Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session L51: Decoherence and Defects in Superconducting Circuits Continued |
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Sponsoring Units: GQI Chair: Frederick Wellstood, University of Maryland Room: 398 |
Wednesday, March 15, 2017 11:15AM - 11:27AM |
L51.00001: Photon Statistics of Propagating Thermal Microwaves F. Deppe, J. Goetz, P. Eder, M. Fischer, S. Pogorzalek, E. Xie, K.G. Fedorov, A. Marx, R. Gross In experiments with superconducting quantum circuits, characterizing the photon statistics of propagating microwave fields is a fundamental task. This task is in particular relevant for thermal fields, which are omnipresent noise sources in superconducting quantum circuits covering all relevant frequency regimes. We quantify the $n^{2}\,{+}\,n$ photon number variance of thermal microwave photons emitted from a black-body radiator for mean photon numbers $0.05\,{\leq}\,n\,{\leq}\,1.5$ [1]. In addition, we also use the fields as a sensitive probe for second-order decoherence effects of the qubit. Specifically, we investigate the influence of thermal fields on the low-frequency spectrum of the qubit parameter fluctuations. We find an enhacement of the white noise contribution of the noise power spectral density. Our data confirms a model of thermally activated two-level states interacting with the qubit [2]. [1] J. Goetz et al., arXiv: 1609.07353 (2016). [2] J. Goetz et al., arXiv: 1609.07351 (2016). [Preview Abstract] |
Wednesday, March 15, 2017 11:27AM - 11:39AM |
L51.00002: Reducing Thermal Photon Dephasing in Transmon Qubits: Microwave Attenuator Design Jen-Hao Yeh, Rui Zhang, Jay Lefebvre, F. C. Wellstood, B. S. Palmer One source of dephasing for superconducting transmon qubits is due to fluctuations in the number of microwave photons in the read-out cavity. Heating in attenuators connected to the input of the cavity can lead to excess non-equilibrium photons. To improve upon the filtering and thermalization of microwave signals going to the cavity, we have designed and fabricated cryogenic microwave attenuators for use in experiments with quantum devices. The thermal photons from the attenuator have been quantified by performing coherence measurements on an Al/AlO$_{\mathrm{x}}$/Al 3D transmon for different attenuator temperatures and power dissipation. In comparison with previous measurements using commercial attenuators, we have observed as large as a factor of two decrease in the effective noise temperature at the output of the attenuator, to T$_{\mathrm{eff}}$ lower than 60 mK, corresponding to a decrease by a factor of 100 in the average number of thermal photons to less than 10$^{\mathrm{-3}}$ photons. Further improvements to the design to increase the cooling power of these attenuators will be described. [Preview Abstract] |
Wednesday, March 15, 2017 11:39AM - 11:51AM |
L51.00003: Reducing Thermal Photon Dephasing in Transmon Qubits: Qubit Measurements Rui Zhang, Jen-Hao Yeh, F. C. Wellstood, B. S. Palmer With recent improvements in the energy relaxation times of superconducting transmon qubits, dephasing due to fluctuations in the number of photons in the cavity has become more noticeable. We have quantified the performance of custom-designed 20 dB and 30 dB microwave attenuators by measuring the coherence of a 3D Al/AlO$_{\mathrm{x}}$/Al transmon qubit as a function of temperature and as a function of dissipated power in the attenuator. Above a temperature of 50 mK, the coherence time T2 of the qubit begins to decrease due to an increase in the fluctuating number of thermal equilibrium photons in the cavity. We find the effective noise temperature T$_{\mathrm{n}}$ of the attenuator depends on the dissipated power P$_{\mathrm{d}}$ as T$_{\mathrm{n}}\propto $ P$_{\mathrm{d}}^{\mathrm{1/5.4}}$. Comparing our results with simulations, this behavior suggests that the hot electron effect (decoupling of the electrons from the phonons) is limiting the cooling power of the attenuator. [Preview Abstract] |
Wednesday, March 15, 2017 11:51AM - 12:03PM |
L51.00004: Cold cavity attenuator for reducing thermal photon population in circuit quantum electrodynamics Z. Wang, S. Shankar, A. Narla, U. Vool, M.H. Devoret Dephasing induced by residual thermal photons in the readout resonator has been identified as one of the leading factors limiting the decoherence time of transmon qubits in the circuit QED architecture. This residual thermal population of the order of $10^{-3}$ is suspected to arise from noise impinging on the resonator from the input and output transmision lines. We have designed and tested a new, ancillary, dissipative brass cavity to protect the readout resonator from input noise. This ancillary cavity, in effect, serves as a narrowband microwave attenuator for superconducting quantum circuits that is well thermalized to the 20 mK stage of the fridge. We study its influence on the thermal photon population in a qubit-3D cavity system by measuring the spectral density of the noise coupled to the transmon qubit using the spin-locking method. [Preview Abstract] |
Wednesday, March 15, 2017 12:03PM - 12:15PM |
L51.00005: Zeno effects from controlled bath interactions Jonathan Monroe, Patrick Harrington, Kater Murch The Zeno and anti-Zeno effects are features of measurement-driven quantum evolution where frequent measurements inhibit or accelerate the decay of a quantum state. Either type of evolution can emerge depending on measurement method and the system-environment interaction. Zeno and anti-Zeno effects are basic examples of quantum control that can arise from dissipative reservoir engineering. In this experiment, we use a superconducting qubit to map out both types of Zeno effects in the presence of structured thermal noise baths and variable measurement rates. We observe both the suppression and acceleration of qubit decay as repeated dephasing measurements modulate the qubit spectrum causing it to sample different portions of the bath. [Preview Abstract] |
Wednesday, March 15, 2017 12:15PM - 12:27PM |
L51.00006: Effect of bath temperature on the decoherence of quantum dissipative system Wei Wu We report an anomalous decoherence phenomenon of quantum dissipative system in the framework of stochastic decoupling scheme along with hierarchical equations of motion formalism without Born, Markovian and weak coupling approximations. It is found that the decoherence of a two-qubit spinboson model can be reduced by increasing the bath temperature in strong coupling regimes. For weak coupling situation, we find that the bath temperature may enhance the decoherence. This result is contrary to the common recognition that a higher bath temperature always induces a severer decoherence and suggests that a decoherence dynamical transition occurs in this two-qubit spin-boson model. We also demonstrate that the critical transition point can be characterized by the behavior of the frequency spectrum of quantum coherence indicator. [Preview Abstract] |
Wednesday, March 15, 2017 12:27PM - 12:39PM |
L51.00007: Proximity Effect in Normal-Metal Quasiparticle Traps Amin Hosseinkhani, Gianluigi Catelani In many superconducting devices, including qubits, quasiparticle excitations are detrimental. A normal metal (N) in contact with a superconductor (S) can trap these excitations. However, the contact between N and S modifies the properties of both materials, a phenomenon known as proximity effect which has drawn attention since the '60s. Despite this long history, we find new analytical results for the density of states, which shows a square root threshold behavior at the minigap energy. In superconducting qubits, the trap must be placed far enough from a Josephson junction in order not to harm the qubit coherence. To estimate the minimum trap-junction separation, we study how the density of states in the superconductor depends on the distance from the trap. For high interface resistance between N and S, a separation of several (5-7) coherence lengths is sufficient. [Preview Abstract] |
Wednesday, March 15, 2017 12:39PM - 12:51PM |
L51.00008: Normal-metal quasiparticle traps for superconducting qubits Roman-Pascal Riwar, Amin Hosseinkhani, Luke D. Burkhart, Yvonne Y. Gao, Robert J. Schoelkopf, Leonid I. Glazman, Gianluigi Catelani The coherence time of superconducting qubits is intrinsically limited by the presence of quasiparticles. While it is difficult to prevent the generation of quasiparticles, keeping them away from active elements of the qubit provides a viable way of improving the device performance. We develop theoretically and validate experimentally a model for the effect of a single small trap on the dynamics of the excess quasiparticles injected in a transmon-type qubit. By means of this model, we show that for small traps, increasing the size shortens the evacuation time of quasiparticles from the transmon. We further identify a characteristic trap size above which the evacuation time saturates to the diffusion time of the quasiparticles. In the diffusion limit, the geometry of the qubit and the trap become relevant. We compute the optimal trap number and placement for several realistic geometries. Finally, our estimates show that the dissipation introduced by the presence of normal metal traps is well below the losses observed in the transmon. [Preview Abstract] |
Wednesday, March 15, 2017 12:51PM - 1:03PM |
L51.00009: Proximitized-superconducting quasiparticle traps for circuit QED devices K. Serniak, G. de Lange, U. Vool, M. Hays, L.D. Burkhart, Y.Y. Gao, I.M. Pop, L. Frunzio, L.I. Glazman, R.J. Schoelkopf, M.H. Devoret Recent experiments have shown that the density of quasiparticles in superconducting quantum circuits exceeds the expected thermal density. In Josephson junction based superconducting qubits, these non-equilibrium quasiparticles can tunnel through the junctions of the circuit, causing decoherence. Quasiparticle traps aim to reduce the density of quasiparticles near the junctions, and therefore the rate of energy loss and dephasing due to tunneling events. Using the proximity effect between titanium and aluminum, one can selectively reduce the superconducting gap away from the Josephson junctions of a circuit, creating quasiparticle traps. In this talk, we will discuss progress in the design and characterization of these proximitized-superconducting quasiparticle traps and their effect on quasiparticle dynamics. [Preview Abstract] |
Wednesday, March 15, 2017 1:03PM - 1:15PM |
L51.00010: Phonon-Mediated Quasiparticle Poisoning of Superconducting Microwave Resonators U. Patel, Ivan V. Pechenezhskiy, K. R. Dodge, B. L. T. Plourde, M. G. Vavilov, R. McDermott Classical Josephson digital logic based on Single Flux Quantum (SFQ) pulses offers a path to high-fidelity coherent control of large-scale superconducting quantum machines. However, an SFQ pulse driver generates nonequilibrium quasiparticles that contribute to qubit relaxation, and steps must be taken to protect the qubit from this decoherence channel. Here we investigate the mechanism of quasiparticle poisoning in devices subjected to local quasiparticle injection. We use NIS junctions to controllably inject quasiparticles into the groundplane of superconducting resonator chips, and we characterize the quasiparticle contribution to dissipation. We examine the effectiveness of groundplane cuts and normal metal quasiparticle traps at protecting the quantum modes against quasiparticle loss. We find that quasiparticle poisoning is dominated by the propagation of pair-breaking phonons across the chip. We characterize the energy dependence of the timescale for quasiparticle poisoning. Finally, we observe that incorporation of extensive normal metal quasiparticle traps leads to a more than order of magnitude reduction in quasiparticle loss for a given injected quasiparticle power. [Preview Abstract] |
Wednesday, March 15, 2017 1:15PM - 1:27PM |
L51.00011: Microwave response of vortices in superconducting Nb resonators K. Dodge, B. L. T. Plourde Magnetic flux vortices driven by microwave currents are an important loss mechanism in superconducting resonators and qubits. Prior work has focused on the behavior of vortices trapped in aluminum thin films. Here we present measurements of vortices trapped in superconducting coplanar waveguide resonators fabricated from other superconducting thin films, including niobium. Field cooling of multiple resonators with different parameters is used to study the magnetic field and frequency dependence of the microwave vortex response. [Preview Abstract] |
Wednesday, March 15, 2017 1:27PM - 1:39PM |
L51.00012: Magnetic-field compatibility of SNS transmon qubits Florian Luthi, Thijs Stavenga, Alessandro Bruno, Christian Dickel, Nathan Langford, Adriaan Rol, David Thoen, Akira Endo, Thomas Jespersen, Jesper Nygard, Peter Krogstrup, Leo DiCarlo We present an experimental investigation of the magnetic-field resilience of superconductor-semiconductor-superconductor (SNS) transmon qubits. Our study includes fixed-frequency and gate-tuneable single-junction transmons and flux-tunable, two-junction variants. The clean interface between the InAs nanowires and epitaxially-grown aluminium shells that constitute the Josephson element give these transmons energy relaxation times T$_1$ up to 15 $\mu$s and echo dephasing times T$_{2\mathrm{e}}$ up to 30 $\mu$s at zero field. We track the evolution of transition frequency and coherence at in-plane fields up to 70 mT, using standard spectroscopy and time-domain techniques to identify dominant sources of relaxation and dephasing. [Preview Abstract] |
Wednesday, March 15, 2017 1:39PM - 1:51PM |
L51.00013: Decoupling a superconducting qubit from dielectric loss and other sources of linear dissipation Yen-Hsiang Lin, BaoLong Nguyen, Nick Grabon, Jon Migue, Vladimir Manucharyan The inter-well "fluxon" transition of a fluxonium circuit has a dipole matrix element that decreases drastically as the ratio EJ/EC becomes large due to a weak overlap of wavefunctions localized in the two Josephson wells. Thus naturally suppresses all linear energy relaxation mechanisms, such as dielectric loss. Despite the vanishing transition dipole of such a qubit, there is still a finite dispersive shift due to the presence of strongly-coupled intra-well "plasmon" transitions in the circuit. By tuning EJ/EC ratio with an external magnetic flux we observed a factor of 100 enhancement of qubit lifetime from about 20 microseconds to over 2 millisecond for a nearly the same transition frequency. Our experiment demonstrates that a highly-decoupled, long-lived qubit can still be coherently manipulated and read out in a multi-level superconducting circuit [Preview Abstract] |
Wednesday, March 15, 2017 1:51PM - 2:03PM |
L51.00014: Decoherence mechanisms in a fluxonium qubit Long Nguyen, Yen-Hsiang Lin, Nicholas Grabon, Jonathan San Miguel, Vladimir Manucharyan |
Wednesday, March 15, 2017 2:03PM - 2:15PM |
L51.00015: Abstract Withdrawn As part of an experiment to optically trap $^{\mathrm{87}}$Rb atoms near a superconducting device, we have coupled an optical fiber to a translatable thin-film lumped-element superconducting Al microwave resonator that is cooled to 15 mK in a dilution refrigerator. The lumped-element resonator has a resonance frequency of 6.15~GHz, a quality factor of 8 x 10$^{\mathrm{5}}$ at high powers, and is mounted inside a superconducting aluminum 3D cavity. The 60-\textmu m-diameter optical fiber passes through small openings in the cavity and close to the lumped-element resonator. The 3D cavity is mounted on an x-z Attocube-translation stage that allows the lumped-element resonator and optical fiber to be moved relative to each other. When the resonator is brought near to the fiber, we observe a shift in resonance frequency, of up to 8 MHz, due to the presence of the fiber dielectric. When optical power is sent through the fiber, Rayleigh scattering in the fiber causes a position-dependent weak illumination of the thin-film resonator affecting its resonance frequency and Q. We model the optical response of the resonator by taking into account optical production, recombination, and diffusion of quasiparticles as well as the non-uniform position-dependent illumination of the resonator. |
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