Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session B39: Superconducting Circuits: Decoherence I |
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Sponsoring Units: GQI Chair: Ted Thorbeck, University of Wisconsin-Madison Room: 213AB |
Monday, March 2, 2015 11:15AM - 11:27AM |
B39.00001: Comparison of 2D transmon coherence for different capacitive shunt fabrication methods Jonilyn Yoder, Archana Kamal, Fei Yan, Theodore Gudmundsen, Paul Welander, Simon Gustavsson, David Hover, Andrew Kerman, Adam Sears, William Oliver Improvements in superconducting qubit coherence times and reproducibility have been demonstrated using capacitive shunting.~ In this study, we present a side-by-side comparison of two distinct methods for preparing the aluminum shunt capacitor material for 2D transmon superconducting qubit devices. The first method involved \textit{in situ} wafer outgassing prior to molecular beam epitaxy aluminum evaporation. The second method involved \textit{ex situ} wafer annealing prior to electron gun aluminum evaporation. Materials analysis for each process will be detailed. Experimental results, including qubit coherence times and superconducting coplanar waveguide resonator internal quality factors, will be presented for representative devices prepared using both methods. This work is sponsored by 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] |
Monday, March 2, 2015 11:27AM - 11:39AM |
B39.00002: Optimizing Hardware Compatibility for Scaling Up Superconducting Qubits Michael Fang, Brooks Campbell, Zijun Chen, Ben Chiaro, Andrew Dunsworth, Julian Kelly, Anthony Megrant, Charles Neill, Peter O'Malley, Chris Quintana, Amit Vainsencher, Jim Wenner, Ted White, Rami Barends, Yu Chen, Austin Fowler, Evan Jeffrey, Josh Mutus, Pedram Roushan, Daniel Sank, John Martinis Since quantum computation relies on the manipulation of fragile quantum states, qubit devices must be isolated from the noisy environment to prevent decoherence. Custom made components make isolation from thermal and infrared radiation possible, but have been unreliable, massive, and show sub-ideal microwave performance. Infrared isolation for large scale experiments (> 8 qubits) was achieved with compact impedance matched microwave filters which attenuate stray infrared signals on cryogenic cables with only -25 dB reflection up to 7.5 GHz. In addition, a thermal anchoring system was designed to effectively transfer unwanted heat from more than 100 coaxial cables in the dilution refrigerator and yielded a 33 percent improvement in base temperature and 50\% improvement in hold time. [Preview Abstract] |
Monday, March 2, 2015 11:39AM - 11:51AM |
B39.00003: Experiments on Interaction of Quasiparticles with Two-Level-Systems in a Superconducting Phase Qubit Alexander Bilmes, J\"urgen Lisenfeld, Andreas Heimes, Sebastian Zanker, Gerd Sch\"on, Alexey Ustinov Two-Level-Systems (TLS) are one of the main sources of decoherence in superconducting qubits. Some individual and coherent TLS, present in the tunnel barrier of the qubit's Josephson junction, can be coherently operated via the qubit. In the past, experiments on superconducting glasses indicated that quasiparticles may give rise to TLS energy loss similar to Korringa relaxation [1]. We will present experiments in which we use a phase qubit to explore the interaction of single TLS with non-equilibrium quasiparticles. We have implemented in-situ quasiparticle injection by using an on-chip dc-SQUID that is pulse-biased beyond its critical current. The quasiparticle density is calibrated by measuring associated characteristic changes to the qubit resonance frequency and energy relaxation rate [2]. The coherence times of individual TLS is measured in dependence of the non-equilibrium quasiparticle density and compared to thermally generated quasiparticles.\\[4pt] [1] J. L. Black, Low Energy Excitations in Metallic Glasses, Springer, 1981\\[0pt] [2] M. Lenander, H. Wang, Radoslaw C. Bialczak et al., Phys.Rev.Lett B 84, 024501 (2011) [Preview Abstract] |
Monday, March 2, 2015 11:51AM - 12:03PM |
B39.00004: Characterization of a Scalable Chip Mount Using a 5 Xmon Qubit Chain Brooks Campbell, Z. Chen, B. Chiaro, A. Dunsworth, I.-C. Hoi, J. Kelly, A. Megrant, C. Neill, P. J. J. O'Malley, C. Quintana, A. Vainsencher, J. Wenner, T. White, R. Barends, Y. Chen, A. Fowler, E. Jeffrey, J. Mutus, P. Roushan, D. Sank, John M. Martinis Superconducting quantum computing technology has progressed to the point that experiments involving the full control more than ten qubits will be realized in the next few years.~ As such, a scalable chip mount, able to accommodate dozens of microwave signal lines, will likely become necessary since current Xmon technology requires two control lines per qubit.~ Additionally, understanding parasitic coupling of Xmon qubits to control lines will aid in the proper design of both chips and chip mounts for even higher density circuits.~ I will present coherence, gate fidelity, and qubit cross-talk benchmark measurements from a high performance 5 Xmon chain in various chip mount designs and materials. [Preview Abstract] |
Monday, March 2, 2015 12:03PM - 12:15PM |
B39.00005: Influence of Non-equilibrium Noise on Quantum Superconducting Devices Jen-Hao Yeh, Jay Lefebvre, Baladitya Suri, Sergey Novikov, Frederick Wellstood, Benjamin Palmer Non-equilibrium noise from temperatures larger than T $\sim$ 20 mK coupled to quantum superconducting devices can cause energy relaxation [1], dephasing [2], and initialization errors [1]. In particular at low temperatures, any dissipated power can drive the electrons out of equilibrium with the phonons and produce thermal noise [3]. To understand thermal noise driven out of equilibrium, we have created both finite element simulations using COMSOL and some simple analytical models. Based on these thermal simulations and models as well as microwave simulations, we have designed and fabricated some devices to decrease the amount of non-equilibrium noise influencing our devices. The design of these devices as well as preliminary characterization using a transmon device will be discussed. \\[4pt] [1] R. J. Schoelkopf \textit{et al.,} Quantum Noise in Mesoscopic Physics, NATO Science Series, \textbf{97}, pp 175-203 (2003).\\[0pt] [2] A. P. Sears \textit{et al.}, Phys. Rev. B \textbf{86,} 180504 (2012).\\[0pt] [3] F. C. Wellstood, C. Urbina, and John Clarke, Phys. Rev. B \textbf{49}, 5942 (1994). [Preview Abstract] |
Monday, March 2, 2015 12:15PM - 12:27PM |
B39.00006: Non-exponential energy decay and quasi-particle fluctuations in a superconducting flux qubit Simon Gustavsson, Fei Yan, Gianluigi Catelani, Archana Kamal, Jonas Bylander, Fumiki Yoshihara, Yasunobu Nakamura, Terry Orlando, William Oliver We measure pronounced non-exponential energy relaxation in a superconducting flux qubit, observing a decay function that exhibits a fast initial decay followed by a much slower decay for long times. When applying a sequence of pi pulses to the qubit and measuring the decay after the last pi pulse, we observe strong modifications to the decay function, including a slow-down of the fast initial decay and a three-fold increase of the 1/e-time. If we attribute the non-exponential decay to quasiparticle number fluctuations, we speculate that the improvements in T1 are due to a qubit-mediated shuffling of quasiparticles between the metallic islands of the device, which will eventually pump them away from the Josephson junctions to a larger ground plane where their contribution to qubit energy relaxation become negligible. [Preview Abstract] |
Monday, March 2, 2015 12:27PM - 12:39PM |
B39.00007: Laser made from a superconducting lumped-element resonator and random defects Yaniv Rosen, Moe Khalil, Alex Burin, Kevin Osborn Random two-level system defects in dielectrics absorb energy and limit the quality factors of superconducting qubits and resonators used in quantum computing applications. We have found a method to invert the population of these random defects and pass them through resonance with a lumped-element superconducting microwave resonator. Stimulated emission of the defects causes the internal quality factor of the resonator, measured by an injection locking tone, to cross through an infinitely large value before becoming negative. In the latter case the defects emit more than they absorb and the internal lasing threshold is reached. With further population inversion, amplification of the injection locking tone is observed. [Preview Abstract] |
Monday, March 2, 2015 12:39PM - 12:51PM |
B39.00008: Investigation of Limiting Decoherence Mechanisms in Xmon Qubits C. M. Quintana, R. Barends, B. Campbell, Y. Chen, Z. Chen, B. Chiaro, A. Dunsworth, A. G. Fowler, I.-C. Hoi, E. Jeffrey, J. Kelly, A. Megrant, J. Mutus, C. Neill, P. J. J. O'Malley, P. Roushan, D. Sank, A. Vainsencher, J. Wenner, T. C. White, A. N. Cleland, J. M. Martinis Xmon-style transmon qubits have demonstrated a high level of coherence and controllability, enabling high-fidelity quantum gates and measurement at the levels required for surface code error correction. However, decoherence is still a limiting factor for fidelities, and further improvements to coherence could significantly reduce the overhead required to build a fault-tolerant quantum computer. We report on relaxation and dephasing mechanisms relevant to the Xmon qubit. In particular, we discuss dielectric loss from stray Josephson junctions and the dependence of dephasing on qubit temperature. [Preview Abstract] |
Monday, March 2, 2015 12:51PM - 1:03PM |
B39.00009: The temperature dependence of decoherence in superconducting flux qubits coupled to microwave resonators Jean-Luc Orgiazzi, Chunqing Deng, Feiruo Shen, Nicolas Gonzalez, Adrian Lupascu We present experiments on decoherence of superconducting flux qubits coupled to superconducting resonators [1]. We characterized decoherence over a temperature range between 35 and 150 mK. The energy relaxation rate increases sharply with temperatures beyond 120 mK due to thermal quasiparticles. The Ramsey and spin echo relaxation times, measured at the flux insensitive point, only have a weak dependence on temperature. Coherence measurements are also used to determine the power spectral density of flux noise at low and high frequencies, using Ramsey sampling and dynamical decoupling respectively. The flux noise is found to only weakly depend on temperature over the explored range. We will discuss the relevance of these results for improving coherence times in flux qubits and for understanding flux noise in superconducting devices.\\[4pt] [1] \textit{J.-L. Orgiazzi, C. Deng, D. Layden, R. Marchildon, F. Kitapli, F. Shen, M. Bal, F. R. Ong, A. Lupascu, arXiv preprint arXiv:1407.1346 (2014).} [Preview Abstract] |
Monday, March 2, 2015 1:03PM - 1:15PM |
B39.00010: Collective modes in the fluxonium qubit Gianluigi Catelani, Giovanni Viola In the fluxonium qubit, an array comprising a large number of identical Josephson junctions form a so-called superinductance. The superinductance is connected to a junction - the phase slip element - with a smaller Josephson energy and a different charging energy. We investigate the effects of unavoidable capacitive couplings to ground as well as non-linearities of the superinductance: they both introduce interactions between the low-energy qubit degree of freedom and higher-energy collective modes of the circuit. We also consider the role of the additional capacitances that are used to couple the qubit to a resonator for driving and read-out. We show that the interactions with the collective modes can affect not only the spectrum of the qubit but also its coherence. [Preview Abstract] |
Monday, March 2, 2015 1:15PM - 1:27PM |
B39.00011: Spurious modes in 3D multi-qubit circuits Martin Sandberg, Douglas Mcclure, Hanhee Paik, Daniela F. Bogorin, B.L.T. Plourde, Oliver Dial, Baleegh Abdo In superconducting 3D circuits coherence times exceeding 100 microseconds are readily achieved for qubits in single cavities. One approach to building more complex circuitry in the 3D architecture is to use ``bridge'' qubits that span into two adjacent cavities. It has been found that these qubits exhibit reduced coherence compared to single cavity qubits. Significant effort has been put into understanding and improving the coherence of the bridge qubit. So far the mechanisms behind the reduced coherence have remained somewhat unclear. Here we present simulations and measurements indicating that stray modes in the microwave environment are one contributing factor to the reduced coherence. One potential location of such stray modes is at the boundary regions between sections of the cavity enclosure, where both machining imperfections and dielectric layers such as oxides can prevent perfect electrical contact. As these systems are scaled up, the spectrum of the modes becomes increasingly dense, presenting an increasing challenge. We present multiple methods that can be implemented to mitigate these modes. [Preview Abstract] |
Monday, March 2, 2015 1:27PM - 1:39PM |
B39.00012: Cavity-induced decoherence in a long-lived superconducting flux qubit Fei Yan, Archana Kamal, Theodore Gudmundsen, Jonilyn Yoder, Simon Gustavsson, William Oliver We implement a circuit QED system with a capacitively shunted (C-shunt) flux qubit coupled to a transmission line resonator. The improved design substantially enhances qubit coherences, achieving T1 of 55us at degeneracy. Dephasing is also improved, giving a spin-echo decay of 40us. We found that the dephasing is limited by photon shot noise at a residual thermal photon population of 0.006. The driven-evolution T1rho-spectroscopy and free-evolution CPMG results both validate the finding. [Preview Abstract] |
Monday, March 2, 2015 1:39PM - 1:51PM |
B39.00013: Quantum optical theory of electronic noise in coherent conductors Farzad Qassemi, Bertrand Reulet, Alexandre Blais We investigate the quantum properties of radiated electromagnetic field from a quantum conductor, i.e., a conductor where the electron transport is governed by quantum mechanics. In particular, using tools borrowed from quantum optics such as input-output theory and Lindblad master equations, we demonstrate how the electron shot noise in the conductor tailors the properties of radiated field, leading to nonclassical electromagnetic radiation. Our results allow us to calculate the outcome of any measurement on the electromagnetic field in terms of the statistical properties of the current in the conductor. As an example, we explain the existence of squeezing, recently observed by G. Gasse et al [1]. \\[4pt] [1] G. Gasse et al Phys. Rev. Lett. 111, 136601 (2013) [Preview Abstract] |
Monday, March 2, 2015 1:51PM - 2:03PM |
B39.00014: Spectroscopy of Nanoscale Two-Level Systems in Insulating Films Bahman Sarabi, Aruna Ramanayaka, Frederick Wellstood, Kevin Osborn Nanoscale tunneling two level systems (TLSs) are viewed as defects in dielectric films because they are parasitic to the performance of superconducting qubits and resonators. Using a calibrated and uniform dc electric field within a special superconducting resonator we modify the energy potential of random TLSs in amorphous insulating films and measure them. As the dc electric field is applied, TLS energies are observed near the tunneling energies from their double-well degeneracy. From these measurements, the dipole moment projected along the field axis is directly extracted for each TLS. The random distribution of projected dipole moments shows that there are multiple dipole sizes in a silicon nitride film. This contrasts other techniques which find a single dipole size in an amorphous film. Spectroscopic splittings are observed which arise from a coherent exchange of a single photon between a TLS and the resonator, and they allow an overconstrained validation of cavity quantum electrodynamics with a TLS. The method used to measure multiple dipole moments is believed to be generally useful for the classification of TLSs, which can be used to test and screen films fabricated for coherent superconducting devices. [Preview Abstract] |
Monday, March 2, 2015 2:03PM - 2:15PM |
B39.00015: Lift-Off Processing and Aluminum on Silicon Superconducting Circuit Coherence Andrew Dunsworth, Anthony Megrant, Chris Quintana, Zijun Chen, Rami Barends, Yu Chen, Austin Fowler, Evan Jeffrey, Josh Mutus, Pedram Roushan, Daniel Sank, IoChun Hoi, Brooks Campbell, Ben Chiaro, Julian Kelly, Charels Neil, Peter O'Malley, Amit Vainsencher, Jim Wenner, Ted White, Andrew Cleland, John Martinis Dielctric loss from two level states (TLS's) are a limiting decoherence method in planar superconducting qubits. Previously we have shown that liftoff deposited metal has more loss than etched devices. Current fabrication techniques of Xmon qubit devices limit this loss by using liftoff metal on only a small area of the transmon including the Josephson junctions. However this method leads to excess loss when used on a silicon substrate. I have used quality factor measurments of coplanar waveguide resonator circuits as a tool to measure isolated steps in the liftoff processes. I will report on the effects of these steps and their added loss. [Preview Abstract] |
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