Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session L17: Superconducting Flux Qubits and Qubit Amplifiers and Readouts |
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Sponsoring Units: GQI Chair: Joe Aumentado, National Institute of Standards and Technology Room: 318 |
Tuesday, March 17, 2009 2:30PM - 2:42PM |
L17.00001: Behavior of a Josephson Flux Qubit on a Sapphire Substrate Anthony Przybysz, E. Crowe, H. Kwon, B.K. Cooper, R.M. Lewis, B.S. Palmer, J.R. Anderson, C.J. Lobb, F.C. Wellstood We discuss the design, fabrication, and testing of a Nakamura- style [1] flux qubit. The device consists of a four-Josephson junction qubit loop that is directly coupled to a small dc SQUID, which is used for detection. The device was built on a sapphire substrate using electron beam lithography and double angle evaporation to form the Al/AlOx/Al tunnel junctions. A 200 nm thick layer of aluminum was deposited on the e-beam resist in order to counteract charging effects during the lithography. Three of the junctions in the qubit loop were 100 nm x 250 nm, and the fourth was 100 nm x 150 nm. The large junctions are the main contribution to the inductance of the qubit loop, and the smaller junction creates an energy splitting of 1-10 GHz between the two circulating current states. The SQUID junctions were 100 nm x 2000 nm, and the critical current of the detection SQUID was 240 nA. We present the results of ongoing measurements on the behavior of the device at 25 mK. This project was funded by the JQI, LPS, and CNAM. [1] F. Yoshihara, Y. Nakamura, et al.,``Decoherence of Flux Qubit Due to 1/f Flux Noise,'' PRL 97, 167001 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 2:42PM - 2:54PM |
L17.00002: IBM's experimental quantum computing effort. Matthias Steffen, David DiVincenzo, Matthew Farinelli, George Keefe, Mark Ketchen, Shwetank Kumar, Frank Milliken, Mary Beth Rothwell, Jim Rozen We present our experimental quantum computing effort and discuss results on flux and phase qubits. [Preview Abstract] |
Tuesday, March 17, 2009 2:54PM - 3:06PM |
L17.00003: Tuning the gap of the superconducting flux qubit Arkady Fedorov, Floor Paauw, Kees Harmans, Hans Mooij Recent advances in experiments with the flux qubits include demonstration of single and two qubit quantum gates as well as a coupling between the flux qubit and a harmonic oscillator. It was also experimentally confirmed that the best coherence properties were achieved when the qubit was kept at the symmetry point. For these conditions the qubit's energy level splitting is minimal (the gap) and determined solely by the quantum tunnelling in the double-well potential. However, since the potential barrier and the gap of the conventional flux qubit are fully fixed by the fabrication, one needs to tune the qubit out of the symmetry point in order to bring it in resonance with another quantum system. We overcame this limitation by introducing the change in the qubit design and demonstrated the tuning of the gap over a range of several Gigahertz within a few nanoseconds. We believe this could be an important step toward the coupling of the flux qubit to another qubit or the quantum bus. This control also allows a more extensive study of the relaxation time of the qubit as a function of the gap size within a constant environment. [Preview Abstract] |
Tuesday, March 17, 2009 3:06PM - 3:18PM |
L17.00004: Ultra-strong coupling regime of cavity QED with flux qubits Jerome Bourassa, Alexandre Blais With improved dephasing rate and coupling strength, the transmon qubit has recently been used to reach the strong coupling regime of cavity QED [1,2]. With the transmon however, these improvements are done at the expense of lower anharmonicity compared to the Cooper-pair box. Here we present an alternative approach where a flux qubit is coupled to the transmission line. As was recently shown experimentally [1], very strong coupling can be obtained by directly connecting the qubit loop to the center conductor of the resonator whose local inductance is tuned to maximize the coupling. We will discuss how this system can be used to study the breakdown of the rotating-wave approximation and how the $\Lambda$-configuration of the energy levels of the flux qubit can be exploited. \\[3pt] [1] J. Koch et al, Phys. Rev. A, 2007, 76, 042319 (2007)\\[0pt] [2] J. A. Schreier et al, PRB 77, 180502 (2008)\\[0pt] [3] A. A. Abdumalikov, et al., PRB 78, 180502 (2008)\\[0pt] [Preview Abstract] |
Tuesday, March 17, 2009 3:18PM - 3:30PM |
L17.00005: Amplitude spectroscopy of a superconducting artificial atom William Oliver, David Berns, Sergio Valenzuela, Mark Rudner, Leonid Levitov, Terry Orlando We introduce and demonstrate amplitude spectroscopy in a superconducting artificial atom [1]. A harmonic field at a fixed frequency drives the artificial atom through its energy-level avoided crossings. Spectroscopic information is obtained from the amplitude dependence of the system response. The resulting ``spectroscopy diamonds,'' regions of parameters space in which state transitions occur, exhibit quantum interference patterns and population inversion which serve as a fingerprint of the atom's energy spectrum. Using this approach, we determined the energy spectrum of a manifold of states with energies from $h $x 0.01 GHz to $h $x 120 GHz for a fixed driving frequency near only 0.16 GHz. The amplitude spectroscopy technique is complementary to frequency spectroscopy, providing a means to access, manipulate, and characterize quantum systems over broad bandwidths while using only a single drive frequency that may be orders of magnitude smaller than the energy scales being probed. [1] Berns et al., Nature 455, 51 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 3:30PM - 3:42PM |
L17.00006: Phase tomography of a strongly driven superconducting artificial atom Mark Rudner, Andrei Shytov, Leonid Levitov, David Berns, William Oliver, Terry Orlando, Sergio Valezuela In a recent experiment [1], amplitude spectroscopy of a superconducting qubit was demonstrated by driving the system with a strong rf field through a manifold of states spanning energies up to 120 GHz. The interference between repeated Landau-Zener transitions in a qubit swept through an avoided level crossing results in Stueckelberg oscillations in qubit magnetization. The resulting oscillatory patterns are a hallmark of the coherent strongly-driven regime in qubits, quantum dots and other two-level systems. The two-dimensional Fourier transforms of these patterns are found to exhibit a family of one-dimensional curves in Fourier space [2], in agreement with experiment [1]. We interpret these images in terms of the time evolution of the quantum phase of the qubit state and show that they can be used to probe dephasing mechanisms in the qubit. [1] D. M. Berns et al., Nature 455, 51 (2008). [2] M. S. Rudner et al., Phys. Rev. Lett. 101, 190502 (2008) [Preview Abstract] |
Tuesday, March 17, 2009 3:42PM - 3:54PM |
L17.00007: Simultaneous cooling of an artificial atom and its neighboring quantum system Jianqiang You, Yu-xi Liu, Franco Nori We propose an approach for cooling both an artificial atom (e.g., a flux qubit) and its neighboring quantum system, the latter modeled by either a quantum two-level system or a quantum resonator. The flux qubit is cooled by manipulating its states, following an inverse process of state population inversion, and then the qubit is switched on to resonantly interact with the neighboring quantum system. By repeating these steps, the two subsystems can be simultaneously cooled. Our results show that this cooling is robust and effective, irrespective of the chosen quantum systems connected to the qubit. [Preview Abstract] |
Tuesday, March 17, 2009 3:54PM - 4:06PM |
L17.00008: Chirped nonlinear cavity for digital quantum state readout without switching Ofer Naaman, Jos\'e Aumentado, Lazar Friedland, Jonathan Wurtele, Irfan Siddiqi We observe a new phase-locking effect in a high-$Q$ cavity embedding a Josephson junction driven with a chirped microwave signal. Above a critical drive amplitude, the cavity phase-locks to the drive and its oscillation amplitude grows with time. Below threshold, the cavity dephases from the drive and its amplitude remains small. The transition to phase-locking is associated with a sharp threshold sensitive to the junction $I_0$, and can be used for digital detection of quantum states. This detector smoothly evolves into one oscillation state or the other without relying on any switching process. [Preview Abstract] |
Tuesday, March 17, 2009 4:06PM - 4:18PM |
L17.00009: Lumped-element microwave resonant circuit with a dc SQUID M.P. DeFeo, C. Song, T.W. Heitmann, K. Yu, B.L.T. Plourde, R. McDermott We have fabricated lumped-element microwave resonant circuits consisting of a dc SQUID shunted with a capacitor formed from superconducting layers. Adjusting the SQUID bias conditions changes its Josephson inductance, thus varying the resonant frequency. We discuss the possibility of time-domain monitoring of the oscillations in these circuits and their potential use in a new readout scheme for superconducting qubits. [Preview Abstract] |
Tuesday, March 17, 2009 4:18PM - 4:30PM |
L17.00010: Qubit decoherence due to a Josephson bifurcation amplifier trapped in one of Frank Wilhelm, Ioana Serban, Mark Dykman We investigate the relaxation of a superconducting flux qubit for the case when its detector, the Josepshon bifurcation amplifier, remains latched in one of its two (meta)stable states. We observe a qualitatively different behavior for the two different attractors, and interpret the result as the combined effect of the amplitude of the detector's response to external driving and the effective curvature of the detector's basins of attraction in a rotating frame, in the proximity of the stable points. We address the question of the proper version of detailed balance for the qubit. [Preview Abstract] |
Tuesday, March 17, 2009 4:30PM - 4:42PM |
L17.00011: Parametric Resonators for Quantum Information Applications C.M. Wilson, M. Sandberg, F. Persson, I. Hoi, G. Johansson, V. Shumeiko, P. Delsing, T. Duty We have fabricated and characterized tunable superconducting transmission line resonators. To change the resonance frequency, we modify the boundary condition at one end of the resonator through the tunable Josephson inductance of a SQUID. We demonstrate a large tuning range, high quality factors and that we can change the frequency of a few-photon field on a time scale orders of magnitude faster than the photon lifetime. When parametrically pumped at twice their resonance frequency, the devices can act as parametric amplifiers. When pumped strongly, a threshold is crossed where the resonators oscillate spontaneously. Within this regime of parametric oscillations, the devices can exist in a variety of dynamical states. We observe a rich pattern in the dynamics of switching between these states. We study the possibility of using this dynamical bifurcation for qubit readout. Finally, recent theoretical work has suggested that it may be easier to observe dynamical tunneling in this system than in the Duffing oscillator. [Preview Abstract] |
Tuesday, March 17, 2009 4:42PM - 4:54PM |
L17.00012: The effects of higher-harmonic mode coupling in quarter-wave SQUID parametric amplifiers Minhyea Lee, Lafe Spietz, Jose Aumentado Recent interest in quarter-wave SQUID-based parametric amplifiers has motivated concerns regarding the coupling of higher harmonic modes to the operating frequency mode. We will present experiments in which the harmonic mode coupling is attempted to measure. We will also discuss the effect of higher mode coupling on noise performance and gain, focusing on whether this mode coupling limits practical amplifier performance. [Preview Abstract] |
Tuesday, March 17, 2009 4:54PM - 5:06PM |
L17.00013: Nonlinear dissipative filters for measurement protection on superconducting qubits Pol Forn-Diaz, Raymond Schouten, Kees Harmans, Hans Mooij Measurements on superconducting qubits require the system to be well isolated from noise sources if its quantum state is not being accessed. This ensures that decoherence induced by the measurement apparatus is minimized. The need to have slow (sub-GHz) and fast (GHz) lines to measure and control the state of the qubit is difficult to combine with the requirement to attenuate the noise over a broad spectral range. To overcome this problem, we have built a new type of non-linear coaxial copper powder filter with a Josephson junction in its inside. The junction in the filter acts as a shorting switch. For low frequencies, the junction acts as a shortcut to ground, and high frequencies are absorbed in the metallic powder. The Josephson junction critical current is taken such that when sending a pulse to probe the measurement device (a DC SQUID in our case), the junction in the filter switches to the voltage state, thus reaching the SQUID to perform the measurement. A minimum noise suppression of 40 dB is obtained, while allowing ns pulses to be transported. [Preview Abstract] |
Tuesday, March 17, 2009 5:06PM - 5:18PM |
L17.00014: Development of a Microwave Resonator for Qubit Read-out Zaeill Kim, V. Zaretskey, K. D. Osborn, F. C. Wellstood, B. S. Palmer We have designed and fabricated a ``lumped-element'' thin-film superconducting Al microwave resonator on sapphire to be used to read out a Cooper-pair box. The resonator consists of a meandering inductor and an interdigitated capacitor coupled to a transmission line. At T=30 mK and on resonance at 5.578 GHz, the transmission through the transmission line decreases by 15 dB and the loaded quality factor is 60,000. We have studied the temperature dependence of our resonator at temperatures as high as 500 mK and compared it to the Mattis-Bardeen theory. Coupling of this resonator to a Cooper-pair box qubit will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 5:18PM - 5:30PM |
L17.00015: Resolving Vacuum Fluctuations in an Electrical Circuit by Measuring the Lamb Shift Andreas Fragner, Martin Goppl, Alexandre Blais, Andreas Wallraff Quantum theory predicts that empty space is not truly empty. Even in the absence of any particles or radiation, in pure vacuum, virtual particles are constantly created and annihilated. In an electromagnetic field, the presence of virtual photons manifests itself as a small renormalization of the energy of a quantum system, known as the Lamb shift. We present an experimental observation of the Lamb shift in a solid-state system. The strong dispersive coupling of a superconducting electronic circuit acting as a quantum bit (qubit) to the vacuum field in a transmission-line resonator leads to measurable Lamb shifts of up to 1.4\% of the qubit transition frequency. The qubit is also observed to couple more strongly to the vacuum field than to a single photon inside the cavity, an effect that is explained by taking into account the limited anharmonicity of the higher excited qubit states. [Preview Abstract] |
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