2,848 research outputs found
Qubit quantum-dot sensors: noise cancellation by coherent backaction, initial slips, and elliptical precession
We theoretically investigate the backaction of a sensor quantum dot with
strong local Coulomb repulsion on the transient dynamics of a qubit that is
probed capacitively. We show that the measurement backaction induced by the
noise of electron cotunneling through the sensor is surprisingly mitigated by
the recently identified coherent backaction [PRB 89, 195405] arising from
quantum fluctuations. This renormalization effect is missing in semiclassical
stochastic fluctuator models and typically also in Born-Markov approaches,
which try to avoid the calculation of the nonstationary, nonequilibrium state
of the qubit plus sensor. Technically, we integrate out the current-carrying
electrodes to obtain kinetic equations for the joint, nonequilibrium
detector-qubit dynamics. We show that the sensor-current response, level
renormalization, cotunneling, and leading non-Markovian corrections always
appear together and cannot be turned off individually in an experiment or
ignored theoretically. We analyze the backaction on the reduced qubit state -
capturing the full non-Markovian effects imposed by the sensor quantum dot on
the qubit - by applying a Liouville-space decomposition into quasistationary
and rapidly decaying modes. Importantly, the sensor cannot be eliminated
completely even in the simplest high-temperature, weak-measurement limit: The
qubit state experiences an initial slip that persists over many qubit cycles
and depends on the initial preparation of qubit plus sensor quantum dot. A
quantum-dot sensor can thus not be modeled as a 'black box' without accounting
for its dynamical variables. We furthermore find that the Bloch vector relaxes
(T1) along an axis that is not orthogonal to the plane in which the Bloch
vector dephases (T2), blurring the notions of T1 and T2 times. Finally, the
precessional motion of the Bloch vector is distorted into an ellipse in the
tilted dephasing plane.Comment: This is the version published in Phys. Rev.
Thermodynamics of a three-flavor nonlocal Polyakov--Nambu--Jona-Lasinio model
The present work generalizes a nonlocal version of the Polyakov loop-extended
Nambu and Jona-Lasinio (PNJL) model to the case of three active quark flavors,
with inclusion of the axial U(1) anomaly. Gluon dynamics is incorporated
through a gluonic background field, expressed in terms of the Polyakov loop.
The thermodynamics of the nonlocal PNJL model accounts for both chiral and
deconfinement transitions. Our results obtained in mean-field approximation are
compared to lattice QCD results for quark flavors. Additional
pionic and kaonic contributions to the pressure are calculated in random phase
approximation. Finally, this nonlocal 3-flavor PNJL model is applied to the
finite density region of the QCD phase diagram. It is confirmed that the
existence and location of a critical point in this phase diagram depends
sensitively on the strength of the axial U(1) breaking interaction.Comment: 31 pages, 15 figures, minor changes compared to v
Thermodynamics and quark susceptibilities: a Monte-Carlo approach to the PNJL model
The Monte-Carlo method is applied to the Polyakov-loop extended
Nambu--Jona-Lasinio (PNJL) model. This leads beyond the saddle-point
approximation in a mean-field calculation and introduces fluctuations around
the mean fields. We study the impact of fluctuations on the thermodynamics of
the model, both in the case of pure gauge theory and including two quark
flavors. In the two-flavor case, we calculate the second-order Taylor expansion
coefficients of the thermodynamic grand canonical partition function with
respect to the quark chemical potential and present a comparison with
extrapolations from lattice QCD. We show that the introduction of fluctuations
produces only small changes in the behavior of the order parameters for chiral
symmetry restoration and the deconfinement transition. On the other hand, we
find that fluctuations are necessary in order to reproduce lattice data for the
flavor non-diagonal quark susceptibilities. Of particular importance are pion
fields, the contribution of which is strictly zero in the saddle point
approximation
Monte-Carlo simulations of QCD Thermodynamics in the PNJL model
We apply a Monte-Carlo method to the two flavor Polyakov loop extended Nambu
and Jona-Lasinio (PNJL) model. In such a way we can go beyond mean field
calculations introducing fluctuations of the fields. We study the impact of
fluctuations on the thermodynamics of the model. We calculate the second
derivatives of the thermodynamic grand canonical partition function with
respect to the chemical potential and present a comparison with lattice data
also for flavor non-diagonal susceptibilities.Comment: Contribution to Cortona 2008, Theoretical nuclear physics in Ital
Calibration of Deformable Mirrors for Open-Loop Control
Deformable mirrors enable the control of wave fronts for the compensation of aberrations in optical systems and/or for beam scanning. Manufacturers of deformable mirrors typically provide calibration data that encode for the fabrication tolerances among the actuators and mirror segments to support open-loop control with high wave front fidelity and accuracy. We report a calibration method that enables users of the deformable mirrors to measure the response of the mirror itself to validate and improve the calibration data. For this purpose, an imaging off-axis Michelson interferometer was built that allowed measuring the mirror topography with high accuracy and sufficient spatial resolution. By calibrating each actuator over its entire range, the open-loop performance for our deformable mirror was improved
Sub-nanometer free electrons with topological charge
The holographic mask technique is used to create freely moving electrons with
quantized angular momentum. With electron optical elements they can be focused
to vortices with diameters below the nanometer range. The understanding of
these vortex beams is important for many applications. Here we present a theory
of focused free electron vortices. The agreement with experimental data is
excellent. As an immediate application, fundamental experimental parameters
like spherical aberration and partial coherence are determined.Comment: 4 pages, 5 figure
On Approximating Restricted Cycle Covers
A cycle cover of a graph is a set of cycles such that every vertex is part of
exactly one cycle. An L-cycle cover is a cycle cover in which the length of
every cycle is in the set L. The weight of a cycle cover of an edge-weighted
graph is the sum of the weights of its edges.
We come close to settling the complexity and approximability of computing
L-cycle covers. On the one hand, we show that for almost all L, computing
L-cycle covers of maximum weight in directed and undirected graphs is APX-hard
and NP-hard. Most of our hardness results hold even if the edge weights are
restricted to zero and one.
On the other hand, we show that the problem of computing L-cycle covers of
maximum weight can be approximated within a factor of 2 for undirected graphs
and within a factor of 8/3 in the case of directed graphs. This holds for
arbitrary sets L.Comment: To appear in SIAM Journal on Computing. Minor change
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