1,863 research outputs found
Response functions of an artificial Anderson atom in the atomic limit
We consider the spin and pseudospin (charge) response functions of the
exactly soluble Anderson atom model. We demonstrate, in particular, that a
deviation from the magnetic Curie-law behaviour, appropriate for a free spin
one-half, increases with increasing asymmetry and temperature. In general,
oscillator strength is transferred from the spin degrees of freedom to the
pseudospin modes. We also consider the negative-U Anderson atom and demonstrate
that the pseudospin modes are the relevant low-energy excitations in this case.
Especially, the roles of the spin and charge excitations are interchanged upon
reversal of the intrasite Coulomb repulsion, U.Comment: 23 pages, 12 figures. Accepted for publication in J. Low Temp. Phy
Internal Magnus effects in superfluid 3A
Orbital angular momentum of the coherently aligned Cooper pairs in superfluid 3A is encountered by an object immersed in the condensate. We evaluate the associated quasiparticle-scattering asymmetry experienced by a negative ion; this leads to a measureable, purely quantum-mechanical reactive force deflecting the ion’s trajectory. Possible hydrodynamic Magnus effects are also discussed.Peer reviewe
Quantum, Stochastic, and Pseudo Stochastic Languages with Few States
Stochastic languages are the languages recognized by probabilistic finite
automata (PFAs) with cutpoint over the field of real numbers. More general
computational models over the same field such as generalized finite automata
(GFAs) and quantum finite automata (QFAs) define the same class. In 1963, Rabin
proved the set of stochastic languages to be uncountable presenting a single
2-state PFA over the binary alphabet recognizing uncountably many languages
depending on the cutpoint. In this paper, we show the same result for unary
stochastic languages. Namely, we exhibit a 2-state unary GFA, a 2-state unary
QFA, and a family of 3-state unary PFAs recognizing uncountably many languages;
all these numbers of states are optimal. After this, we completely characterize
the class of languages recognized by 1-state GFAs, which is the only nontrivial
class of languages recognized by 1-state automata. Finally, we consider the
variations of PFAs, QFAs, and GFAs based on the notion of inclusive/exclusive
cutpoint, and present some results on their expressive power.Comment: A new version with new results. Previous version: Arseny M. Shur,
Abuzer Yakaryilmaz: Quantum, Stochastic, and Pseudo Stochastic Languages with
Few States. UCNC 2014: 327-33
Half-Quantum Vortices in Thin Film of Superfluid He
Stability of a half-quantum vortex (HQV) in superfluid He has been
discussed recently by Kawakami, Tsutsumi and Machida in Phys. Rev. B {\bf 79},
092506 (2009). We further extend this work here and consider the A phase of
superfluid He confined in thin slab geometry and analyze the HQV realized
in this setting. Solutions of HQV and singly quantized singular vortex are
evaluated numerically by solving the Ginzburg-Landau (GL) equation and
respective first critical angular velocities are obtained by employing these
solutions. We show that the HQV in the A phase is stable near the boundary
between the A and A phases. It is found that temperature and magnetic
field must be fixed first in the stable region and subsequently the angular
velocity of the system should be increased from zero to a sufficiently large
value to create a HQV with sufficiently large probability. A HQV does not form
if the system starts with a fixed angular velocity and subsequently the
temperature is lowered down to the A phase. It is estimated that the
external magnetic field with strength on the order of 1 T is required to have a
sufficiently large domain in the temperature-magnetic field phase diagram to
have a stable HQV.Comment: 5 pages, 5 figure
Unconventional Vortices and Phase Transitions in Rapidly Rotating Superfluid ^{3}He
This paper studies vortex-lattice phases of rapidly rotating superfluid ^3He
based on the Ginzburg-Landau free-energy functional. To identify stable phases
in the p-Omega plane (p: pressure; Omega: angular velocity), the functional is
minimized with the Landau-level expansion method using up to 3000 Landau
levels. This system can sustain various exotic vortices by either (i) shifting
vortex cores among different components or (ii) filling in cores with
components not used in the bulk. In addition, the phase near the upper critical
angular velocity Omega_{c2} is neither the A nor B phases, but the polar state
with the smallest superfluid density as already shown by Schopohl. Thus,
multiple phases are anticipated to exist in the p-Omega plane. Six different
phases are found in the present calculation performed over 0.0001 Omega_{c2} <=
Omega <= Omega_{c2}, where Omega_{c2} is of order (1- T/T_c) times 10^{7}
rad/s. It is shown that the double-core vortex experimentally found in the B
phase originates from the conventional hexagonal lattice of the polar state
near Omega_{c2} via (i) a phase composed of interpenetrating polar and
Scharnberg-Klemm sublattices; (ii) the A-phase mixed-twist lattice with polar
cores; (iii) the normal-core lattice found in the isolated-vortex calculation
by Ohmi, Tsuneto, and Fujita; and (iv) the A-phase-core vortex discovered in
another isolated-vortex calculation by Salomaa and Volovik. It is predicted
that the double-core vortex will disappear completely in the experimental p-T
phase diagram to be replaced by the A-phase-core vortex for Omega >~ 10^{3} ~
10^{4} rad/s. C programs to minimize a single-component Ginzburg-Landau
functional are available at {http://phys.sci.hokudai.ac.jp/~kita/index-e.html}.Comment: 13 pages, 9 figure
Fermion zero modes at the boundary of superfluid 3He-B
Superfluid 3He-B belongs to the important special class of time-reversal
invariant topological superfluids. It has Majorana fermions as edge states on
the surface of bulk 3He-B. On the rough wall these fermion zero modes have
finite density of states at E=0. It is possible that Lancaster experiments with
a wire vibrating in 3He-B have already probed Majorana fermions living on the
surface of the wire.Comment: 4 pages, no Figures, JETP Letters style, version to be published in
JETP Letter
Quantum circuits with uniformly controlled one-qubit gates
Uniformly controlled one-qubit gates are quantum gates which can be
represented as direct sums of two-dimensional unitary operators acting on a
single qubit. We present a quantum gate array which implements any n-qubit gate
of this type using at most 2^{n-1} - 1 controlled-NOT gates, 2^{n-1} one-qubit
gates and a single diagonal n-qubit gate. The circuit is based on the so-called
quantum multiplexor, for which we provide a modified construction. We
illustrate the versatility of these gates by applying them to the decomposition
of a general n-qubit gate and a local state preparation procedure. Moreover, we
study their implementation using only nearest-neighbor gates. We give upper
bounds for the one-qubit and controlled-NOT gate counts for all the
aforementioned applications. In all four cases, the proposed circuit topologies
either improve on or achieve the previously reported upper bounds for the gate
counts. Thus, they provide the most efficient method for general gate
decompositions currently known.Comment: 8 pages, 10 figures. v2 has simpler notation and sharpens some
result
Stability of multi-electron bubbles in liquid helium
The stability of multi-electron bubbles in liquid helium is investigated
theoretically. We find that multi-electron bubbles are unstable against fission
whenever the pressure is positive. It is shown that for moving bubbles the
Bernoulli effect can result in a range of pressures over which the bubbles are
stable.Comment: 7 pages, 5 figure
Resonant Tunneling through Multi-Level and Double Quantum Dots
We study resonant tunneling through quantum-dot systems in the presence of
strong Coulomb repulsion and coupling to the metallic leads. Motivated by
recent experiments we concentrate on (i) a single dot with two energy levels
and (ii) a double dot with one level in each dot. Each level is twofold
spin-degenerate. Depending on the level spacing these systems are physical
realizations of different Kondo-type models. Using a real-time diagrammatic
formulation we evaluate the spectral density and the non-linear conductance.
The latter shows a novel triple-peak resonant structure.Comment: 4 pages, ReVTeX, 4 Postscript figure
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