12,463 research outputs found
Engineering fast and stable splitting of matter waves
When attempting to split coherent cold atom clouds or a Bose-Einstein
condensate (BEC) by bifurcation of the trap into a double well, slow adiabatic
following is unstable with respect to any slight asymmetry, and the wave
"collapses" to the lower well, whereas a generic fast chopping splits the wave
but it also excites it. Shortcuts to adiabaticity engineered to speed up the
adiabatic process through non-adiabatic transients, provide instead quiet and
robust fast splitting. The non-linearity of the BEC makes the proposed shortcut
even more stable
Dynamical Properties of Multi-Armed Global Spirals in Rayleigh-Benard Convection
Explicit formulas for the rotation frequency and the long-wavenumber
diffusion coefficients of global spirals with arms in Rayleigh-Benard
convection are obtained. Global spirals and parallel rolls share exactly the
same Eckhaus, zigzag and skewed-varicose instability boundaries. Global spirals
seem not to have a characteristic frequency or a typical size ,
but their product is a constant under given experimental
conditions. The ratio of the radii of any two dislocations (,
) inside a multi-armed spiral is also predicted to be constant. Some of
these results have been tested by our numerical work.Comment: To appear in Phys. Rev. E as Rapid Communication
Simultaneous cooling of an artificial atom and its neighboring quantum system
We propose an approach for cooling both an artificial atom (e.g., a flux
qubit) and its neighboring quantum system, the latter modeled by either a
quantum two-level system or a quantum resonator. The flux qubit is cooled by
manipulating its states, following an inverse process of state population
inversion, and then the qubit is switched on to resonantly interact with the
neighboring quantum system. By repeating these steps, the two subsystems can be
simultaneously cooled. Our results show that this cooling is robust and
effective, irrespective of the chosen quantum systems connected to the qubit.Comment: 5 pages, 3 figure
Transient energy excitation in shortcuts to adiabaticity for the time dependent harmonic oscillator
There is recently a surge of interest to cut down the time it takes to change
the state of a quantum system adiabatically. We study for the time-dependent
harmonic oscillator the transient energy excitation in speed-up processes
designed to reproduce the initial populations at some predetermined final
frequency and time, providing lower bounds and examples. Implications for the
limits imposed to the process times and for the principle of unattainability of
the absolute zero, in a single expansion or in quantum refrigerator cycles, are
drawn.Comment: 7 pages, 6 figure
Variable-frequency-controlled coupling in charge qubit circuits: Effects of microwave field on qubit-state readout
To implement quantum information processing, microwave fields are often used
to manipulate superconuducting qubits. We study how the coupling between
superconducting charge qubits can be controlled by variable-frequency magnetic
fields. We also study the effects of the microwave fields on the readout of the
charge-qubit states. The measurement of the charge-qubit states can be used to
demonstrate the statistical properties of photons.Comment: 7 pages, 3 figure
Measuring the quality factor of a microwave cavity using superconduting qubit devices
We propose a method to create superpositions of two macroscopic quantum
states of a single-mode microwave cavity field interacting with a
superconducting charge qubit. The decoherence of such superpositions can be
determined by measuring either the Wigner function of the cavity field or the
charge qubit states. Then the quality factor Q of the cavity can be inferred
from the decoherence of the superposed states. The proposed method is
experimentally realizable within current technology even when the value is
relatively low, and the interaction between the qubit and the cavity field is
weak.Comment: 8 page
Optimal measurements to access classical correlations of two-qubit states
We analyze the optimal measurements accessing classical correlations in
arbitrary two-qubit states. Two-qubit states can be transformed into the
canonical forms via local unitary operations. For the canonical forms, we
investigate the probability distribution of the optimal measurements. The
probability distribution of the optimal measurement is found to be centralized
in the vicinity of a specific von Neumann measurement, which we call the
maximal-correlation-direction measurement (MCDM). We prove that for the states
with zero-discord and maximally mixed marginals, the MCDM is the very optimal
measurement. Furthermore, we give an upper bound of quantum discord based on
the MCDM, and investigate its performance for approximating the quantum
discord.Comment: 8 pages, 3 figures, version accepted by Phys. Rev.
Fast and robust spin manipulation in a quantum dot by electric fields
We apply an invariant-based inverse engineering method to control by
time-dependent electric fields electron spin dynamics in a quantum dot with
spin-orbit coupling in a weak magnetic field. The designed electric fields
provide a shortcut to adiabatic processes that flips the spin rapidly, thus
avoiding decoherence effects. This approach, being robust with respect to the
device-dependent noise, can open new possibilities for the spin-based quantum
information processing.Comment: 7 pages, 6 figures, with supplemental material. Errors in the
published version have been correcte
LDA+Gutzwiller Method for Correlated Electron Systems: Formalism and Its Applications
We introduce in detail our newly developed \textit{ab initio} LDA+Gutzwiller
method, in which the Gutzwiller variational approach is naturally incorporated
with the density functional theory (DFT) through the "Gutzwiller density
functional theory (GDFT)" (which is a generalization of original Kohn-Sham
formalism). This method can be used for ground state determination of electron
systems ranging from weakly correlated metal to strongly correlated insulators
with long-range ordering. We will show that its quality for ground state is as
high as that by dynamic mean field theory (DMFT), and yet it is computationally
much cheaper. In additions, the method is fully variational, the charge-density
self-consistency can be naturally achieved, and the quantities, such as total
energy, linear response, can be accurately obtained similar to LDA-type
calculations. Applications on several typical systems are presented, and the
characteristic aspects of this new method are clarified. The obtained results
using LDA+Gutzwiller are in better agreement with existing experiments,
suggesting significant improvements over LDA or LDA+U.Comment: 20 pages, 11 figure
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