Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session Y40: Solid State Quantum Computing I |
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Sponsoring Units: TGQI DCMP Chair: John Martinis, University of California, Santa Barbara Room: Baltimore Convention Center 343 |
Friday, March 17, 2006 8:00AM - 8:12AM |
Y40.00001: Landau-Zener interferometry in a Cooper pair box Mika Sillanp\"{a}\"{a}, Teijo Lehtinen, Antti Paila, Yuriy Makhlin, Pertti Hakonen Quantum-mechanical systems having two crossing energy levels are ubiquitous in nature. The rate $v = d (E_1 - E_0)/dt$ at which such levels in a driven system approach each other determines the probability $P_{LZ}$ of a Landau-Zener (LZ) tunneling between them. The traditional treatment of the LZ process, however, ignores quantum-mechanical interference. Here we report an observation of phase-sensitive interference between consecutive LZ tunneling attempts in an artificial two-state system, a superconducting charge qubit. We interpret the experiment in terms of a multi-pass analog to the optical Mach- Zehnder interferometer: The beam splitting occurs by LZ tunneling at the charge degeneracy, while the arms of the Mach- Zehnder interferometer in energy space are represented by the ground and excited state. In accord with theory, we observe constructive interference when the Stokes phase $\phi_S$ picked up during the LZ interaction, and the dynamical phase of one drive period $\phi = \int (E_1 - E_0) dt$ satisfy the condition: $(\phi - 2 \phi_S) = m \cdot 2\pi$. Our LZ interferometer can be used as a high-resolution detector for phase and charge owing to interferometric sensitivity- enhancement. [Preview Abstract] |
Friday, March 17, 2006 8:12AM - 8:24AM |
Y40.00002: Measuring the environmental impedance of the Cooper-pair box Benjamin Turek, Johannes Majer, John Teufel, Aashish Clerk, Steven Girvin, Robert Schoelkopf The Cooper-pair box qubit measured by the SET can have long decoherence times that are limited by the quantum noise of the environment. Qubits fabricated at Yale are designed with 50 ohm transmission lines that control this environmental impedance to very high frequencies. We use the AC Josephson effect of a hysteretic DC-Squid to measure the frequency dependence of this impedance. At frequencies where the real part of the environmental impedance is large, the AC Josephson effect causes the small-junction analogue of self-induced Shapiro steps in the IV curves of a DC Squid [T. Holst et al., PRL 73, 3455 (1994)]. We determine that the environmental impedance of our qubit is well behaved at frequencies less than 20 GHz. [Preview Abstract] |
Friday, March 17, 2006 8:24AM - 8:36AM |
Y40.00003: Anomalously low tunneling escape rates from the excited states of an inductively--isolated current--biased Josephson junction phase qubit R.M. Lewis, T.A. Palomaki, Hanhee Paik, S.K. Dutta, A. Przybysz, B.K. Cooper, J.R. Anderson, A.J. Dragt, C.J. Lobb, F.C. Wellstood We present measurements of an inductively-isolated current-biased Nb/AlOx/Nb Josephson junction quantum bit at 20 mK. Density matrix fits of Rabi oscillations in our system suggest that the tunneling rate ($\Gamma_1$) from the first excited state is an order of magnitude lower than expected from a single current-biased junction. Furthermore, measurements of the energy relaxation time, $T_1$, through both pulse/decay and thermal population \footnote{S. K. Dutta {\it et al.\ }, Phys. Rev. B {\bf 70} 140502(R) (2004).} techniques only agree if $\Gamma_1$ is approximately an order of magnitude lower than our single junction model predicts. To test for low $\Gamma_1$, we use a fast-ramp technique ($\alpha= d(ln \Gamma )/dt > 1/T_1$) to directly measure $\Gamma_1$. We propose that an increase in the Josephson inductance of the qubit junction when in the excited state causes this effective reduction in $\Gamma_1$. [Preview Abstract] |
Friday, March 17, 2006 8:36AM - 8:48AM |
Y40.00004: Variable coupling between the inductively isolated current-biased Josephson junction qubit and the current bias leads Hanhee Paik, S. K. Dutta, R. M. Lewis, R. C. Ramos, H. Xu, T. A. Palomaki, B. K. Cooper, A. J. Przybysz, A. J. Dragt, J. R. Anderson, C. J. Lobb, F. C. Wellstood We examined the behavior of inductively isolated Josephson junction qubits in which the coupling to the bias leads could be varied in situ. The variable coupling was achieved by using a second Josephson junction and an inductor that act as an inductive current divider. The coupling between the current bias leads and the qubit was varied by changing the current through the second junction, altering its Josephson inductance. We measured the tunneling escape rates of Al/Al$O_x$/Al and Nb/Al$O_x$/Nb junctions with continuous or pulsed microwave power, showing the allowed energy transitions and coherent Rabi oscillations. We found that $T_2$, $T_2^*$ and $T_1$ did not change significantly as the coupling to the current bias leads was varied. [Preview Abstract] |
Friday, March 17, 2006 8:48AM - 9:00AM |
Y40.00005: Analysis of Rabi Oscillations of a Josephson Phase Qubit S. K. Dutta, H. Xu, Frederick W. Strauch, Philip R. Johnson, R. C. Ramos, Hanhee Paik, T. A. Palomaki, R. M. Lewis, J. R. Anderson, Alex J. Dragt, C. J. Lobb, F. C. Wellstood We have experimentally studied asymmetric Nb/AlOx/Nb dc SQUID qubits at 25 mK. The two lowest metastable levels localized within a single well of the complex two-dimensional potential of the device can serve as qubit states, if they are not unduly perturbed by resonant coupling to higher states of the full potential. Rabi oscillations between the qubit states can be driven with a microwave bias current. State readout is performed by measuring the tunneling rate from all energy levels with non-zero occupation probability to the finite voltage state. To interpret the results of our Rabi oscillation measurements, we have used a multi-level density matrix simulation to extract the populations of the individual quantum states from this total rate. We can then calculate the visibility of the oscillations and determine the effects of the higher levels and multi-photon transitions. [Preview Abstract] |
Friday, March 17, 2006 9:00AM - 9:12AM |
Y40.00006: Two Coupled Inductively-Isolated Josephson Junction Qubits Tauno Palomaki, Sudeep Dutta, Hanhee Paik, Rupert Lewis, Roberto Ramos, Huizhong Xu, Bob Anderson, Chris Lobb, Fred Wellstood We report experimental measurements on coherent quantum oscillations and entangled macroscopic quantum states in two capacitively-coupled inductively-isolated Nb/Al$_{2}$O$_{3}$/Nb Josephson qubits at 25mK. The interaction between the two qubits is controlled by tuning the energy level spacings of the junctions using the bias current and applied flux. We discuss transitions to various states of the coupled device, show Rabi oscillations, and analyze the spectroscopy of the system when the junctions are in and out of resonance with each other. [Preview Abstract] |
Friday, March 17, 2006 9:12AM - 9:24AM |
Y40.00007: Flux Noise in an Inductively Isolated Josephson Junction Qubit B. K. Cooper, Hanhee Paik, R. M. Lewis, S. K. Dutta, T. A. Palomaki, A. J. Przybysz, J. R. Anderson, Alex J. Dragt, C. J. Lobb, F. C. Wellstood Martinis et al. [1] first proposed a technique for inductively isolating a Josephson junction qubit from the bias leads. It involves using one junction of a dc SQUID as a qubit, and the SQUID inductance and second junction of the SQUID as an inductance divider. This arrangement allows for isolation from current bias lines but potentially introduces greater sensitivity to flux noise. By introducing counterwound inductors on the qubit arm of the SQUID, we can reduce spatially uniform flux noise. We compare experimental coherence times for a Nb/Al$_{2}$O$_{3}$/Nb qubit in such gradiometer designs to similar devices lacking the counterwound inductors. No significant difference is seen, suggesting that uniform flux noise is not the major source of decoherence in our system. [1] J. M. Martinis et al., PRL \textbf{89}, 117901 [Preview Abstract] |
Friday, March 17, 2006 9:24AM - 9:36AM |
Y40.00008: Measurement of Microwave Resonators for Improved $T_{1}$ in Josephson Qubits Matthew Neeley, M. Ansmann, R. Bialczak, N. Katz, E. Lucero, R. McDermott, M. Steffen, E. Weig, A. Cleland, J. M. Martinis To realize a quantum computer with Josephson qubits, the energy relaxation time $T_{1}$ must be increased by an order of magnitude or more over current qubits. One of the dominant sources of energy decoherence is dielectric loss due to two-level defect states in the Josephson junction and in wiring crossovers. Easily-fabricated microwave resonators provide a convenient way to measure the dielectric loss of candidate materials before incorporating them into qubits. We describe the measurement process and the results for several candidate dielectric materials. The loss tangents have been observed to vary with applied magnetic field. This variation is explained by a simple model of flux vortex trapping in the type I superconducting aluminum films of the resonators. [Preview Abstract] |
Friday, March 17, 2006 9:36AM - 9:48AM |
Y40.00009: High Q Dielectrics for Josephson Phase Qubits E. M. Weig, M. Ansmann, R. Bialczak, N. Katz, E. Lucero, R. McDermott, M. Neeley, M. Steffen, J. M. Martinis, A. N. Cleland Dielectric loss in the bulk insulating material surrounding a superconducting phase qubit has recently attracted attention as a major source of decoherence. Dissipation arises from the excitation of a bath of two-level defects that is unsaturated in the limit of low microwave power and low temperature. This gives rise to much lower intrinsic Q factors than expected from material characterization typically performed in the saturated higher power or temperature regime. The density of these two-level systems in the insulator can be distinctively reduced by carefully choosing and engineering the dielectric material. We have investigated LC resonators fabricated using various amorphous dielectrics ranging from silicon dioxide or silicon nitride to silicon hydride (a-Si:H). The intrinsic Q factors of the materials have been measured at microwave frequencies in the unsaturated regime. The data demonstrates that by proper choice of material the dielectric loss tangent can be dramatically reduced, thus allowing for long coherence time phase qubits. [Preview Abstract] |
Friday, March 17, 2006 9:48AM - 10:00AM |
Y40.00010: Fabrication and Testing of AlN Josephson Junction Qubits Radoslaw Bialczak, Markus Ansmann, Nadav Katz, Erik Lucero, Robert McDermott, Matthew Neeley, Matthias Steffen, Ewa Weig, Andrew Cleland, John Martinis Recently, it has been shown that a major source of decoherence in Josephson junction (JJ) qubits comes from coupling to two-level systems (TLS) in the dielectric materials used to construct these qubits. These TLS's result from defects in the dielectric material. In our previous work we have shown that the energy relaxation times of our JJ phase qubits improve 20-fold when we substitute SiO2 with SiN as the dielectric material used for cross-over wiring. This shows that nitride based dielectrics might be less prone to defects and suggests that the next logical step would be to replace the Al2O3 tunnel barrier dielectric of the JJ with AlN. We have used atomic nitrogen to successfully fabricate a JJ phase qubit with AlN as the JJ tunnel barrier material. Through spectroscopy measurements, we have found that qubits made with AlN as the tunnel barrier material have only slightly lower concentrations of defects compared with previously studied qubits made with Al2O3 grown by natural oxidation. Also, the measured T1 times for these AlN qubits were low ($\sim $15ns). This agrees with theoretical predictions of Ioffe et al. which state that piezoelectric materials, such as AlN, might have other loss mechanisms due to phonon radiation. [Preview Abstract] |
Friday, March 17, 2006 10:00AM - 10:12AM |
Y40.00011: Evolution and decay of a superconducting Josephson junction qubit due to partial measurement N. Katz, M. Ansmann, R. Bialczek, E. Lucero, R. McDermott, M. Neeley, M. Steffen, E. Weig, A. Cleland, J. M. Martinis, A. Korotkov Superconducting Josephson phase qubits have been shown to be a promising candidate for scalable quantum computing. In many such quantum computing algorithms, partial measurement of the quantum state is used to project the system into a required subspace. We experimentally study the effect of a partial measurement on our Josephson phase qubit using state tomography and high fidelity measurement capabilities. We also explore related multi-photon effects which appear naturally in such a system during state preparation, evolution and measurement. [Preview Abstract] |
Friday, March 17, 2006 10:12AM - 10:24AM |
Y40.00012: Quantized Rabi oscillation observed in the superconducting flux qubit LC-harmonic oscillator system K. Semba, J. Johansson, S. Saito, T. Meno, H. Nakano, M. Ueda, H. Takayanagi Superconducting circuit containing Josephson junctions is one of the promising candidates as a quantum bit (qubit) which is an essential building block for quantum computation. A flux qubit is represented by energetically lowest two collective states of macroscopic numbers of Cooper pairs which are linear combination of clockwise and counterclockwise persistent-current states. By replacing an atom with a flux qubit (artificial atom), and a high-Q cavity with an LC-circuit, quantum optics type experiments are possible on a superconductor chip. We have observed, for the first time, the vacuum Rabi oscillations in a superconducting flux qubit LC-oscillator coupled system [1]. We have also obtained evidence of level quantization of the LC circuit by observing the change in quantum oscillation frequency when the LC circuit was not initially in the vacuum state. Sharing a single superconducting LC-circuit with many flux qubits as a quantum information bus, spatially separated multiple qubits can be controlled by a set of microwave pulses. [1] J. Johansson et al., arXiv:cond-mat/0510457, http://www.brl.ntt.co.jp/group/shitsuryo-g/index.html [Preview Abstract] |
Friday, March 17, 2006 10:24AM - 10:36AM |
Y40.00013: Quantum Computing Architectural Design Jacob West, Geoffrey Simms, Mark Gyure Large scale quantum computers will invariably require scalable architectures in addition to high fidelity gate operations. Quantum computing architectural design (QCAD) addresses the problems of actually implementing fault-tolerant algorithms given physical and architectural constraints beyond those of basic gate-level fidelity. Here we introduce a unified framework for QCAD that enables the scientist to study the impact of varying error correction schemes, architectural parameters including layout and scheduling, and physical operations native to a given architecture. Our software package, aptly named QCAD, provides compilation, manipulation/transformation, multi-paradigm simulation, and visualization tools. We demonstrate various features of the QCAD software package through several examples. [Preview Abstract] |
Friday, March 17, 2006 10:36AM - 10:48AM |
Y40.00014: Threshold calculation and optimization for measurementless quantum error correction Geoffrey Simms, Mark Gyure, Jacob West General-purpose quantum computing will rely on measurement as a primitive operation, but the operations of measurement and classical feed-forward are not necessary to perform certain useful computations, including quantum error correction. Measurementless quantum computation is appealing because it reduces the classical control system to an automaton, having no conditional operations. Measurementless, fault-tolerant quantum error correction (MFTQEC) of Calderbank-Shor-Steane (CSS) encoded logical qubits requires logical zero states to be prepared with high fidelity as an initial step, and this logical zero preparation has a threshold of its own, analogous to, but not identical to, the threshold of the entire error correction algorithm. In this talk, we present the results of mapping the MFTQEC algorithm onto a specific semiconductor-based qubit system using the Quantum Computing Architectural Design (QCAD) program, discussed in another talk in this session. The algorithm is translated from the set of ``design gates'' to the set of accessible ``physical gates,'' and the resulting quantum circuit is optimized to improve the threshold. Limited 2-dimensional connectivity is assumed, making this well suited to the ``enhancement mode'' quantum dot qubits described in other talks in this session. [Preview Abstract] |
Friday, March 17, 2006 10:48AM - 11:00AM |
Y40.00015: Shift register in a SQUID architecture with untunable couplings Preethika Gagnebin, Steven Skinner, Elizabeth Behrman, James Steck A scheme to implement a qubit shift register in a one-dimensional series of superconducting quantum interference devices (SQUIDs), using a sequence of pulsed biases, is described. Each SQUID is coupled to its neighbors through an untunable coupling parameter. The only variable parameter of the system is the bias on each SQUID, which is pulsed low during a shift operation. Our design requires only two bias control signals for any size of shift register, with an additional one on the output qubit to shift out the data. The shift register operation is realized by copying the state of one qubit onto another, in the direction of the shift, during the bias pulse. As the no-cloning theorem prohibits the cloning of an unknown arbitrary quantum state, this device works as a classical shift register or, in other words\textbf{, }a binary wire. We show here how to find the time duration of the bias pulse and the minimum value of the bias during the pulse given the fixed physical parameters of the system. [Preview Abstract] |
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