Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session P33: Focus Session: Superconducting Qubits IV |
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Sponsoring Units: GQI Chair: Charles Bennett, IBM Yorktown Heights Room: Colorado Convention Center 403 |
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P33.00001: A new type of superconducting qubit: How the transmon thwarts the $T_2$ problem Terri M. Yu, Jens Koch, Jay Gambetta, Andrew A. Houck, David I. Schuster, Johannes Majer, Robert J. Schoelkopf, Steven M. Girvin Superconducting qubits have long been dogged by small energy relaxation ($T_1$) and dephasing times ($T_2$). Here we propose a new type of superconducting qubit that we call the ``transmon.'' This device consists of a Cooper pair box shunted by a large capacitance. The two quantities crucial to the operating the transmon as a qubit are a) energy level anharmonicity and b) charge noise sensitivity. Sufficient anharmonicity is required to prevent transitions out of the qubit two-level system. Low sensitivity is desired so that fluctuations of the gate charge do not appreciably change the qubit transition frequency. Decreasing (increasing) the Josephson energy to charging energy ratio ($E_J$/$E_C$) of the transmon raises (reduces) anharmonicity and charge noise sensitivity. By operating the transmon in a radically different parameter regime $10^1 < E_J/E_C \ll 10^3$, the qubit becomes exponentially more stable to charge fluctuations compared to the Cooper pair box, yet it retains enough anharmonicity for fast qubit operations. For large enough $E_J/E_C$, dephasing due to charge noise becomes completely negligible so that greatly enhanced $T_2$ times should be achievable. [Preview Abstract] |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P33.00002: Circuit QED with a new type of qubit: The transmon coupled to a transmission line resonator Jens Koch, Terri M. Yu, Jay Gambetta, Andrew A. Houck, David I. Schuster, Johannes Majer, Robert J. Schoelkopf, Steven M. Girvin The idea of coupling a superconducting qubit to a one-dimensional transmission line resonator, termed circuit QED [1], has evolved into an important paradigm in the ongoing pursuit of quantum computing. Recent experiments using Cooper pair boxes (CPBs) have revealed impressive results ranging from the realization of the strong-coupling limit [2] to the observation of the ac Stark shift and measurement-induced dephasing [3]. Here, we present theoretical and experimental results on the circuit-QED physics of a new type of qubit - the transmon. We show that the reduced anharmonicity gives rise to a generalized Jaynes-Cummings model, whose coupling strength can be even larger as compared to typical CPB qubits. In the dispersive limit, the transmon displays an intriguing regime of large positive dispersive shifts. \newline [1] A. Blais et al., Phys. Rev. A 69, 062320 (2004) \newline [2] A. Wallraff et al., Nature (London) 431, 162 (2004) \newline [3] D. I. Schuster et al., Phys. Rev. Lett. 94, 123602 (2005) [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P33.00003: Sources of Decoherence in the Transmon Qubit. Joseph Schreier, Steve Girvin, Rob Schoelkopf Here we discuss a new type of superconduting qubit known as the transmon, a Cooper Pair Box in the high E$_{j}$/E$_{c}$ limit. This qubit offers insensivity to 1/f noise in charge while maintaining sufficient anharmonicity to be treated as a two level system. In this talk we consider other experimentally important sources of dephasing and relaxation including: substrate/dielectric loss, flux coupling through the SQUID loop, radiation to parasitic modes, vortices, phonons, and quasiparticles. Order of magnitude estimates for these sources of decoherence indicate that T$_{1 }$and T$_{2}$ coherence times of at least 10 $\mu $s should be attainable. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P33.00004: Macroscopic quantum tunneling in high-T$_c$ superconducting rings with intrinsic Josephson junction stacks X.Y. Jin, J. Lisenfeld, Y. Koval, A.V. Ustinov, P. M\"uller The properties of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ superconducting rings broken by intrinsic Josephson junction stacks were studied. The stack height was in between 4 and 50 junctions. SQUID behavior was observed in all devices. The modulation depth of critical currents increased with decreasing number of junctions in the stack, and conformed to the $\beta_L$ values. Furthermore, switching current distributions were investigated as a function of magnetic field and temperature. Crossover temperatures were in the range of 300 to 600 mK. Whereas the small stacks behaved like series arrays of independent junctions, the larger stacks were uniform and showed anomalous enhancement of escape rates. An unconventional coherent retrapping was observed, i.e., the retrapping probability decayed exponentially with the trapped flux. Possible implications for the realization of high-T$_c$ phase qubits are discussed. [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P33.00005: Observation of macroscopic resonant tunneling in a superconducting flux qubit Bo Mao, Wei Qiu, Siyuan Han It has been argued recently that various coherent phenomena observed in superconducting phase and flux qubits could be due to classical phase-locking between oscillators (qubits) and sinusoidal driving (microwave) [1]. We report observation of macroscopic resonant tunneling (MRT) in a weakly damped flux qubit, i.e., a radio-frequency superconducting quantum interference device (rf SQUID). Since no microwave was involved in the experiment the observation of MRT unambiguously confirms that dynamics of superconducting flux qubits are governed by quantum rather than classical physics and that superconducting flux qubits are good candidates for implementing quantum computing. The measured tunneling rate as a function of flux bias agrees with the energy level structure calculated from independently determined physical parameters of the qubit. \newline \newline [1] N. Gronbech-Jensen and M. Cirillo, Phys. Rev. Lett. 95, 067001 (2005). [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P33.00006: Effects of a resonant cavity on macroscopic quantum tunneling of fluxons in long Josephson junctins Ju Kim, Ramesh Dhungana We investigate the effects of a resonant cavity on the tunneling rate of a Josephson vortex (i.e., fluxon) which is pinned by a microresistor in long Josephson junction (LJJ). In a single LJJ, we find that the tunneling rate can be enhanced significantly when the fluxon couples to the electromagnetic field of the resonant cavity. Here the main effect of the cavity is reducing the barrier potential for the trapped fluxon. In a two LJJs that are coupled by the magnetic induction effect, the tunneling rate is determined by the competition between the strength of pinning due to the microresistor which tends to break the phase-locking behavior of the fluxons and the cavity mode which tends to promote collective motion of these fluxons. We discuss the effects of the resonant cavity on the tunneling of phase-locked fluxon-fluxon and fluxon-antifluxon pairs. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P33.00007: rf-reflectrometry measurements of a Josephson junction oscillator circuit at milliKelvin temperatures R. M. Lewis, B. K. Cooper, B. Palmer, Hanhee Paik, S. K. Dutta, T. A. Palomaki, A. J. PrzyPysz, H. Kwon, J. R. Anderson, A. J. Dragt, C. J. Lobb, F. C. Wellstood We report on rf-reflectometry measurements on a Nb/AlOx/Nb Josephson junction tank circuit. The junction has nominal critical current of 5 $\mu$A and is loaded with an on chip capacitance of 50 pF to suppress the plasma frequency to $f_p \approx 2$ GHz. Measurements were performed at temperature $T \approx 100$ mK in a dilution refrigerator. Reflection data show a clear rf absorption resonance and concomitant phase change about the resonant frequency. We will discuss use of this circuit for state readout \footnote{I. Siddiqi {\it et al.}, Phys.\ Rev.\ Lett.\, {\bf 93} 207002 (2004).} of a phase qubit and as a device for measuring critical current noise in Josephson junctions. [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P33.00008: For Improving Superconducting Qubits. Raymond W. Simmonds, M.S. Allman, F. Altomare, K. Cicak, K.D. Osborn, A.J. Sirois, J.A. Strong, J.D. Whittaker Josephson junction-based superconducting qubits are still a very promising platform for creating quantum computers of the future. We have created a strategy to improve the coherence of superconducting phase qubits, through the removal of unwanted two-level system defects known to be a significant source of decoherence. Through creating dielectric free fabrication techniques and vaccum gap capacitors, we can remove a considerable amount of troublesome defects in the construction of phase qubits. Here, we discuss some results and obstacles still facing the design and fabrication of phase qubits. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P33.00009: Energy Decay in Josephson Qubits from Off-Resonant Coupling to Two-Level States J. Martinis, M. Ansmann, R. Bialczek, N. Katz, E. Lucero, R. McDermott, M Neeley, A. O'Connell, M. Steffen, E. Weig, A. Cleland Decoherence of Josephson qubits is thought to be protected from dielectric loss of two-level states by using sub-micrometer tunnel junctions that statistically avoids resonant coupling. Here, we calculate that off-resonant coupling and the subsequent phonon radiation of the two-level states may produce significant energy loss even for ultra-small junctions. This theory possibly explains several key features in a variety of experimental data for phase, flux, and charge qubits, such as the magnitude of the observed energy decay time, its statistical variation, and the increased decay rate with qubit area and frequency. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P33.00010: Quantum Process Tomography Using Superconducting Qubits Radoslaw C. Bialczak, M. Ansmann, N. Katz, E. Lucero, R. McDermott, M. Neeley, A. D. O'Connell, M. Steffen, E. Weig, A. Cleland, J. Martinis Due to recent advances in device design and materials, universal quantum gates using Josephson junction phase qubits are now feasible. To measure gate performance other quantum computation architectures have utilized standard quantum process tomography (SQPT). In SQPT one obtains a process matrix with which gate operations for arbitrary input states can be predicted and performance measures such as fidelity and entangling capability can be obtained. Here we demonstrate how to implement SQPT with our Josephson junction phase qubits and use it to characterize a CNOT gate. We show how to obtain the process matrix of a CNOT gate and extract its fidelity and entangling capability. This allows us to compare our gate performance to that of quantum gates in other architectures. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P33.00011: A Josephson junction~resonator to test~the quality of superconducting qubit circuits Kevin Osborn, Josh Strong, Adam Sirois, Raymond Simmonds Superconducting Josephson junction~resonators can~probe nonlinear oscillators~such~as~qubit~readout amplifiers and~qubits themselves.~We have fabricated~weakly coupled resonators~with a~flux tunable resonant frequency of over 1 GHz.~ At high powers the Josephson junction~resonators become nonlinear and two stable oscillation states are observed which can be harnessed to readout qubits.~ At sufficiently low powers, the~resonators~can probe a qubit since they store no more than~one photon of energy.~~At these low powers we observe two-level system defects attributed to the Josephson junction,~similar to those observed in the phase qubit.~ We have fabricated and measured~resonators with different~device parameters, such as the junction area and the critical current density.~ We will report on~the quality of the~resonators as a function of the device parameters. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P33.00012: Optimal Control of a Qubit coupled to a Two-Level Fluctuator Patrick Rebentrost, Ioana Serban, Frank K. Wilhelm, Thomas Schulte-Herbr\"uggen Experimental realizations of superconducting qubits are prone to decoherence from a fluctuating environment. An important source of charge and critical current noise are two-level fluctuators with a Lorentzian noise spectrum. We apply a recent generalization of quantum optimal control in presence of decoherence, the openGRAPE algorithm [1], to a microscopic model of a qubit coupled to a single two-level fluctuator. We find pulses that decrease the error of single qubit quantum gates and quantum memory up to a magnitude compared with conventional pulses. The qubit is pulsed in a way that the effect of the fluctuator is minimized. We investigate system and optimized pulses in various parameter regimes. [1] T. Schulte-Herbr\"uggen et al., quant-ph/0609037 [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P33.00013: A two-qubit gate based on a multi-terminal double barrier Josephson Serhii Shafraniuk A multi-terminal double barrier SISIS junction (S and I denote a superconductor and an insulating barrier respectively) is suggested as a two-qubit gate with tunable intrinsic coupling. Two quantum wells are formed in vicinities of the left and right SIS subjunctions. This gives two individual qubits, which are intrinsically coupled via the middle S layer due to phase coherence. The inter-qubit coupling $J$ is tuned by two bias supercurrents $I_1$ and $I_2$ across each of the SIS subjunctions independently. Additional coupling is accomplished by transport supercurrents $I^{\mathrm{tr}}_l$ along adjacent S layers. Using a microscopic model we compute major qubit characteristics and study sources of the intrinsic decoherence. We compute the entanglement of the two qubit states, leakage and fidelity characteristics versus $J$, and discuss the readout process. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P33.00014: Non-Linear, DC-Biased, Vacuum-Gap Capacitor LC Oscillators M.S. Allman, K. Cicak, K.D. Osborn, J.A. Strong, R.W. Simmonds We have observed non-linear behavior at higher powers in vacuum- gap capacitor LC resonators. These non-linear effects are a result of electrostatic forces on the vacuum capacitors. Electrostatic forces cause the distance between the capacitor plates to contract, resulting in a power-dependent resonant frequency. In an effort to characterize these non-linear effects, we have incorporated a bias T on chip allowing us to apply a DC voltage-bias to a fabricated vacuum-gap LC resonator. We then measure the resonant frequency of the circuit as a function of applied bias-voltage or equivalently, applied force on the vacuum-gap capacitor. This information allows us to predict the non-linear effects in our LC resonators as well as gives insight into the structural integrity of the vacuum-gap capacitors. These devices can lead to future applications of DC-biased vacuum-gap capacitors as tuneable superconducting phase qubit couplers. [Preview Abstract] |
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