255 research outputs found
Locally addressable tunnel barriers within a carbon nanotube
We report the realization and characterization of independently controllable
tunnel barriers within a carbon nanotube. The nanotubes are mechanically bent
or kinked using an atomic force microscope, and top gates are subsequently
placed near each kink. Transport measurements indicate that the kinks form
gate-controlled tunnel barriers, and that gates placed away from the kinks have
little or no effect on conductance. The overall conductance of the nanotube can
be controlled by tuning the transmissions of either the kinks or the
metal-nanotube contacts.Comment: related papers at http://marcuslab.harvard.ed
Decoherence due to elastic Rayleigh scattering
We present theoretical and experimental studies of the decoherence of
hyperfine ground-state superpositions due to elastic Rayleigh scattering of
light off-resonant with higher lying excited states. We demonstrate that under
appropriate conditions, elastic Rayleigh scattering can be the dominant source
of decoherence, contrary to previous discussions in the literature. We show
that the elastic-scattering decoherence rate of a two-level system is given by
the square of the difference between the elastic-scattering \textit{amplitudes}
for the two levels, and that for certain detunings of the light, the amplitudes
can interfere constructively even when the elastic scattering \textit{rates}
from the two levels are equal. We confirm this prediction through calculations
and measurements of the total decoherence rate for a superposition of the
valence electron spin levels in the ground state of Be in a 4.5 T
magnetic field.Comment: 5 pages, 3 figure
Robust randomized benchmarking of quantum processes
We describe a simple randomized benchmarking protocol for quantum information
processors and obtain a sequence of models for the observable fidelity decay as
a function of a perturbative expansion of the errors. We are able to prove that
the protocol provides an efficient and reliable estimate of an average
error-rate for a set operations (gates) under a general noise model that allows
for both time and gate-dependent errors. We determine the conditions under
which this estimate remains valid and illustrate the protocol through numerical
examples.Comment: 4+ pages, 1 figure, and 1 tabl
The effect of noise correlations on randomized benchmarking
Among the most popular and well studied quantum characterization,
verification and validation techniques is randomized benchmarking (RB), an
important statistical tool used to characterize the performance of physical
logic operations useful in quantum information processing. In this work we
provide a detailed mathematical treatment of the effect of temporal noise
correlations on the outcomes of RB protocols. We provide a fully analytic
framework capturing the accumulation of error in RB expressed in terms of a
three-dimensional random walk in "Pauli space." Using this framework we derive
the probability density function describing RB outcomes (averaged over noise)
for both Markovian and correlated errors, which we show is generally described
by a gamma distribution with shape and scale parameters depending on the
correlation structure. Long temporal correlations impart large nonvanishing
variance and skew in the distribution towards high-fidelity outcomes --
consistent with existing experimental data -- highlighting potential
finite-sampling pitfalls and the divergence of the mean RB outcome from
worst-case errors in the presence of noise correlations. We use the
Filter-transfer function formalism to reveal the underlying reason for these
differences in terms of effective coherent averaging of correlated errors in
certain random sequences. We conclude by commenting on the impact of these
calculations on the utility of single-metric approaches to quantum
characterization, verification, and validation.Comment: Updated and expanded to include full derivation. Related papers
available from http://www.physics.usyd.edu.au/~mbiercuk/Publications.htm
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