Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session Y16: Superconducting Single and Coupled Qubits |
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Sponsoring Units: DCMP Chair: Guido Burkard, University of Basel Room: LACC 404A |
Friday, March 25, 2005 11:15AM - 11:27AM |
Y16.00001: Measurement Crosstalk in the Josephson Phase Qubit R. McDermott, K.B. Cooper, M. Steffen, M. Ansmann, J.M. Martinis, K. Osborn, K. Cicak, S. Oh, D.P. Pappas, R.W. Simmonds In order to accurately assess the fidelilty of quantum gates, or to perform quantum state tomography and thereby definitively prove entanglement, it is necessary to measure the states of all qubits in the system (wordwise readout). In multi-qubit circuits with fixed couplings – a common architecture for superconducting qubits – realization of this goal is complicated by measurement crosstalk: the measurement of one qubit perturbs the states of the other qubits, destroying information about quantum correlations. For the flux-biased Josephson phase qubit, the measurement of a $|1\rangle$ state implies a tunneling transition between local minima of the qubit potential. The resulting time-varying voltage across the measured qubit junction couples a transient current to other qubits, which can induce transitions between the qubit $|0\rangle$ and $|1 \rangle$ states. We present a semiclassical model which quantitatively accounts for the observed measurement crosstalk in our circuit, and describe how fast, simultaneous state measurement can circumvent this problem. [Preview Abstract] |
Friday, March 25, 2005 11:27AM - 11:39AM |
Y16.00002: Flux Qubits and Readout Device with Two Independent Flux Lines B.L.T. Plourde, T.L. Robertson, T. Hime, P.A. Reichardt, C.-E. Wu, John Clarke Circuits involving multiple flux qubits require an architecture which is scalable. In particular, the flux bias must be settable for each element individually. We report measurements on two superconducting flux qubits coupled to a readout Superconducting QUantum Interference Device (SQUID). The devices were fabricated with Al-AlOx-Al tunnel junctions using electron-beam lithography. Two on-chip flux bias lines allowed independent flux control of any two of the three elements. By rastering the currents in these two flux lines, we observed the modulation of the SQUID critical current due to the applied flux as well as the changing screening currents in the two qubits. These results are illustrated by a two-dimensional qubit flux map. By varying the flux bias currents to move along a line of constant flux applied to the SQUID, we could measure the qubit transitions while maintaining a fixed SQUID sensitivity. When combined with variable qubit-qubit coupling based on the circulating currents in the readout SQUID, this architecture should be scalable to many qubits and SQUIDs on a single chip. [Preview Abstract] |
Friday, March 25, 2005 11:39AM - 11:51AM |
Y16.00003: Quantum Coherence in a Superconducting Flux Qubit T. Hime, B.L.T. Plourde, P.A. Reichardt, T.L. Robertson, C.-E. Wu, John Clarke We report observations of quantum coherence in a superconducting flux qubit. As the flux applied to the qubit was swept through the degeneracy point, $(n+1/2)\Phi_0$, we could resolve the change in qubit screening flux produced by the reversal of the qubit circulating current. By applying microwave radiation to the qubit, we observed resonant excitation when the qubit level splitting matched the energy of the microwave photons, corresponding to a change in the qubit screening flux. We varied the microwave frequency and mapped out the dispersion of the excited state transition which fit well to the expected hyperbolic dependence. With high-resolution spectroscopy, we measured anomalous structure and splittings on the excited state line, which may correspond to coupling to defect states in the junction tunnel barriers. We performed coherent manipulation of the qubit state by applying microwave pulses of fixed amplitude and frequency, but variable width. This resulted in Rabi oscillations with a Rabi frequency which scaled linearly with the amplitude of the microwave pulses. [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:03PM |
Y16.00004: Measurements of Dephasing in Superconducting Flux Qubits C.-E. Wu, T. Hime, B.L.T. Plourde, P.A. Reichardt, T.L. Robertson, John Clarke The time over which a superposition of qubit states maintains phase coherence is an important figure of merit for a qubit. One technique for measuring this dephasing time is the Ramsey fringe, consisting of two $\pi/2$ pulses detuned from resonance. Varying the time between the pulses produces a damped oscillatory fringe, with the frequency equal to the detuning from resonance and the decay time given by the dephasing time. The dephasing time can also be extracted from measurements of the spectroscopic linewidths for different excitation amplitudes. We report measurements using both techniques in a superconducting flux qubit, giving dephasing times of the order of 10 ns. We present the variation of the dephasing time with various parameters, such as qubit level splitting, readout SQUID operating point, and temperature. We compare our results with expected levels of low frequency noise in the qubit environment and discuss possible methods for enhancing the coherence, including spin echo pulse sequences. [Preview Abstract] |
Friday, March 25, 2005 12:03PM - 12:15PM |
Y16.00005: Measurements of Relaxation in Superconducting Flux Qubits P.A. Reichardt, T. Hime, B.L.T. Plourde, T.L. Robertson, C.-E. Wu, John Clarke The exchange of energy between a qubit and its environment leads to the decay of an excited state, characterized by the inelastic relaxation time. This relaxation is determined by the noise in the qubit environment at the level splitting frequency. We have measured the relaxation time in a superconducting flux qubit by exciting the qubit with a resonant microwave pulse, and varying the time following the excitation before the qubit is read out with a Superconducting QUantum Interference Device (SQUID). We present the variation of the relaxation time with various parameters, such as the qubit level splitting, the readout SQUID operating point, measurement repetition time, and temperature. We compare our results with expected levels of noise in the qubit environment and discuss possible sources of the qubit relaxation. [Preview Abstract] |
Friday, March 25, 2005 12:15PM - 12:27PM |
Y16.00006: Spectroscopic study of energy levels and transition rates in large area flux qubit Jaan Mannik, Douglas Bennett, Vladimir Kuznetsov, Vijay Patel, Wei Chen, James Lukens The level structure and transition rates between fluxoid states of a large area flux qubit have been studied using microwave spectroscopy and resonant tunneling spectroscopy. This qubit uses an rf-SQUID in a gradiometer configuration and has independent, \textit{in situ}, controls for the barrier height between fluxoid wells and the relative positions of levels in different wells. This design makes it well suited for the study of decoherence mechanisms that adversely affect the operation of flux as well as phase qubits based on fluxoid states. This work was supported in part by NSF and by AFOSR and ARDA through a DURINT program. [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 12:39PM |
Y16.00007: Junction Resonances in Josephson Phase Qubits John Martinis, R. McDermott, K.B. Cooper, M. Steffen, M. Ansmann, K. Osborn, K. Cicak, S. Oh, D.P. Pappas, R.W. Simmonds Careful spectroscopic measurements of Josephson phase qubits have revealed avoided crossings characteristic of the qubit interacting with a set of two-level resonators. As these extra splittings degrade the quality of the qubit, it is extremely important to characterize and understand their origin. We will present data from a variety of qubits that show these resonators have an amplitude probability distribution which scales inversely with splitting magnitude up to a certain splitting size. This data can be explained with a model based on two-level critical-current fluctuators in the junction. This model is also reasonably consistent with previous measurements of 1/f (low frequency) critical-current noise. [Preview Abstract] |
Friday, March 25, 2005 12:39PM - 12:51PM |
Y16.00008: Time-Domain Investigation of Coupled Superconducting Qubits Andrew Berkley, Sergei Govorkov, Murray Thom, Brock Wilson We report on measurements of two coupled rf SQUID phase qubits. With both qubits initially in their ground states, the first is excited with a pi pulse. Being initially detuned they are brought into resonance with a coupling pulse that allows the qubits to be put into a maximally entangled state. By varying the length of the interaction time we are able to perform a full logical SWAP gate operation. The coherence times and Rabi amplitudes measured on these qubits are small when compared to quantum error correction limits. [Preview Abstract] |
Friday, March 25, 2005 12:51PM - 1:03PM |
Y16.00009: Energy Relaxation in Josephson Phase Qubits Raymond Simmonds, K. Cicak, S. Oh, K. Osborn, J. A. Strong, D. P. Pappas, M. Ansmann, K. B. Cooper, R. McDermott, M. Steffen, John M. Martinis The characteristic energy relaxation time $T_1$ of any ``isolated" quantum system depends on the degree of isolation, or the strength with which a number of ``environmental" degrees of freedom couple to that system. In order for superconducting based quantum bits to be a viable technology for quantum computing, these systems must be sufficiently isolated from their environment to enable them to undergo relatively undisturbed quantum evolutions. Additionally, they must be controlled precisely in order to perform quantum operations. This operation time is ultimately limited by the single qubit $T_1$ time. Recently, we have investigated a number of different Josephson phase qubit design geometries in order to identify what, where, and how environmental degrees of freedom couple to individual qubits, reducing $T_1$. These measurements will help to improve the operation of future coupled phase qubit systems. [Preview Abstract] |
Friday, March 25, 2005 1:03PM - 1:15PM |
Y16.00010: Decay of Rabi Oscillations in Josephson Charge Qubits. Roman Lutchyn, Leonid Glazman, Anatoly Larkin We analyze the decay of Rabi oscillations in a charge qubit consisting from a Cooper pair box connected by a Josephson junction to a finite-size superconductor.\footnote{A. Blais \emph{et. al.}, cond-mat 0402216.; A. Wallraff \emph{et. al.}, Nature (London) 431,162 (2004)} The dominant mechanisms of the decay are still debated, and we concentrate on the contribution of quasiparticles to the decay rate. Passing of a quasiparticle through the Josephson junction tunes the qubit away from the charge degeneracy, thus spoiling the Rabi oscillations. We find the temperature dependence of the quasiparticle contribution to the decay rate. This dependence has three distinct regimes defined by two temperature scales, $T^*_{1,2}=\Delta/\ln(\Delta/\delta_{1,2})$, where $\Delta$ is the superconducting gap, and $\delta_{1,2}$ are the one-electron level spacings in the electrodes forming the qubit. Temperatures $T^*_{1,2}$ characterize the appearance of thermally excited quasiparticles in the parts of qubit; typically $T^*_{1,2}\ll E_c, E_j$, where $E_c$ and $E_j$ are the charging and Josephson energies, respectively. [Preview Abstract] |
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