Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session B25: Superconducting Qubits: Loss Mechanisms (TLS) and Novel Materials |
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Sponsoring Units: GQI Chair: David Pappas, National Institute of Standards and Technology Room: 327 |
Monday, March 18, 2013 11:15AM - 11:27AM |
B25.00001: Delocalised oxygen as the origin of two-level defects in Josephson junctions Jared Cole, Timothy DuBois, Manolo Per, Salvy Russo One of the key problems facing superconducting qubits and other Josephson junction devices is the decohering effects of bi-stable material defects. Although a variety of phenomenological models exist, the true microscopic origin of these defects remains elusive. We show that these defects can arise from delocalisation of the atomic position of the oxygen in the oxide forming the Josephson junction barrier. Using a microscopic model, we compute experimentally observable parameters for phase qubits. Such defects are charge neutral but have non-zero response to both applied electric field and strain. This explains the observed long coherence time of two-level defects in the presence of charge noise, while still coupling to the junction electric field and substrate phonons. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B25.00002: Noise from Two-Level Systems in Superconducting Resonators C. Neill, R. Barends, Y. Chen, B. Chiaro, E. Jeffrey, J. Kelly, M. Mariantoni, A. Megrant, J. Mutus, S. Ohya, D. Sank, A. Vainsencher, J. Wenner, T. White, A. N. Cleland, J. M. Martinis Two-level systems (TLSs) present in amorphous dielectrics and surface interfaces are a significant source of decoherence in superconducting qubits. Linear microwave resonators offer a valuable instrument for characterizing the strongly power-dependent response of these TLSs. Using quarter-wavelength coplanar waveguide resonators, we monitored the microwave response of the resonator at a single near-resonant frequency versus time at varying microwave drive powers. We observe a time dependent variation of the resonator's internal dissipation and resonance frequency. The amplitude of these variations saturates with power in a manner similar to loss from TLSs. These results provide a means for quantifying the number and distribution of TLSs. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B25.00003: Universal dielectric loss in amorphous solids in Josephson qubits from simultaneous bias and microwave fields Alexander Burin, Kevin Osborn, Khalil Moe We calculate the microwave dielectric loss of an ensemble of two-level systems in amorphous solids within superconducting qubits during the application of a time-varying electric bias field. We find that this loss becomes universal in a wide range of temperatures and frequencies of the AC drive field, corresponding to the bare linear dielectric permittivity in the low-temperature limit. This non-equilibrium theory allows the separate extraction of the TLS density and their dipole size in experiments and can be used to reduce the destructive effect of decoherence. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B25.00004: Polaronic model of Two Level Systems in amorphous solids Kartiek Agarwal, Ivar Martin, Eugene Demler, Mikhail Lukin Motivated by recent experiments studying effects of elastic strain on two level systems (TLSs) in Josephson Junctions, we consider interaction of the electronic TLS with phonons. We demonstrate that including strong polaronic effects is crucial for analyzing these systems. Our model not only gives a quantitative understanding of the TLS relaxation and dephasing as probed in Josephson junction qubits, but also provides a microscopic justification for phenomenological models used to describe experiments with bulk amorphous solids. Our model explains such surprising observations of recent experiments as the existence of minima in the energy of some TLSs as a function of strain and maximum of the relaxation time in such minima. We argue that better understanding of the microscopic nature of TLSs can be used to improve properties of quantum devices, from dramatic enhancement of TLS relaxation time by putting them inside phononic crystals to creating new types of strongly interacting optomechanical systems. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B25.00005: Superconducting Titanium Nitride Coplanar Resonators: Relationships between performance and deposition parameters B. Chiaro, S. Ohya, A. Megrant, C. Neill, R. Barends, B. Campbell, Y. Chen, J. Kelly, M. Mariantoni, J. Mutus, P. O'Malley, P. Roushan, D. Sank, A. Vainsencher, J. Wenner, T. White, C.J. Palmstrom, B.A. Mazin, A.N. Cleland, J.M. Martinis Superconducting coplanar waveguide (CPW) resonators are widely used structures in the fields of photon detection and quantum information processing. Recently, there has been a growing interest in titanium nitride (TiN) thin films due to their widely tunable critical temperature, large surface inductance, and ability to produce high intrinsic quality factor (Q$_{i}$) resonators. We have deposited nearly stoichiometric TiN films on Si substrates by reactive magnetron sputtering. By increasing the deposition pressure and adjusting the N2 flow rate to maintain stoichiometry, the film stress was changed from $\sim 100$ MPa to $>3000$ MPa and the Q$_{i}$ of CPW resonators made from these films increased from $\sim 10^{4}$ to $\sim 10^{6}$ for single photon excitations measured at $\sim 100$ mK. In this talk, we discuss relationships between deposition parameters, film properties, and microwave electrodynamic responses in these resonators. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B25.00006: Characterization of quantum-regime dielectric loss of aluminum oxide using superconducting LC resonators Chunqing Deng, Martin Otto, Adrian Lupascu We report low-temperature measurements of dielectric loss of thin layers of aluminum oxide. The experiments are performed by measuring the microwave transmission of coplanar waveguides coupled to LC resonators where the capacitor contains the dielectric to be characterized. We develop a method, based on systematic approximations of transfer functions, to analyze the measured transmission curves. The fit of the resonance curves yields not only the loss tangent of the dielectric, but also the relation between the voltage on the capacitor and the excitation voltage. The latter is a nonlinear relation which has to be properly taken into account when analyzing the power dependence of dielectric loss. We find that the loss tangent of the aluminum oxide increases with decreasing capacitor voltage and temperature and reaches a constant value around $2\times 10^{-3}$ at sub-single photon levels. Our results are qualitatively in agreement with the two-level system defect model. Despite large loss, compact resonators based on these dielectrics have potential applications in microwave amplifiers. These results are relevant to understanding decoherence in superconducting quantum devices. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B25.00007: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 12:39PM - 12:51PM |
B25.00008: Observation of Cavity QED in thick dielectric films Bahman Sarabi, A.N. Ramanayaka, S. Gladchenko, M.J.A. Stoutimore, M.S. Khalil, K.D. Osborn Cavity QED in amorphous dielectrics is investigated by measuring five linear superconducting resonators with thick dielectric films and capacitor volumes ranging from 80$\mu $m$^{3}$ to 5000$\mu $m$^{3}$. In the smallest volume dielectrics we observe additional resonances which may be explained by CQED, despite the dielectric volume which is many orders of magnitude larger than Josephson junction barrier volumes. In addition to the volume dependence of the CQED resonances, we will report on the stability of the resonances in time and the phase noise. This research allows new fundamental studies on TLS phenomena in meso-volume amorphous dielectrics. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B25.00009: TLS-like temperature and power dependence for loss in superconducting coplanar resonators S. Gladchenko, M.J.A. Stoutimore, M. Khalil, K. D. Osborn Loss in 2D superconducting coplanar resonators and qubits is often limited by two-level systems thought to be on the metal and substrate surfaces. While these TLSs are thought to be similar to those found in amorphous dielectrics, their nature is generally different. In most experiments, loss in coplanar resonators shows power and temperature dependence which disagrees with TLS theory. Here we will show new data from high-quality Al on sapphire coplanar resonators which is in qualitative agreement with TLS theory, and discuss the quantitative differences to TLS theory. The data on surface TLS behavior will be compared to resonator measurements of ALD-grown thin films. [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B25.00010: Non-equilibrium two-level system dynamics probed with a biased bridge resonator Moe S. Khalil, Sergiy Gladchenko, M.J.A. Stoutimore, F.C. Wellstood, K.D. Osborn We have designed a biased bridge resonator (BBR), which allows us to probe amorphous dielectric films by simultaneously applying a quasi-static electric bias field in addition to a microwave electric field. The BBR is made with a bridge arrangement of capacitors using superconducting aluminum electrodes and operated at millikelvin temperatures. Measurements of a universal amorphous dielectric film at high microwave amplitudes and a sufficiently fast bias field ramp reveals a non-equilibrium dielectric loss equal to its intrinsic steady state value. This phenomenon is explained by a theory which uses the dynamics of charged two-level systems undergoing Landau-Zener transitions to remain in their ground state. We will compare the experimental data to Monte Carlo simulations of the theory which allow for the separate extraction of the dipole moment and the spectral density of two-level systems. [Preview Abstract] |
Monday, March 18, 2013 1:15PM - 1:27PM |
B25.00011: Observation of the dynamics of two-state parametric fluctuators in superconducting flux qubits Adrian Lupascu, Mustafa Bal, Mohammad Ansari Spectroscopic measurements of a few persistent current qubit samples yield data in which the spectroscopic lines are doublets. The doublet splitting decreases with increasing qubit transition frequency. In three devices with a relatively low Josephson to charging energy ratio $E_J/E_c$, the maximum splitting ranges between 30 and 270 MHz. The splitting value is found to have variations over time scales of the order of days. The doublet structure was not observed in two other samples with larger $E_J/E_c$. Assuming a model in which the qubit experiences a parametric fluctuation that changes its frequency, we perform an experiment to probe the time scale of this fluctuation. We repeat a sequence in which the qubit is reset by energy relaxation, then driven with weak Rabi $\pi$ pulses on one of the spectroscopy lines, and finally measured. The time correlation of the series of measurement results displays an exponential decay, consistent with a telegraph noise component in the qubit frequency. The correlation does not depend on time if the qubit is either not excited or driven with a strong Rabi pulse. The transition rate was found to vary between 8 kHz and 38 kHz for temperatures between 43 and 165 mK. We discuss quasiparticle poisoning and other possible source of this effect. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B25.00012: Low-frequency two level systems and $1/f$ noise in Al/AlO$_{x}$/Al Josephson junctions for superconducting qubits: achieving a noiseless junction Christopher Nugroho, Vladimir Orlyanchik, Dale Van Harlingen The characterization of low-frequency two level systems (TLS) provides a connection between the generic $1/f$ noise in Josephson junctions to the TLSs observed in qubit energy spectroscopy. We present measurements of the tunneling-resistance noise in nanoscale Al/AlO$_{x}$/Al shadow evaporated junctions with areas $<(100~\textrm{nm})^{2}$. As the junction area or the temperature is decreased we observed a crossover from ensemble-averaged $1/f$ noise to a random telegraph noise from isolated TLSs. From the area threshold for the onset of non-gaussianity, we estimate a density of TLSs in the amorphous AlO$_{x}$ barrier consistent with the magnitude of $1/f$ noise in larger junctions and the density of high frequency TLSs from qubit spectroscopy. Furthermore we may deduce the potential landscape of the TLSs by characterizing the switching times and signal variance as a function of voltage bias and temperature. In some junctions no fluctuators are active, giving rise to immeasurably small noise signal. We discuss the implication of our findings to qubit coherence times. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B25.00013: Josephson Phase Qubits Incorporating Novel Coherent Materials U. Patel, Y. Gao, D. Hover, G. Ribeill, S. Sendelbach, R. McDermott The Josephson phase qubit is an attractive candidate for scalable quantum information processing in the solid state; however, qubit coherence is currently limited by coupling to spurious microscopic defects in the materials used to realize the circuit. Here we demonstrate that the incorporation of crystalline, defect-free dielectrics into the circuit leads to a dramatic enhancement of energy relaxation times. In addition we describe the realization of improved superconductor-insulator interfaces with extremely low levels of excess low-frequency flux noise, and we discuss efforts to incorporate these interfaces into the qubit circuit in order to extend pure dephasing times. We describe qubit fabrication and tomographic characterization and discuss ultimate limits to qubit coherence. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B25.00014: Optimization of Transmon Qubit Fabrication Josephine Chang, Mary Beth Rothwell, George Keefe Rapid advances in the field of superconducting transmon qubits have refined our understanding of the role that substrate and interfaces play in qubit decoherence. Here, we review strategies for enhancing coherence times in both 2D and 3D transmon qubits through substrate design, structural improvements, and process optimization. Results correlating processing techniques to decoherence times are presented, and some novel structures are proposed for further consideration. [Preview Abstract] |
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