges for Superconducting Qubit Circuits Zijun Chen, Anthony Megrant, Julian Kelly, Rami Barends, Joerg Bochmann, Yu Chen, Benjamin Chiaro, Andrew Dunsworth, Evan Jeffrey, Joshua Mutus, Peter O'Malley, Charles Neill, Pedram Roushan, Daniel Sank, Amit Vainsencher, James Wenner, Theodore White, Andrew Cleland, John Martinis Superconducting circuits based on coplanar waveguides (CPWs) are susceptible to parasitic slotline modes which can lead to loss and decoherence. We motivate the use of superconducting airbridges as a reliable method for preventing the propagation of these modes. We describe the fabrication of these airbridges on superconducting resonators, which we use to measure the loss due to placing airbridges over CPW lines. We find that the additional loss at single photon levels is small, and decreases at higher drive powers. These results pave the way for building airbridge crossovers on more complex qubit circuits. [Preview Abstract] |
Wednesday, March 5, 2014 1:39PM - 1:51PM |
M36.00013: Substrate removal near superconducting qubits and resonators in circuit QED processors Alessandro Bruno, Leonardo DiCarlo We investigate the effect of etching away substrate volumes in the capacitive gaps of transmon qubits and resonators in planar circuit QED quantum processors. We use cryogenic and deep reactive-ion etching techniques to control the etching depth, profile, wall roughness, and passivation chemistries in high-resistivity silicon substrates. Two independent etching steps allow freedom to choose which areas to etch using standard fluorine etch (i.e., feeds and readout resonators), and which volumes to deeply etch (i.e., capacitive gaps in transmons and bus resonators). We study the effects of different etching techniques on losses in superconducting resonators operating in the quantum regime and on relaxation times of transmon qubits. [Preview Abstract] |
Wednesday, March 5, 2014 1:51PM - 2:03PM |
M36.00014: Chip Mount Design as a Dissipation-Limiting Factor in High Quality Superconducting Resonators Brooks Campbell, R. Barends, J. Bochmann, Yu Chen, Z. Chen, B. Chiaro, A. Dunsworth, I. Hoi, E. Jeffrey, J. Kelly, A. Megrant, J. Mutus, C. Neil, P. O'Malley, C. Quintana, P. Roushan, D. Sank, A. Vainsencher, J. Wenner, T. White, A.N. Cleland, J.M. Martinis Superconducting quantum computing technology continues to make progress with regards to both materials quality and circuit complexity. We have found that chip mount design can become a coherence-limiting factor for superconducting coplanar resonators with an internal quality factor above 1 million. Understanding the impact of chip-to-mount coupling will aid in both proper mount design for higher density circuits as well as the further improvement of coherence times. These coplanar resonators provide an ideal test circuit as they are sensitive to a variety of loss mechanisms including radiation, infrared light, and magnetic fields which also affect more complex superconducting circuits. I will present results relating the coherence and performance of resonators to box design, box material, and chip layout. [Preview Abstract] |
Wednesday, March 5, 2014 2:03PM - 2:15PM |
M36.00015: Metamaterials for circuit QED: Quantum simulations and other applications Bruno G. Taketani, Frank K. Wilhelm The ability to design periodically structured materials not present in nature provides scientists with new tools, ranging from sub-wavelength imaging to well controlled band structures for wave propagation in photonic crystals. Superconducting metamaterials have been recently proposed to manipulate the density-of-modes of transmission lines [D. J. Egger and F. K. Wilhelm, Phys. Rev. Letters {\bf 111}, 163601 (2013)]. We further build on these ideas and develop a toolbox for environment manipulation based on nano-structured, periodic, lossless, superconducting circuits. In particular we show that high density of low energy states can be achieved using a superlattice arrangement of left-handed circuit elements. Multimode, ultra-strong coupling of superconducing qubits to such engineered environments thus allow for experimental implementation of quantum simulation of interesting new phenomena as well as for complex quantum state engineering. [Preview Abstract] |
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