16,269 research outputs found
Continuous vortex pumping into a spinor condensate with magnetic fields
We study the mechanisms and the limits of pumping vorticity into a spinor
condensate through manipulations of magnetic (B-) fields. We discover a
fundamental connection between the geometrical properties of the magnetic
fields and the quantized circulation of magnetically trapped atoms, a result
which generalizes several recent experimental and theoretical studies. The
optimal procedures are devised that are capable of continuously increasing or
decreasing a condensate's vorticity by repeating certain two step B-field
manipulation protocols. We carry out detailed numerical simulations that
support the claim that our protocols are highly efficient, stable, and robust
against small imperfections of all types. Our protocols can be implemented
experimentally within current technologies.Comment: 9 pages, 6 figure
Optical selection rules and phase-dependent adiabatic state control in a superconducting quantum circuit
We analyze the optical selection rules of the microwave-assisted transitions
in a flux qubit superconducting quantum circuit (SQC). We show that the
parities of the states relevant to the superconducting phase in the SQC are
well-defined when the external magnetic flux , then the
selection rules are same as the ones for the electric-dipole transitions in
usual atoms. When , the symmetry of the potential of
the artificial "atom'' is broken, a so-called -type "cyclic"
three-level atom is formed, where one- and two-photon processes can coexist. We
study how the population of these three states can be selectively transferred
by adiabatically controlling the electromagnetic field pulses. Different from
-type atoms, the adiabatic population transfer in our three-level
-atom can be controlled not only by the amplitudes but also by the
phases of the pulses
A qubit strongly-coupled to a resonant cavity: asymmetry of the spontaneous emission spectrum beyond the rotating wave approximation
We investigate the spontaneous emission spectrum of a qubit in a lossy
resonant cavity. We use neither the rotating-wave approximation nor the Markov
approximation. The qubit-cavity coupling strength is varied from weak, to
strong, even to lower bound of the ultra-strong. For the weak-coupling case,
the spontaneous emission spectrum of the qubit is a single peak, with its
location depending on the spectral density of the qubit environment. Increasing
the qubit-cavity coupling increases the asymmetry (the positions about the
qubit energy spacing and heights of the two peaks) of the two spontaneous
emission peaks (which are related to the vacuum Rabi splitting) more.
Explicitly, for a qubit in a low-frequency intrinsic bath, the height asymmetry
of the splitting peaks becomes larger, when the qubit-cavity coupling strength
is increased. However, for a qubit in an Ohmic bath, the height asymmetry of
the spectral peaks is inverted from the same case of the low-frequency bath,
when the qubit is strongly coupled to the cavity. Increasing the qubit-cavity
coupling to the lower bound of the ultra-strong regime, the height asymmetry of
the left and right peak heights are inverted, which is consistent with the same
case of low-frequency bath, only relatively weak. Therefore, our results
explicitly show how the height asymmetry in the spontaneous emission spectrum
peaks depends not only on the qubit-cavity coupling, but also on the type of
intrinsic noise experienced by the qubit.Comment: 10pages, 5 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
Effects of critical temperature inhomogeneities on the voltage-current characteristics of a planar superconductor near the Berezinskii-Kosterlitz-Thouless transition
We analyze numerically how the voltage-current (V-I) characteristics near the
so-called Berezinskii-Kosterlitz-Thouless (BKT) transition of 2D
superconductors are affected by a random spatial Gaussian distribution of
critical temperature inhomogeneities with long characteristic lengths (much
larger than the in-plane superconducting coherence length amplitude). Our
simulations allow to quantify the broadening around the average BKT transition
temperature of both the exponent alpha in V I^alpha and of the resistance V/I.
These calculations reveal that strong spatial redistributions of the local
current will occur around the transition as either I or the temperature T are
varied. Our results also support that the condition alpha=3 provides a good
estimate for the location of the average BKT transition temperature, and that
extrapolating to alpha->1 the alpha(T) behaviour well below the transition
provides a good estimate for the average mean-field critical temperature.Comment: 18 pages; pdfLaTeX; 1 TeX file + 8 PDF files for figures
(figs.1,2,3a,3b,4,5a,5b,6
Resonant peak splitting for ballistic conductance in magnetic superlattices
We investigate theoretically the resonant splitting of ballistic conductance
peaks in magnetic superlattices. It is found that, for magnetic superlattices
with periodically arranged identical magnetic-barriers, there exists a
general -fold resonant peak splitting rule for ballistic conductance,
which is the analogy of the -fold resonant splitting for transmission in
-barrier electric superlattices (R. Tsu and L. Esaki, Appl. Phys. Lett. {\bf
22}, 562 (1973)).Comment: 9 pages, 3 figures, latex forma
Genghis Khan's death (AD 1227): An unsolvable riddle or simply a pandemic disease?
The article examines Genghis Khan's death from the historico-medical perspective. Although several etiologies have been proposed over the years, most of these at a closer look appear to be later inventions by historians. A reassessment of the available evidence suggests instead bubonic plague as the most likely clinical scenario. Genghis Khan's death is also a reflection on the impact of pandemic diseases on leadership in ancient times as well as nowadays
Probing tiny motions of nanomechanical resonators: classical or quantum mechanical?
We propose a spectroscopic approach to probe tiny vibrations of a
nanomechanical resonator (NAMR), which may reveal classical or quantum behavior
depending on the decoherence-inducing environment. Our proposal is based on the
detection of the voltage-fluctuation spectrum in a superconducting transmission
line resonator (TLR), which is {\it indirectly} coupled to the NAMR via a
controllable Josephson qubit acting as a quantum transducer. The classical
(quantum mechanical) vibrations of the NAMR induce symmetric (asymmetric) Stark
shifts of the qubit levels, which can be measured by the voltage fluctuations
in the TLR. Thus, the motion of the NAMR, including if it is quantum mechanical
or not, could be probed by detecting the voltage-fluctuation spectrum of the
TLR.Comment: 4 pages, 3 figures. to appear in Physical Review Letter
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