478 research outputs found
Decoupling a Cooper-pair box to enhance the lifetime to 0.2 ms
We present a circuit QED experiment in which a separate transmission line is
used to address a quasi-lumped element superconducting microwave resonator
which is in turn coupled to an Al/AlO/Al Cooper-pair box (CPB) charge
qubit. In our measurements we find a strong correlation between the measured
lifetime of the CPB and the coupling between the qubit and the transmission
line. By monitoring perturbations of the resonator's 5.44 GHz resonant
frequency, we have measured the spectrum, lifetime (), Rabi, and Ramsey
oscillations of the CPB at the charge degeneracy point while the CPB was
detuned by up to 2.5 GHz . We find a maximum lifetime of the CPB was s for to 4.5 GHz. Our measured 's are consistent with
loss due to coupling to the transmission line, spurious microwave circuit
resonances, and a background decay rate on the order of
s of unknown origin, implying that the loss tangent in the AlO
junction barrier must be less than about at 4.5 GHz, about 4
orders of magnitude less than reported in larger area Al/AlO/Al tunnel
junctions
Quantum and thermal spin relaxation in diluted spin ice: Dy(2-x)MxTi2O7 (M = Lu, Y)
We have studied the low temperature a.c. magnetic susceptibility of the
diluted spin ice compound Dy(2-x)MxTi2O7, where the magnetic Dy ions on the
frustrated pyrochlore lattice have been replaced with non-magnetic ions, M = Y
or Lu. We examine a broad range of dilutions, 0 <= x <= 1.98, and we find that
the T ~ 16 K freezing is suppressed for low levels of dilution but re-emerges
for x > 0.4 and persists to x = 1.98. This behavior can be understood as a
non-monotonic dependence of the quantum spin relaxation time with dilution. The
results suggest that the observed spin freezing is fundamentally a single spin
process which is affected by the local environment, rather than the development
of spin-spin correlations as earlier data suggested.Comment: 26 pages, 9 figure
Quantum-Classical Reentrant Relaxation Crossover in Dy2Ti2O7 Spin-Ice
We have studied spin relaxation in the spin ice compound Dy2Ti2O7 through
measurements of the a.c. magnetic susceptibility. While the characteristic spin
relaxation time is thermally activated at high temperatures, it becomes almost
temperature independent below Tcross ~ 13 K, suggesting that quantum tunneling
dominates the relaxation process below that temperature. As the low-entropy
spin ice state develops below Tice ~ 4 K, the spin relaxation time increases
sharply with decreasing temperature, suggesting the emergence of a collective
degree of freedom for which thermal relaxation processes again become important
as the spins become highly correlated
Decoherence induced deformation of the ground state in adiabatic quantum computation
Despite more than a decade of research on adiabatic quantum computation
(AQC), its decoherence properties are still poorly understood. Many theoretical
works have suggested that AQC is more robust against decoherence, but a
quantitative relation between its performance and the qubits' coherence
properties, such as decoherence time, is still lacking. While the thermal
excitations are known to be important sources of errors, they are predominantly
dependent on temperature but rather insensitive to the qubits' coherence. Less
understood is the role of virtual excitations, which can also reduce the ground
state probability even at zero temperature. Here, we introduce normalized
ground state fidelity as a measure of the decoherence-induced deformation of
the ground state due to virtual transitions. We calculate the normalized
fidelity perturbatively at finite temperatures and discuss its relation to the
qubits' relaxation and dephasing times, as well as its projected scaling
properties.Comment: 10 pages, 3 figure
Surface effects on nanowire transport: numerical investigation using the Boltzmann equation
A direct numerical solution of the steady-state Boltzmann equation in a
cylindrical geometry is reported. Finite-size effects are investigated in large
semiconducting nanowires using the relaxation-time approximation. A nanowire is
modelled as a combination of an interior with local transport parameters
identical to those in the bulk, and a finite surface region across whose width
the carrier density decays radially to zero. The roughness of the surface is
incorporated by using lower relaxation-times there than in the interior.
An argument supported by our numerical results challenges a commonly used
zero-width parametrization of the surface layer. In the non-degenerate limit,
appropriate for moderately doped semiconductors, a finite surface width model
does produce a positive longitudinal magneto-conductance, in agreement with
existing theory. However, the effect is seen to be quite small (a few per cent)
for realistic values of the wire parameters even at the highest practical
magnetic fields. Physical insights emerging from the results are discussed.Comment: 15 pages, 7 figure
Thermal transport measurements of individual multiwalled nanotubes
The thermal conductivity and thermoelectric power of a single carbon nanotube
were measured using a microfabricated suspended device. The observed thermal
conductivity is more than 3000 W/K m at room temperature, which is two orders
of magnitude higher than the estimation from previous experiments that used
macroscopic mat samples. The temperature dependence of the thermal conductivity
of nanotubes exhibits a peak at 320 K due to the onset of Umklapp phonon
scattering. The measured thermoelectric power shows linear temperature
dependence with a value of 80 V/K at room temperature.Comment: 4 pages, figures include
Quantum Zeno stabilization in weak continuous measurement of two qubits
We have studied quantum coherent oscillations of two qubits under continuous
measurement by a symmetrically coupled mesoscopic detector. The analysis is
based on a Bayesian formalism that is applicable to individual quantum systems.
Measurement continuously collapses the two-qubit system to one of the
sub-spaces of the Bell basis. For a detector with linear response this
corresponds to measurement of the total spin of the qubits. In the other
extreme of purely quadratic response the operator \sigma_y^1 \sigma_y^2 +
\sigma_z^1 \sigma_z^2 is measured. In both cases, collapse naturally leads to
spontaneous entanglement which can be identified by measurement of the power
spectrum and/or the average current of the detector. Asymmetry between the two
qubits results in evolution between the different measurement subspaces.
However, when the qubits are even weakly coupled to the detector, a kind of
quantum Zeno effect cancels the gradual evolution and replaces it with rare,
abrupt switching events. We obtain the asymptotic switching rates for these
events and confirm them with numerical simulations. We show how such switching
affects the observable power spectrum on different time scales.Comment: 18 pages, 8 eps figures, reference adde
The Phonon Drag Effect in Single-Walled Carbon Nanotubes
A variational solution of the coupled electron-phonon Boltzmann equations is
used to calculate the phonon drag contribution to the thermopower in a 1-D
system. A simple formula is derived for the temperature dependence of the
phonon drag in metallic, single-walled carbon nanotubes. Scattering between
different electronic bands yields nonzero values for the phonon drag as the
Fermi level varies.Comment: 8 pages, 4 figure
Comment on `A scattering quantum circuit for measuring Bell's time inequality: a nuclear magnetic resonance demonstration using maximally mixed states'
A recent paper by Souza, Oliveira and Sarthour (SOS) reports the experimental
violation of a Leggett-Garg inequality (sometimes referred to as a temporal
Bell inequality). The inequality tests for quantum mechanical superposition: if
the inequality is violated, the dynamics cannot be explained by a large class
of classical theories under the heading of macrorealism. Experimental tests of
the LG inequality are beset by the difficulty of performing the necessary
so-called 'non-invasive' measurements (which for the macrorealist will extract
information from a system of interest without disturbing it). SOS argue that
they nevertheless achieve this difficult goal by putting the system in a
maximally mixed state. The system then allegedly undergoes no perturbation
during their experiment. Unfortunately the method is ultimately unconvincing to
a skeptical macrorealist, and so the conclusions drawn by SOS are unjustified.Comment: 6 pages, 1 figur
H2 in the interstitial channels of nanotube bundles
The equation of state of H2 adsorbed in the interstitial channels of a carbon
nanotube bundle has been calculated using the diffusion Monte Carlo method. The
possibility of a lattice dilation, induced by H2 adsorption, has been analyzed
by modeling the cohesion energy of the bundle. The influence of factors like
the interatomic potentials, the nanotube radius and the geometry of the channel
on the bundle swelling is systematically analyzed. The most critical input is
proved to be the C-H2 potential. Using the same model than in planar graphite,
which is expected to be also accurate in nanotubes, the dilation is observed to
be smaller than in previous estimations or even inexistent. H2 is highly
unidimensional near the equilibrium density, the radial degree of freedom
appearing progressively at higher densities.Comment: Accepted for publication in PR
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