Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session N2: Progress in Superconducting Quantum Computing |
Hide Abstracts |
Sponsoring Units: GQI DCMP Chair: Robert Schoelkopf, Yale University Room: Colorado Convention Center Four Seasons 4 |
Wednesday, March 7, 2007 8:00AM - 8:36AM |
N2.00001: Superconducting qubits on the way to a quantum processor Invited Speaker: Experimental research on supeconducting qubits has seen an enormous progress in recent years. About 10 years after its first theoretical proposals, experiments have demonstrated the necessary building blocks for the exploration of quantum information along several avenues: Single qubit-rotations, long coherence times, high-fidelity nondemolition readout, two-qubit interactions and gates, coupling to delocalized qubit modes. With this progress, analogies to other qubit candidates such as magnetic resonance systems, atomic, and optical systems are evident, but we also see the specific strengths of superconducting qubits play out - in situ tunable strong qubit-qubit coupling, strong coupling between qubits and the quantized electromagnetic field, strong intrinsic nonlinearity, and the possibility to fabricate large circuits. Most of these achievements will be discussed later in the session. I will give an introduction to superconducting qubits in the perspective of quantum information processing [1] accessible to outsiders in the field. I will put the different elements of the session in the perspective of an actual scalable architecture which allows for fault-tolerant quantum computation [1,2]. In order to make further progress in direction, the fidelities of quantum operations need to be improved. I will discuss the crucial topic of understanding and further supressing noise from material defects in these systems, which can influence both the phase and bit-flip error rate [3,4]. I will show, how optimal control theory can help to find fast and high-fidelity shaped pulses for superconducting qubits, even though they, other than spin 1/2 systems, have relatively close leakage levels outside the qubit manyfold [5,6]. This technique also allows to optimize pulses in the presence of telegraph noise [6]. Finally, I will describe how the strong nonlinearity of Josephson circuit can be used for the generation of single microwave photons [7] and lead to a nonlinear generalization of cavity quantum electrodynamics effects [8].\newline \newline [1] M.R. Geller, E.J. Pritchett, A.T. Sornborger, and F.K. Wilhelm quant-ph/0603224 \newline [2] A.G. Fowler, W. Thompson, Z. Yan, A.H. Majedi, and F.K. Wilhelm, in preparation\newline [3] R. de Sousa, K.B. Whaley, F.K. Wilhelm, and J. von Delft, Phys. Rev. Lett 95, 247006 (2005)\newline [5] A.K. Sporl, T. Schulte-Herbrueggen, S.J. Glaser, V. Bergholm, M.J. Storcz, J. Ferber, and F.K. Wilhelm quant-ph/0504202\newline [6] P. Rebentrost, I. Serban, T. Schulte-Herbrueggen, and F.K. Wilhelm, in preparation\newline [7] M. Mariantoni, M.J. Storcz, F.K. Wilhelm, W.D. Oliver, A. Emmert, A. Marx, R. Gross, H. Christ, and E. Solano, cond-mat/0509737\newline [8] I. Serban, E. Solano, F.K. Wilhelm, cond-mat/0606734. [Preview Abstract] |
Wednesday, March 7, 2007 8:36AM - 9:12AM |
N2.00002: Solid State Qubits with Current-Controlled Coupling Invited Speaker: The ability to switch the coupling between quantum bits (qubits) on and off is essential for implementing many quantum computing algorithms. We have demonstrated such control with two, three-junction flux qubits coupled together via their mutual inductances and via the dc SQUID (Superconducting Quantum Interference Device) that reads out their magnetic flux states. The flux in each qubit was controlled by an on-chip loop, and the chip was surrounded by a superconducting cavity that eliminates fluctuations in the ambient magnetic field. By applying microwave radiation to the device, we observed resonant absorption in each of the qubits when the level splitting in the qubit matched the energy of the microwave photons. With the qubits biased at the same frequency, the interaction produced an avoided crossing in their energy spectrum. At the avoided crossing transitions to the first excited state were suppressed and transitions to the second excited state enhanced, indicating formation of singlet and triplet states in the coupled-qubit system. The observed peak amplitudes were consistent with calculated matrix elements. When both qubits were biased at their degeneracy points, a level repulsion was observed in the energy spectrum. A bias current applied to the SQUID in the zero-voltage state prior to measurement induced a change in its dynamic inductance, reducing the coupling energy controllably to zero and even reversing its sign. The dependence of the splitting on the bias current was in good agreement with predictions. This work was performed in collaboration with P.A. Reichardt, B.L.T. Plourde, T.L. Robertson, C.-E. Wu, A.V. Ustinov, and John Clarke, and supported by NSF, AFOSR, ARO and ARDA. [Preview Abstract] |
Wednesday, March 7, 2007 9:12AM - 9:48AM |
N2.00003: Measurement and Generation of Single Photons in a Circuit Invited Speaker: I will describe the measurement and generation of single photons in a circuit quantum electrodynamics system. A one-dimensional transmission line cavity realizing well-defined microwave linear photon modes is coupled to a Cooper-pair box qubit. The qubit-photon coupling is exploited to realize a quantum non-demolition measurement of the qubit state by the photons, resulting in high visibility and long coherence times. The reverse measurement can also be performed: the qubit can used to measure the number of photons in the cavity. In this case, the qubit transition is resolved into separate spectral lines for each photon number, leading to a photon statistics analyzer. The same interaction can also be used to convert qubit states into a flying qubit consisting of superpositions of photon states, and to generate single microwave photons on demand, enabling a full range of quantum optics experiments. Work done in collaboration with D.I. Schuster, A. Wallraff, A. Blais, J. Schreier, L. Frunzio, J.A. Gambetta, J. Koch, J. Majer, B. Johnson, J. Chow, T. Yu, M. Devoret, S.M. Girvin, R.J. Schoelkopf. [Preview Abstract] |
Wednesday, March 7, 2007 9:48AM - 10:24AM |
N2.00004: Direct Measurement of the Entanglement of Two Superconducting Qubits via State Tomography. Invited Speaker: The Josephson phase qubit can be thought of as an electrical ``atom'' whose resonance frequency can be tuned via an external control bias. Owing to its potential compatibility with conventional integrated circuit fabrication techniques, this system is a promising candidate for a scalable architecture for a quantum computer. Currently, the critical path towards a real device consists of understanding all sources of decoherence that destroy the fragile quantum states. Recently, dielectric loss was identified as the main source of decoherence for phase qubits. By employing techniques to minimize dielectric loss we improved the performance of our quantum bit, which enabled us to show quantum-mechanical entanglement between two phase qubits and identify the generation of a Bell state with a fidelity of up to 0.87, still limited by decoherence effects. We detail the experiment and outline further progress on reducing dielectric loss, leading to an improvement of the measured energy relaxation time by a factor of five. We also identified other insulating materials, which should improve the energy relaxation time by an additional factor of two, resulting in overall coherence times of about one microsecond. [Preview Abstract] |
Wednesday, March 7, 2007 10:24AM - 11:00AM |
N2.00005: Flux qubits: quantum nondemolition readout and controlled-not gate Invited Speaker: Superconducting flux qubits have of a loop with three Josephson junctions, biased at about half a flux quantum. Basic states have opposite persistent currents, readout is by inductive coupling to a SQUID magnetometer. The following results have been obtained in a bias flux regime where the qubit energy states closely resemble the current states. Coherence was significantly lower than for the best samples. A dispersive method for readout was developed, where the inductance of the SQUID is measured rather than the critical current. The SQUID together with an on-chip capacitance forms a nonlinear oscillator where the resonant frequency depends on the flux in the SQUID, in turn influenced by the qubit. For high driving, two oscillation modes exist with low and high amplitude with a hysteretic transition. A short microwave pulse is applied and the probability that the oscillator switches to the high-amplitude mode is determined. This readout method yields a fidelity of 87{\%} without any corrections for relaxation. We have performed series of two consecutive measurements on a qubit in various superposition states and correlations between the outcomes were determined. Between the first measurement and the second a Rabi pulse was applied. Results were consistent with fully projective measurement, with a quantum nondemolition fidelity of 88{\%} without corrections. We have also studied a system of two permanently coupled flux qubits. For each qubit, the energy splitting is shifted by the other qubit to plus or minus 200 MHz. When a suitable pulse is applied to a target qubit, it acts as a pi-pulse when the control qubit is in one state, and does nothing in the opposite case. This controlled-not operation that consists of a single microwave pulse has been performed for arbitrary superposition states of the two qubits. We have determined the phase reliability of the operation as well as its amplitude response. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700