Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D39: Superconducting Circuits: Decoherence II |
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Sponsoring Units: GQI Chair: Robert McDermott, University of Wisconsin-Madison Room: 213AB |
Monday, March 2, 2015 2:30PM - 2:42PM |
D39.00001: Al/AlOx/Al Josephson junctions fabricated without double-angle evaporation Kyle Sundqvist, Pranav Sharma, Michael Babb, Jae Woo Suh, H. Rusty Harris Superconducting circuits are a common means to produce quantum-mechanically coherent structures. It is possible to produce superconducting circuits which may sustain and even amplify coherent states of microwaves close to the quantum limit. To this end, work is underway at Texas A\&M University to locally implement Josephson junctions in our research. We have developed our own process flow for Josephson junction fabrication. This technique does not rely on the commonly used Dolan-bridge double-angle evaporation technique, and is easily incorporated into the process flow of other solid-state devices at the AggieFab fabrication facility. [Preview Abstract] |
Monday, March 2, 2015 2:42PM - 2:54PM |
D39.00002: Dependence of transmon qubit relaxation rate on cavity photon population S.O. Mundhada, S. Shankar, Y. Liu, M. Hatridge, A. Narla, K.M. Sliwa, S.M. Girvin, M.H. Devoret In circuit QED experiments, a qubit is dispersively coupled to a cavity such that the cavity frequency depends on the qubit state. This dispersive shift enables quantum non-demolition readout of the qubit by exciting the cavity with a microwave pulse and detecting the phase shift of the reflected signal. However, this cavity excitation has been observed in experiments to increase the qubit relaxation rate, hence demolishing the qubit state and limiting the maximum measurement strength. Here we experimentally study this effect in a transmon qubit coupled to a three-dimensional superconducting cavity. We also explore alternate qubit circuits designed to mitigate this demolition effect. [Preview Abstract] |
Monday, March 2, 2015 2:54PM - 3:06PM |
D39.00003: Quasiparticles and vortices in superconducting microwave resonators Ibrahim Nsanzineza, B.L.T. Plourde Nonequilibrium quasiparticles and trapped magnetic flux vortices can significantly impact the performance of superconducting microwave resonant circuits and qubits at millikelvin temperatures. Quasiparticles result in excess loss, reducing resonator quality factors and qubit lifetimes. Vortices trapped near regions of large microwave currents also contribute excess loss. However, vortices located in current-free areas can actually trap quasiparticles and lead to a reduction in the quasiparticle loss. We will discuss experiments involving the controlled trapping of vortices for reducing quasiparticle densities as well as the use of normal metal quasiparticle traps in superconducting resonators. In our measurements, quasiparticles are generated either by stray pair-breaking radiation or by direct tunnel-junction injection. [Preview Abstract] |
Monday, March 2, 2015 3:06PM - 3:18PM |
D39.00004: Quantifying Surface Loss Induced by Anti-Vortex Hole Arrays in Planar Superconducting Circuits for Quantum Computation B. Chiaro, A. Megrant, A. Dunsworth, Z. Chen, B. Campbell, I.-C. Hoi, J. Kelly, 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, A. N. Cleland, J. M. Martinis Two important dissipation sources in superconducting circuits operated at low power are surface loss from two level systems (TLS) and magnetic vortex loss. By patterning the superconducting electrodes with an array of holes, it is possible to reduce or eliminate loss due to magnetic vortices. However, since the highest levels of coherence in planar superconducting circuits have been achieved by improving the electrode-substrate interface, it is natural to expect that adding hole arrays to the electrodes may cause excess surface loss. We present simulations predicting the excess loss magnitude to be $<10\%$ for typical ground plane hole arrays, but for extreme cases of hole size or placement the loss may be much greater. We confirm the simulation result with measurements of high quality factor resonators ($Q_{i}$ $>$ $10^{6}$) with and without the hole patterns. [Preview Abstract] |
Monday, March 2, 2015 3:18PM - 3:30PM |
D39.00005: $1/f^\alpha$ noise in interacting spin systems: a real space RG approach Kartiek Agarwal, Ivar Martin, Eugene Demler Localized paramagnetic electrons are believed to be the cause of magnetic flux noise that plagues superconducting qubits, but how such interacting spins generate frequency dependent noise of the form $1/f^\alpha$ is not well understood. We describe a novel real space RG procedure that is equipped to calculate directly various dynamical quantities in a strongly disordered Heisenberg spin system (in arbitrary dimensions), including the `noise' from such systems. In 1-D, we find that the RG procedure describes a fairly temperature-indepedent noise with a power law $\alpha < 1$ that varies smoothly depending on the disorder strength, relative concentration of Ferro/Anti-Ferro bonds and temperature. The dynamic structure factor (of spin-spin correlations) inherits this power law while displaying a crossover to a related power at higher frequencies. In 2-D, the RG results in dynamics that are diffusive at high temperatures but remain anomalous at lower temperatures. A possible connection of the phenomena of $1/f$ noise and Many-Body Localization is also discussed. [Preview Abstract] |
Monday, March 2, 2015 3:30PM - 3:42PM |
D39.00006: Flux noise in SQUIDs: Electron versus nuclear spins Rogerio de Sousa, Stephanie Laforest Superconducting Quantum Interference Devices (SQUIDs) are limited by intrinsic flux noise whose origin is unknown. We develop a method to accurately calculate the flux produced by spin impurities in realistic superconducting thin film wires, and show that the flux produced by each spin is much larger than anticipated by former calculations. Remarkably, the total flux noise power due to electron spins at the thin side surface of the wires is found to be of similar magnitude as the one due to electrons at the wide top surface of the wires. In addition, flux noise due to lattice nuclear spins in the bulk of the wires is found to be a sizable fraction of the total noise for some SQUID geometries. We discuss the relative importance of electron and nuclear spin species in determining the total noise power, and propose strategies to design SQUIDs with lower flux noise. [Preview Abstract] |
Monday, March 2, 2015 3:42PM - 3:54PM |
D39.00007: Adsorbed Oxygen Molecules as a Possible Source of Flux Noise in SQUIDs Chuntai Shi, Hui Wang, Jun Hu, Clare Yu, Ruqian Wu One of the dominant source of flux noise in SQUIDs is flux noise which has been attributed to mysterious fluctuating magnetic spins on the surface. We propose that the spins producing flux noise could be adsorbed O$_2$ molecules that have a magnetic moment of about 2 $\mu_B$. Using density functional calculations, we studied O$_2$ molecules adsorbed on a sapphire surface. We find that the barrier for spin rotation is small enough to allow almost free spin reorientation due to thermal excitations at low temperatures. Monte Carlo simulations of a 2D XY spin model yields $1/f$ noise where $f$ is frequency. [Preview Abstract] |
Monday, March 2, 2015 3:54PM - 4:06PM |
D39.00008: Adsorbed Oxygen Molecules as a Source of Flux Noise in SQUIDs Hui Wang, Zhe Wang, Jun Hu, Chuntai Shi, Clare C. Yu, Ruqian Wu A major obstacle for using superconducting quantum interference devices (SQUIDs) as qubits is the flux noise generated by fluctuating magnetic spins on the surface of SQUIDs. Using density functional theory (DFT) calculations, we investigated O2 adsorbates and various vacancies on an $\alpha $-alumina surface as spin candidates. Their spectroscopic features are directly compared to experimental data using the x-ray magnetic circular dichroism. The calculated magnetic anisotropy energy for the spin of O2 to rotate within a plane perpendicular to the axis of the O-O bond is only about 12 mK (or $\sim$ 1 $\mu $eV) so we believe that O2 molecules are the main source of flux noise in Al SQUIDs. [Preview Abstract] |
Monday, March 2, 2015 4:06PM - 4:18PM |
D39.00009: Qubit freezing under strong 1/f low frequency noise Fedir Vasko, Kostyantyn Kechedzhi, Vadim Smelyanskiy We consider an adiabatic sweep of the effective Zeeman field applied to a single qubit in presence of the noise unavoidable in real life superconduting qubits. Motivated by detailed experiments on flux qubits we consider a model including two types of noise: strong low frequency 1/f noise, and weakly coupled high frequency ohmic or subohmic noise. Noise components with frequencies higher than the qubit tunneling element result in excitation-relaxation dynamics of the qubit. At the same time the low frequency noise (with frequency lower than the qubit tunneling splitting) causes strong modulation of the level splitting between the ground and excited states. We show that the combined effect of the low and high frequency components of the noise results in the qubit freezing in the excited state with a sizable probability. This mechanism may set an upper bound on the quantum annealing computation time. [Preview Abstract] |
Monday, March 2, 2015 4:18PM - 4:30PM |
D39.00010: Exchange interactions of paramagnetic spins on -Al$_{2}$O$_{3}$ Jonathan DuBois, Donghwa Lee, Nicole Adelstein, Vincenzo Lordi Although Superconducting qubits (SQs) represent a promising route to achieving a scalable quantum computer, maintaining coherence remains a major challenge. It is generally accepted that the phase coherence of SQs are reduced by interactions of superconducting states with magnetic spins in the environment. In a previous study we showed that magnetic spins can be induced by ambient molecules on the Al$_{2}$O$_{3}$ substrate and lead to a paramagnetic noise in SQs. [PRL 112, 017001 (2014)] In this follow-on work we show how the induced surface magnetic moments can interact with each other and lead to competing magnetic ordering on the surface In particular, the energy landscape of diverse arrangements of surface magnetic spins are considered to understand spin-spin interaction mechanisms and magnetic ordering boundaries on the Al$_{2}$O$_{3}$ surface. In addition, we will also discuss how the ordered surface spins can be controlled by mechanical strain on the substrate. In conclusion, we propose routes to improving SQ performance by enhancing or minimizing magnetic ordering of induced spins on Al$_{2}$O$_{3}$ surface. [Preview Abstract] |
Monday, March 2, 2015 4:30PM - 4:42PM |
D39.00011: Low frequency magnetic flux noise: role of surface adsorbates Pradeep Kumar, Matthew Beck, John Freeland, Clare Yu, Ruqian Wu, Zhe Wang, David Pappas, Robert McDermott Excess low frequency 1/f flux noise is a major source of decoherence in superconducting quantum devices. It is generally accepted that the noise is due to surface magnetic defects, but the microscopic physics behind the noise mechanism is still unclear. Recent experiments suggest that adsorbates play a dominant role in the surface magnetism. Here, we describe X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) studies of magnetic adsorbates on the surfaces of superconducting thin films. We present the results of SQUID-based susceptibility and noise measurements that are part of an ongoing effort to reduce surface spin density and flux noise by improving the vacuum environment of the superconducting device. [Preview Abstract] |
Monday, March 2, 2015 4:42PM - 4:54PM |
D39.00012: Can magnetic noise from Kondo traps explain high frequency flux noise in superconducting qubits? Luis Dias da Silva, Rogerio de Sousa In solid state devices, charge and magnetic noise have common microscopic origin. Both occur due to the presence of Kondo traps nearby metallic wires. We use numerical renormalization group calculations to show that, despite their common origin, charge and magnetic noise have opposing behavior controlled by completely different energy scales. While magnetic noise follows an universal scaling with the Kondo temperature, charge noise remains well described by non-interacting theory even when the trap is deep into the Kondo regime. We show how these results may explain the high frequency (f= 1-10 GHz) Ohmic flux noise observed in SQUIDs and superconducting qubits. [Preview Abstract] |
Monday, March 2, 2015 4:54PM - 5:06PM |
D39.00013: Unconventional permittivity noise and dielectric loss from tunneling systems A. N. Ramanayaka, B. S. Sarabi, S. Gladchenko, K. D. Osborn The performance of phase-coherent superconducting devices, e.g. resonators and qubits, are limited by tunneling two-level-systems (TLSs) contained within their dielectric layers. We have measured the power loss and 1/f permittivity noise of deposited dielectrics at microwave frequencies over a range of millikelvin temperatures. The experiments were carried out with a uniform microwave field amplitude applied throughout the dielectric as the center layer of a trilayer capacitor contained within a resonator circuit. Measurements of silicon nitride show deviations from the standard model of TLSs. The film absorption shows an unusual temperature dependence. In addition the 1/f permittivity noise increases as the temperature decreases, and also shows a decrease in 1/f noise above a particular electric field amplitude. The results will be compared to recent strong-interaction theories. [Preview Abstract] |
Monday, March 2, 2015 5:06PM - 5:18PM |
D39.00014: Exchange coupling of spins on superconducting qubit substrates from first-principles Nicole Adelstein, Donghwa Lee, Jonathan Dubois, Vincenzo Lordi Magnetic flux noise in superconducting qubits remains a hurdle to the realization of a scalable quantum computer. Current performance as measured by the lifetime of quantum states in these systems is, however, largely limited by an as yet unidentified source of low frequency flux noise. Recent experimental and theoretical results suggest that spin-clustering of paramagnetic defects can explain the measured low frequency noise spectrum. Modeling of spin-ordering requires knowledge of the couplings between spin defects. We present here exchange coupling between spin defects on the surface of common substrate materials calculated using first-principles density functional theory. [Preview Abstract] |
Monday, March 2, 2015 5:18PM - 5:30PM |
D39.00015: Dephasing of Superconducting Asymmetric Transmon Qubits Matt Hutchings, Matthew Ware, Caleb Howington, B.L.T. Plourde Split-junction transmon qubits allow for the tuning of qubit energy levels with a magnetic flux. However, this tunability can lead to excess dephasing due to flux noise. By making the two junctions asymmetric, the modulation range of the qubit energy bands can be reduced along with the sensitivity to flux noise. Such asymmetric transmons have been used previously for demonstrations of flux-modulated first-order sideband transitions between a qubit and cavity. We will describe experiments on asymmetric transmons in a multi-qubit geometry to study the effect of varying the junction asymmetry on qubit dephasing due to flux noise. [Preview Abstract] |
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