Dephasing of a superconducting flux qubit

In order to gain a better understanding of the origin of decoherence in superconducting flux qubits, we have measured the magnetic field dependence of the characteristic energy relaxation time ($T_1$) and echo phase relaxation time ($T_2^{\rm echo}$) near the optimal operating point of a flux qubit. We have measured $T_2^{\rm echo}$ by means of the phase cycling method. At the optimal point, we found the relation $T_2^{\rm echo}\approx 2T_1$. This means that the echo decay time is {\it limited by the energy relaxation} ($T_1$ process). Moving away from the optimal point, we observe a {\it linear} increase of the phase relaxation rate ($1/T_{2}^{\rm echo}$) with the applied external magnetic flux. This behavior can be well explained by the influence of magnetic flux noise with a $1/f$ spectrum on the qubit

Transport properties of double-walled carbon nanotube quantum dots

The transport properties of quantum dot (QD) systems based on double-walled carbon nanotube (DWCNT) are investigated. The interplay between microscopic structure and strong Coulomb interaction is treated within a bosonization framework. The linear and nonlinear G-V-V_g characteristics of the QD system is calculated by starting from the Liouville equation for the reduced density matrix. Depending on the intershell couplings, an 8-electron periodicity of the Coulomb blockade peak spacing in the case of commensurate DWCNT QDs and a 4-electron periodicity in the incommensurate case are predicted. The contribution of excited states of DWCNTs to the nonlinear transport is investigated as well.Comment: 18 pages, 7 figure

Non-additivity of decoherence rates in superconducting qubits

We show that the relaxation and decoherence rates 1/T_1 and 1/T_2 of a qubit coupled to several noise sources are in general not additive, i.e., that the total rates are not the sums of the rates due to each individual noise source. To demonstrate this, we calculate the relaxation and pure dephasing rates 1/T_1 and 1/T_\phi of a superconducting (SC) flux qubit in the Born-Markov approximation in the presence of several circuit impedances Z_i using network graph theory and determine their deviation from additivity (the mixing term). We find that there is no mixing term in 1/T_\phi and that the mixing terms in 1/T_1 and 1/T_2 can be positive or negative, leading to reduced or enhanced relaxation and decoherence times T_1 and T_2. The mixing term due to the circuit inductance L at the qubit transition frequency \omega_{01} is generally of second order in \omega_{01}L/Z_i, but of third order if all impedances Z_i are pure resistances. We calculate T_{1,2} for an example of a SC flux qubit coupled to two impedances.Comment: 5 pages, 2 figure

Integration, Disintegration, and Reintegration via Advanced Information Technology

American culture is shifting from a mass culture toward increasing specialization and diversification. The growth of communication in cyberspace is consistent with this trend inasmuch as it supports the proliferation of interest and affinity groups. Socioeconomic, demographic, and cultural factors affect access to, use of, and behavior within this communication space. Other forms of regulation of the use of cyberspace include the emerging norms of on-line discussion groups and political attempts to regulate the content of the Internet.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68894/2/10.1177_089443939601400103.pd

Heat flux operator, current conservation and the formal Fourier's law

By revisiting previous definitions of the heat current operator, we show that one can define a heat current operator that satisfies the continuity equation for a general Hamiltonian in one dimension. This expression is useful for studying electronic, phononic and photonic energy flow in linear systems and in hybrid structures. The definition allows us to deduce the necessary conditions that result in current conservation for general-statistics systems. The discrete form of the Fourier's Law of heat conduction naturally emerges in the present definition

Phase-Coherent Dynamics of a Superconducting Flux Qubit with Capacitive-Bias Readout

We present a systematic study of the phase-coherent dynamics of a superconducting three-Josephson-junction flux qubit. The qubit state is detected with the integrated-pulse method, which is a variant of the pulsed switching DC SQUID method. In this scheme the DC SQUID bias current pulse is applied via a capacitor instead of a resistor, giving rise to a narrow band-pass instead of a pure low-pass filter configuration of the electromagnetic environment. Measuring one and the same qubit with both setups allows a direct comparison. With the capacitive method about four times faster switching pulses and an increased visibility are achieved. Furthermore, the deliberate engineering of the electromagnetic environment, which minimizes the noise due to the bias circuit, is facilitated. Right at the degeneracy point the qubit coherence is limited by energy relaxation. We find two main noise contributions. White noise is limiting the energy relaxation and contributing to the dephasing far from the degeneracy point. 1/f-noise is the dominant source of dephasing in the direct vicinity of the optimal point. The influence of 1/f-noise is also supported by non-random beatings in the Ramsey and spin echo decay traces. Numeric simulations of a coupled qubit-oscillator system indicate that these beatings are due to the resonant interaction of the qubit with at least one point-like fluctuator, coupled especially strongly to the qubit.Comment: Minor changes. 21 pages, 15 figure

Doppler evaluation of patients with constrictive pericarditis: Use of tricuspid regurgitation velocity curves to determine enhanced ventricular interaction

AbstractObjectives. This study sought to examine the value of analyzing Doppler echocardiographically derived tricuspid regurgitation signals during respiration in relation to the diagnosis of constrictive pericarditis.Background. A physiologic hallmark of constrictive pericarditis is enhanced ventricular interdependence, which produces reciprocal changes in right and left ventricular filling and ejection dynamics during the respiratory cycle. It was hypothesized that these changes could be detected noninvasively by analyzing Doppler echocardiographically derived tricuspid regurgitation signals and that this information could assist in noninvasively diagnosing constrictive pericarditis.Methods. Simultaneous Doppler echocardiography and catheterization studies of the right and left sides of the heart with high fidelity pressure manometers were performed in 5 patients with surgically confirmed constrictive pericarditis and 12 patients (control subjects) with heart failure due to other causes.Results. Changes observed in tricuspid regurgitation Doppler echocardiographic variables from onset to peak inspiration in patients with constrictive pericarditis were significantly different from those in control subjects. Mean (±SD) percent change in maximal tricuspid regurgitation velocity was 13% ± 6% and −8% ± 7% in the constrictive pericarditis and control groups, respectively (p < 0.0001); mean percent change in tricupsid regurgitation signal duration was 18% ± 2% and −2% ± 7%, respectively (p < 0.0001); mean percent change in tricuspid regurgitation time velocity integral was 27% ± 15% and −10% ± 12%, respectively (p < 0.0001).Conclusions. Respiratory changes in Doppler echocardiographically derived tricuspid regurgitation peak velocity and velocity duration are increased in patients with constrictive pericarditis and may be helpful in diagnosing this condition noninvasively