Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F28: Focus Session: Superconducting Qubits: Coherence & Noise |
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Sponsoring Units: GQI Chair: David Schuster, University of Chicago Room: 601 |
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F28.00001: Towards a Spin-Ensemble Quantum Memory for Superconducting Qubits Invited Speaker: P. Bertet A multi-mode quantum memory able to store coherently large numbers of qubit states is a desirable resource for quantum information. We report progress towards this direction, using an ensemble of electronic spins (NV centers in diamond) coupled to a superconducting transmon qubit via a tunable resonator. We demonstrate the reversible coherent storage and retrieval of a single microwave photon from the qubit into the spin ensemble [1]. In this experiment the storage time was however limited by inhomogeneous broadening of the ensemble of spins. We propose a realistic protocol [2] that should extend the ensemble storage time by several orders of magnitude, based on spin-echo like pulse sequences; first experimental results will be presented [3].\\[4pt] [1] Y. Kubo et al., PRL \textbf{107}, 220501 (2011).\\[0pt] [2] B. Julsgaard, C. Grezes, P. Bertet, and K. Moelmer, \textbf{PRL} 110, 205503 (2013).\\[0pt] [3] C. Grezes et al., submitted (2014). [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F28.00002: Quantum memory operations in a flux qubit - spin ensemble hybrid system S. Saito, X. Zhu, R. Amsuss, Y. Matsuzaki, K. Kakuyanagi, T. Shimo-Oka, N. Mizuochi, K. Nemoto, W. J. Munro, K. Semba Superconducting quantum bits (qubits) are one of the most promising candidates for a future large-scale quantum processor. However for larger scale realizations the currently reported coherence times of these macroscopic objects (superconducting qubits) has not yet reached those of microscopic systems (electron spins, nuclear spins, etc). In this context, a superconductor-spin ensemble hybrid system has attracted considerable attention. The spin ensemble could operate as a quantum memory for superconducting qubits. We have experimentally demonstrated quantum memory operations in a superconductor-diamond hybrid system [1]. An excited state and a superposition state prepared in the flux qubit can be transferred to, stored in and retrieved from the NV spin ensemble in diamond. From these experiments, we have found the coherence time of the spin ensemble is limited by the inhomogeneous broadening of the electron spin (4.4 MHz) and by the hyperfine coupling to nitrogen nuclear spins (2.3 MHz). In the future, spin echo techniques could eliminate these effects and elongate the coherence time. Our results are a significant first step in utilizing the spin ensemble as long-lived quantum memory for superconducting flux qubits. [1] S. Saito, et al., Phys. Rev. Lett. 111, 107008 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F28.00003: Improved coherence times for transmon qubits in two-dimensional resonators Simon Gustavsson, Archana Kamal, Theodore Gudmundsen, Jonilyn Yoder, Paul Welander, Xiaoyue Jin, Fei Yan, David Hover, Andrew Kerman, Adam Sears, Terry Orlando, William Oliver We have designed, fabricated and characterized the coherence of transmon qubits coupled to planar microwave resonators. By using high-quality, epitaxially grown aluminum, we see a significant increase in coherence times compared to samples fabricated with evaporated metal. We also study how the coherence time scales with qubit dimensions, and for the device with largest spacing between the fingers of the interdigitated capacitance we report an energy-relaxation time (T1) of 34 us. The Lincoln Laboratory portion of this work was sponsored by the Assistant Secretary of Defense for Research {\&} Engineering under Air Force Contract number FA8721-05-C-0002.~ Opinions, interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the United States Government. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F28.00004: Geometrical description of nonreciprocity in coupled two-mode systems Jose Aumentado, Leonardo Ranzani Traditional microwave and optical devices that break reciprocal symmetry are based on the Faraday effect in anisotropic materials such as ferrites. These devices contain permanent magnets and are therefore not compatible with superconducting quantum circuits. Various nonreciprocal devices that do not employ dc magnetic fields to break reciprocal systems have been discussed in the literature, but it is not obvious if and how these different systems might be connected conceptually. In this talk we explore the concept of nonreciprocity in coupled two-mode systems using a geometric mapping to the Poincar\'{e} sphere. In this picture the evolution of the system is described by a rotation sequence of the state vector, where the axis of rotation is determined by the matrix of the coupled-mode system and a different order for the rotations corresponds to a different direction of propagation of the signal. The requirements for reciprocity are then expressed in terms of geometric properties of the rotation axis of the system. We provide a few examples (the microwave circulator, parametric up/down converter, and traveling wave frequency converter) to demonstrate how this general geometric picture can provide insight into specific physical systems. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F28.00005: Trapping a single vortex in a superconducting microwave resonator Ibrahim Nsanzineza, B.L.T. Plourde Trapped vortices in a superconducting microwave resonant circuit can have a significant influence on the loss and resonance frequency. By varying the linewidths of our resonators in different configurations and weakly coupling them to the measurement circuitry, we are able to resolve the shift of the resonance caused by the addition of individual vortices that become trapped following a field-cooling process. In addition, by probing harmonics with different driving forces on the vortices, we are able to study interactions between the trapped vortices and non-equilibrium quasiparticles in the superconducting film. We will discuss prospects for upcoming microwave experiments based on the trapping of a single vortex. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F28.00006: Quasiparticle Trapping and Dynamics in Superconducting Nanobridges E.M. Levenson-Falk, F. Kos, R. Vijay, L. Glazman, I. Siddiqi Quasiparticle excitations can cause loss and noise in superconducting circuits. Recent experiments [1-4] have probed the bulk density of nonequilibrium quasiparticles and their tunneling rates in aluminum superconducting qubits and resonators at low temperature. We perform dispersive measurements of quasiparticle trapping in phase-biased aluminum nanobridge Josephson junctions incorporated into a superconducting resonator. The trapped quasiparticles populate Andreev states formed in the biased nanobridge. We use our technique to not only infer the quasiparticle density, but also to probe the quasiparticles' energy distribution and trapping statistics, to perform spectroscopy on the trap states, and to measure trapping dynamics. We find that the quasiparticle energy distribution is non-thermal below 75 mK, with non-Poissonian trapping statistics. The evolution of the trapping time with the phase bias is consistent with electron-phonon relaxation as the dominant mechanism for quasiparticle trapping in Andreev states.\\[4pt] [1] Manucharayan et al., Science 326, 113 (2009)\\[0pt] [2] Paik et al., Phys. Rev. Lett. 107, 240501 (2011)\\[0pt] [3] Barends et al., Appl. Phys. Lett. 99, 113507 (2011)\\[0pt] [4] Rist\'e et al., Nature Communications 4, 1913 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F28.00007: Parity switching and decoherence by quasiparticles in single-junction transmons Gianluigi Catelani Transmons are at present among the most coherent superconducting qubits, reaching quality factors of order $10^6$ both in 3D and 2D architectures. These high quality factors enable detailed investigations of decoherence mechanisms. An intrinsic decoherence process originates from the coupling between the qubit degree of freedom and the quasiparticles that tunnel across Josephson junctions. In a transmon, tunneling of a single quasiparticle is associated with a change in parity. I will discuss the theory of the parity switching rate in single-junction transmons, compare it with recent measurements, and consider the role of parity switching in limiting the coherence time. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F28.00008: Progress towards a metastable RF squid (MRFS) qubit Archana Kamal, Andrew Kerman, Simon Gustavsson, Xiaoyue Jin, Fei Yan, Ted Gudmundsen, David Hover, Adam Sears, Jonilyn Yoder, Terry Orlando, William Oliver The MRFS qubit [1] consists of an RF squid with a very high loop inductance, and whose two lowest quantum states are very well-defined, equal and opposite persistent supercurrents. These states can be strongly decoupled from each other, such that spontaneous electromagnetic decay processes of the excited state are extremely slow. Also, the large loop inductance suppresses the magnetic flux sensitivity of the design. We have realized these large inductances with NbN nanowires whose kinetic inductance is around 0.5 ?H. We will discuss experimental progress in measuring MRFS qubits fabricated using these inductors, and expected improvements in coherence. Future directions include studying the dynamics of quantum phase slips through these nanowires. [1] A. J. Kerman, PRL 104, 027002(2010). 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 Asst Secretary of Defense for Research \& 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, ODNI or the US government [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F28.00009: Decoherence of superconducting flux qubits in coplanar waveguide resonators Adrian Lupascu, Jean-Luc Orgiazzi, David Layden, Ryan Marchildon, Mustafa Bal, Chunqing Deng, Florian Ong We present detailed measurements of decoherence of persistent current qubits coupled to coplanar waveguide resonators. We find energy relaxation times reaching up to 10 $\mu$s. Dephasing is characterized in detail for different flux biasing points, corresponding to coupling of flux noise with different strength, using Ramsey, spin-echo, and multiple pulse dynamical decoupling. The coherence decay changes in a continuous manner from Gaussian to exponential as the strength of the coupling to flux noise is reduced. This indicates the presence of a source of noise with a flat spectrum around the flux insensitive point of the qubit, a result which is also confirmed by extracting the spectral density of the noise based on different sets of measurements with decoupling sequences. This noise source limits dephasing times at the flux insensitive point to about 1-2 $\mu$s. In qubits with a smaller Josephson to charging energy ratio, we observe decoherence induced by quasiparticles. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F28.00010: First-principles simulation of magnetic defects on the substrate of noisy superconducting qubits Nicole Adelstein, Jonathan DuBois, Vincenzo Lordi Superconducting qubits represent one of the more promising routes to 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 theory efforts suggest that the noise in flux qubits arises from hopping of unpaired spins on the silica or sapphire substrate. In addition, noise could be due to defects with low energy magnetic excited states, though neither noise source is known at the atomic level. We have performed a comprehensive study of the magnetic defects on the surface of SiO$_{\mathrm{2}}$ and investigated barriers to magnetic fluctuations using first-principles density functional theory. Within this framework, we show how defects, such as oxygen vacancies, and adsorbents, such as water, on the substrate represent possible sources of magnetic flux fluctuations. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F28.00011: 1/f flux noise and field-dependent spin susceptibility Pradeep Kumar, Taylor Klaus, Antonio Puglielli, Steven Sendelbach, Robert McDermott Low-frequency 1/f magnetic flux noise is a dominant source of dephasing in superconducting Qubits. It is believed that the noise originates in a high density of surface magnetic defects, but the microscopic noise mechanism is not understood. Here, we describe investigations of the field-dependent complex susceptibility of the surface magnetic system. We have fabricated and characterized asymmetric dc SQUIDs that allow injection of a low-frequency excitation current directly into the SQUID loop to allow measurement of the SQUID inductance, which contains a contribution from the surface spin system. We observe a strong dependence of the SQUID inductance on applied dc field, which we attribute to field-dependent surface spin susceptibility. The data constrains possible models for 1/f flux noise from surface spin states. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F28.00012: Temperature dependent spin-diffusion as a mechanism of intrinsic flux noise in in SQUIDs Rogerio de Sousa, S.-F. Chen, Stephanie Laforest, Trevor Lanting, Mohammad Amin The intrinsic flux noise observed in superconducting quantum interference devices (SQUIDs) is thought to be due to the fluctuation of electron spin impurities, but the frequency and temperature dependence observed in experiments do not agree with the usual 1/f models. We present theoretical calculations of flux noise in rf-SQUID flux qubits that shows how these observations can be interpreted in terms of a spin-diffusion constant that increases with temperature. A comparison of our theory to measurements of flux noise in the 20-80 mK temperature range allows the extraction of the spin-diffusion constant and its temperature dependence, suggesting that the spin system is close to a phase transition. See our paper at http://arxiv.org/abs/1306.1512. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F28.00013: Geometric inductance effects in the spectrum of split transmon qubits R.T. Brierley, J. Blumoff, K. Chou, R.J. Schoelkopf, S.M. Girvin The low-energy spectra of transmon superconducting qubits in a cavity can be accurately calculated using the black-box quantization approach [1]. This method involves finding the normal modes of the circuit with a linearized Josephson junction and using these as the basis in which to express the non-linear terms. A split transmon qubit consists of two Josephson junctions in a SQUID loop. This configuration allows the Josephson energy to be tuned by applying external flux. Ideally, the system otherwise behaves as a conventional transmon with a single effective Josephson junction [2]. However, the finite geometric inductance of the SQUID loop causes deviations from the simplest ideal description of a split transmon. This alters both the linearized and non-linear behaviour of the Josephson junctions in the superconducting circuit. We study how these changes can be incorporated into the black-box quantization approach and their effects on the low-energy spectrum of the split transmon. \\[4pt] [1] S. E. Nigg et al, Phys. Rev. Lett., 108, 240502 (2012)\\[0pt] [2] J. Koch et al, Phys. Rev. A, 76, 042319 (2007) [Preview Abstract] |
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