555 research outputs found
Deconfinement transition and dimensional crossover in the Bechgaard-Fabre salts: pressure- and temperature-dependent optical investigations
The infrared response of the organic conductor (TMTSF)PF and the Mott
insulator (TMTTF)PF are investigated as a function of temperature and
pressure and for the polarization parallel and perpendicular to the molecular
stacks. By applying external pressure on (TMTTF)PF, the Mott gap
rapidly diminishes until the deconfinement transition occurs when the gap
energy is approximately twice the interchain transfer integral. In its
deconfined state (TMTTF)PF exhibits a crossover from a
quasi-one-dimensional to a higher-dimensional metal upon reducing the
temperature. For (TMTSF)PF this dimensional crossover is observed
either with increase in external pressure or with decrease in temperature. We
quantitatively determine the dimensional crossover line in the
pressure-temperature diagram based on the degree of coherence in the optical
response perpendicular to the molecular stacks.Comment: 12 pages, 15 figure
Charge echo in a Cooper-pair box
A spin-echo-type technique is applied to an artificial two-level system that
utilizes charge degree of freedom in a small superconducting electrode.
Gate-voltage pulses are used to produce the necessary pulse sequence in order
to eliminate the inhomogeneity effect in the time-ensemble measurement and to
obtain refocused echo signals. Comparison of the decay time of the observed
echo signal with estimated decoherence time suggests that low-frequency
energy-level fluctuations due to the 1/f charge noise dominate the dephasing in
the system.Comment: 4 pages, 3 figure
Quantum noise in the Josephson charge qubit
We study decoherence of the Josephson charge qubit by measuring energy
relaxation and dephasing with help of the single-shot readout. We found that
the dominant energy relaxation process is a spontaneous emission induced by
quantum noise coupled to the charge degree of freedom. Spectral density of the
noise at high frequencies is roughly proportional to the qubit excitation
energy.Comment: Submitted to Phys. Rev. Letter
Parity effect in superconducting aluminum single electron transistors with spatial gap profile controlled by film thickness
We propose a novel method for suppression of quasiparticle poisoning in Al
Coulomb blockade devices. The method is based on creation of a proper energy
gap profile along the device. In contrast to the previously used techniques,
the energy gap is controlled by the film thickness. Our transport measurements
confirm that the quasiparticle poisoning is suppressed and clear 2
periodicity is observed only when the island is made much thinner than the
leads. This result is consistent with the existing model and provides a simple
method to suppress quasiparticle poisoning
Parametric coupling for superconducting qubits
We propose a scheme to couple two superconducting charge or flux qubits
biased at their symmetry points with unequal energy splittings. Modulating the
coupling constant between two qubits at the sum or difference of their two
frequencies allows to bring them into resonance in the rotating frame.
Switching on and off the modulation amounts to switching on and off the
coupling which can be realized at nanosecond speed. We discuss various physical
implementations of this idea, and find that our scheme can lead to rapid
operation of a two-qubit gate.Comment: 6 page
Dynamics of coupled spins in the white- and quantum-noise regime
We study the dynamics of dissipative spins for general spin-spin coupling. We
investigate the population dynamics and relaxation of the purity in the white
noise regime, in which exact results are available. Inter alia, we find
distinct reduction of decoherence and slowdown of purity decay around
degeneracy points. We also determine in analytic form the one-phonon exchange
contribution to decoherence and relaxation in the ohmic quantum noise regime
valid down to zero temperature.Comment: 8 pages, 5 figure
Temperature square dependence of the low frequency 1/f charge noise in the Josephson junction qubits
To verify the hypothesis about the common origin of the low frequency 1/f
noise and the quantum f noise recently measured in the Josephson charge qubits,
we study temperature dependence of the 1/f noise and decay of coherent
oscillations. T^2 dependence of the 1/f noise is experimentally demonstrated,
which supports the hypothesis. We also show that dephasing in the Josephson
charge qubits off the electrostatic energy degeneracy point is consistently
explained by the same low frequency 1/f noise that is observed in the transport
measurements.Comment: 4 pages, 2 figure
- …