Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session X16: Superconductors for Quantum Computing: Experimental Techniques and Fabrication |
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Sponsoring Units: DCMP Chair: John Martinis, University of California-Santa Barbara Room: LACC 404A |
Friday, March 25, 2005 8:00AM - 8:12AM |
X16.00001: A General-Purpose Microwave Sequencer for Controlling Josephson Phase Qubits Markus Ansmann, R. McDermott, K.B. Cooper, M. Steffen, J.M. Martinis, K. Osborn, K. Cicak, S. Oh, D.P Pappas, R.W. Simmonds The short coherence times and the sensitivity to noise of quantum states in superconducting Josephson phase qubits call for measurement and control equipment that is both fast and low-noise. At the same time, because these qubits are still in the development phase, this equipment should be maximally flexible to easily allow for new and unexpected experiments. Here, we present a solution based on a custom computer-controlled 64-bit 200MHz FPGA sequencer card that can deliver clean, precisely timed ($\sim $0.6ns) microwave pulses with an amplitude and phase shift that can be adjusted in 5ns intervals. It also provides a buffered readout system to allow for a high measurement repetition rate yielding good statistics quickly. A modular design of the hardware and software allows the entire measurement process to be readily reconfigured. [Preview Abstract] |
Friday, March 25, 2005 8:12AM - 8:24AM |
X16.00002: Real-time observation of quasiparticle poisoning in a single cooper pair transistor Ofer Naaman, Jos\'e Aumentado We probe the charge state of a single cooper-pair transistor (SCPT) via its charge dependent Josephson inductance by measuring the reflected rf power off of a microwave circuit embedding the device. Since the entire measurement is performed without the SCPT ever leaving its supercurrent branch, this readout of the SCPT state does not induce quasiparticle generation. This fact, together with the high bandwidth of the measurement, allows us to observe quasiparticle poisoning events in the time domain. We measure and analyze the temporal distribution of these events to extract the lifetimes of the poisoned (one unpaired electron on the SCPT island) and un- poisoned states, and study their statistics as a function of gate charge, current bias and temperature. We compare the results with data we obtained simultaneously from switching current experiments on the same device. [Preview Abstract] |
Friday, March 25, 2005 8:24AM - 8:36AM |
X16.00003: Superconducting SET backaction on the Cooper-pair box Benjamin Turek, Johannes Majer, Aashish Clerk, Steven Girvin, Robert Schoelkopf, Kevin Bladh, David Gunnarsson, Per Delsing We report on measurements of the backaction of a superconducting single electron transistor (SSET) measuring a Cooper-pair box qubit. During the weak, continuous measurement made by the SSET, the qubit can be relaxed, dephased, and even excited by noise capacitively coupled from the island of the SSET. This backaction is calculated to depend on the operating point of the SSET. We operate the SSET near the double Josephson quasiparticle (DJQP) feature, where the backaction of the SSET is well understood (A. Clerk, et al., Phys. Rev. Lett. \textbf{89}, 176804 (2002)), and where there are no quasiparticle poisoning effects. Measurements of the variation in steady-state polarization and relaxation time of the Cooper-pair box vary as expected with SSET operating point and confirm this model of SSET backaction. [Preview Abstract] |
Friday, March 25, 2005 8:36AM - 8:48AM |
X16.00004: Quasiparticle poisoning induced by a biased SET electrometer Jos\'{e} Aumentado, Ofer Naaman In this talk, we will present measurements of the quasiparticle ``poisoning" rate induced in a superconducting Cooper pair transistor (SCPT) due to the biasing of an adjacent SCPT. We operate the first SCPT as a quasiparticle detector biased on its supercurrent branch, while we bias the drain-source and gate voltages on the second SCPT to generate quasiparticles in the first SCPT.\footnote{this was first noted J.M\"{a}nnik and J.E.Lukens, Phys. Rev. Lett. \textbf{92} 057004 (2004)} Using radio-frequency methods, we are able to study the poisoning in the time domain and correlate it with resonant features in the current-voltage characteristic of the second SCPT. In this way, we study the quasiparticle generation rate that a similarly biased electrometer may induce in a Cooper pair box. We will discuss the possible mechanisms for this quasiparticle generation and its implications for charge qubit readout. [Preview Abstract] |
Friday, March 25, 2005 8:48AM - 9:00AM |
X16.00005: Low back-action magnetometers for high-speed readout of Nb flux qubits Vladimir Kuznetsov, Vijay Patel, Douglas Bennett, Jaan Mannik, James Lukens Hysteretic dc-squid magnetometers are used to measure the state of a large area gradiometric rf-squid qubit. The current bias and flux bias of the magnetometer are designed to avoid direct inductive coupling to the qubit. However the current bias affects the circulating current in the magnetometer and hence indirectly couples to the qubit. By designing an asymmetry in the two junctions of the magnetometer we can achieve bias points where fluctuations in the bias currents do not change the circulating current and effectively decouple the readout from the qubit. High-speed readout is achieved by pulsing the flux in the magnetometer, which is inductively coupled to microstrip lines on a separate chip. Resonant tunneling between the quantum wells of the qubit is measured at 10 mK using different sequences of controlling pulses including 10ns flux bias pulses. The effect of bias conditions of the magnetometer on the shape and positions of tunneling peaks indicated the low back-action of the measurements. [Preview Abstract] |
Friday, March 25, 2005 9:00AM - 9:12AM |
X16.00006: The symmetry of phase-coherent thermopower oscillations in Andreev interferometers Paul Cadden-Zimansky, Zhigang Jiang, Venkat Chandrasekhar We study the thermopower of diffusive Andreev interferometers, which are hybrid loops with one normal-metal arm and one superconducting arm. The thermopower oscillates as a function of the magnetic flux through the loop with a fundamental period corresponding to one flux quantum $\Phi_0=h/2e$. These oscillations may be symmetric or antisymmetric with respect to the magnetic flux. New measurements on double-loop Andreev interferometers in which the flux through each interferometer can be independently controlled by on-chip field coils show that the symmetry of the thermopower oscillations is related to the distribution of the supercurrent in the device. We compare our experimental results with recent theoretical work. [Preview Abstract] |
Friday, March 25, 2005 9:12AM - 9:24AM |
X16.00007: Inductive Isolation of a Current-Biased Josephson Junction Qubit S.K. Dutta, H. Xu, R.C. Ramos, T.A. Palomaki, H. Paik, R.M. Lewis, A.J. Berkley, J.R. Anderson, C.J. Lobb, F.C. Wellstood Although the current-biased Josephson junction is a promising qubit candidate, it has the drawback that its bias leads directly connect it to a dissipative environment. With measurements on a Niobium device at 20 mK, we demonstrate the effectiveness of a broadband isolation scheme composed of an inductive divider with an auxiliary detection junction. In addition, by manipulating the state of the asymmetric SQUID that results from the isolation network, we show how the qubit's coupling to its bias leads can be varied, which provides a way of evaluating the importance of this source of dissipation. This work is supported by the Department of Defense, the National Science Foundation, and the Center for Superconductivity Research. [Preview Abstract] |
Friday, March 25, 2005 9:24AM - 9:36AM |
X16.00008: Study of Dissipation in Phase Qubits Using High-Q Resonators K. D. Osborn, K. Cicak, S. Oh, D. P. Pappas, J. A. Strong, R. W. Simmonds, R. McDermott, M. Steffen, K. B. Cooper, Markus Ansmann, John M. Martinis The coherence time in a qubit is adversely affected by dissipation. Superconducting phase qubits are made with microelectronic circuits that have finite dissipation due to losses from dielectrics, radiation, and the barrier of the aluminum-oxide-based Josephson junction. We have measured coplanar-waveguide and lumped-element high-Q microwave resonators operating at the transition frequency of phase qubits. The coplanar-waveguide resonator is used to test the quality of the silicon dioxide used in the qubit circuit, whereas the lumped-element resonator is used to test losses associated with the qubit inductor. These resonator experiments suggest that our phase qubits are not currently limited by dissipation from the dielectrics or radiation loss. [Preview Abstract] |
Friday, March 25, 2005 9:36AM - 9:48AM |
X16.00009: Epitaxial superconducting refractory metals for quantum computing D. P. Pappas, S. Oh, K. Osborn, K. Cicak, R.W. Simmonds, R. McDermott, M. Steffen, K. B. Cooper, M. Ansmann, J. M. Martinis The base layer for a Josephson tunnel junction is important because it defines the nanoscale morphology and roughness of the device. This can affect the critical current and the spectroscopic nature of the barrier. In this study we discuss the growth of epitaxial Re as a base layer for tunnel junctions. Studies were conducted using UHV sputtering of the metal, with in-situ characterization using STM, LEED, RHEED, and Auger. Ex-situ characterization was also conducted using AFM. We find that the Re grows oriented even at room temperature for very slow deposition. The grain size grows significantly with relatively low temperature anneals (850 C). After these anneals, we find that the grains form around a screw dislocation, with a tendency for bi-layer step bunching. However, there is evidence for occasional steps with a higher number of atomic layers (5 -- 10). The impact of these steps on the IV characteristics of the tunnel barriers will be discussed. [Preview Abstract] |
Friday, March 25, 2005 9:48AM - 10:00AM |
X16.00010: Growth of Epitaxial Re/AlOx/Re(or Al) Trilayers for Josephson Junction Phase Qubits S. Oh, K. Cicak, K. Osborn, J. A. Strong, R. W. Simmonds, D. P. Pappas, R. McDermott, K. B. Cooper, M. Steffen, M. Ansmann, J. M. Martinis Our group has recently shown that there exist two-state fluctuators in amorphous AlO$_{x}$ based superconducting qubits, which adversely affect the qubit behavior. So far researchers in superconducting qubit community have been focusing on improving circuit designs and measurement schemes to minimize decoherences, while relying on amorphous AlO$_{x}$ barriers. However, sooner or later these intrinsic materials-based decoherence sources could become a major bottleneck for any serious qubit operations. In order to solve this problem, we have started to develop a new type of Josephson junctions with epitaxial AlO$_{x}$ barriers. We describe our effort to grow epitaxial Re/AlO$_{x}$/Re(or Al) system using various analysis tools such as LEED, RHEED, AFM, STM and AES. Preliminary Josephson junctions made out of these trilayers have already shown as good I-V characteristics as the conventional Al/AlO$_{x}$/Al junctions in terms of sub-gap quasiparticle current. [Preview Abstract] |
Friday, March 25, 2005 10:00AM - 10:12AM |
X16.00011: Fabrication and Testing of Re-AlOx-Re(or Al) Josephson Junctions for Phase Qubits K. Cicak, S. Oh, K. Osborn, J.A. Strong, D.P. Pappas, R.W. Simmonds, R. McDermott, K.B. Cooper, M. Steffen, John M. Martinis We have successfully fabricated and characterized new Josephson junctions from UHV grown Re-AlOx-Re and Re-AlOx-Al trilayers with both epitaxial and amorphous base Re electrodes. Fabrication utilized optical lithography, ion-milling, and reactive-ion-etching techniques. When exposed to air ambient for days or even hours, Re films develop defect-like structures. Simple tests, optical, and AFM inspection showed that these defect-like ``dots'' are not true disorder in the Re film, and that they can be completely removed from films by solvents. 4-probe resistance measurements at room T show that junction resistance scales properly with the junction area. Preliminary devices show I-V characteristics with subgap leakage comparable to Al-AlOx-Al junctions. Junctions with epitaxial Re base layer and oxide barrier annealed in O$_{2}$ ambient show far superior I-V signatures (well defined voltage gap, small sub-gap current) compared to junctions with amorphous Re base layer and barrier annealed in vacuum. These investigations are presently underway in an attempt to eliminate the spurious resonators found in Josephson junction phase qubits. [Preview Abstract] |
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