13,558 research outputs found
Metastable Quantum Phase Transitions in a One-Dimensional Bose Gas
This is a chapter for a book. The first paragraph of this chapter is as
follows: "Ultracold quantum gases offer a wonderful playground for quantum many
body physics, as experimental systems are widely controllable, both statically
and dynamically. One such system is the one-dimensional (1D) Bose gas on a
ring. In this system binary contact interactions between the constituent
bosonic atoms, usually alkali metals, can be controlled in both sign and
magnitude; a recent experiment has tuned interactions over seven orders of
magnitude, using an atom-molecule resonance called a Feshbach resonance. Thus
one can directly realize the Lieb-Liniger Hamiltonian, from the weakly- to the
strongly-interacting regime. At the same time there are a number of experiments
utilizing ring traps. The ring geometry affords us the opportunity to study
topological properties of this system as well; one of the main properties of a
superfluid is the quantized circulation in which the average angular momentum
per particle, L/N, is quantized under rotation. Thus we focus on a tunable 1D
Bose system for which the main control parameters are interaction and rotation.
We will show that there is a critical boundary in the interaction-rotation
control-parameter plane over which the topological properties of the system
change. This is the basis of our concept of \textit{metastable quantum phase
transitions} (QPTs). Moreover, we will show that the finite domain of the ring
is necessary for the QPT to occur at all because the zero-point kinetic
pressure can induce QPTs, i.e., the system must be finite; we thus seek to
generalize the concept of QPTs to inherently finite, mesoscopic or nanoscopic
systems."Comment: 29 pages, 12 figures, book will appear later this year; v2 is in
improved format and includes small corrections for final versio
Resonant tunneling and Fano resonance in quantum dots with electron-phonon interaction
We theoretically study the resonant tunneling and Fano resonance in quantum
dots with electron-phonon (e-ph) interaction. We examine the bias-voltage ()
dependence of the decoherence, using Keldysh Green function method and
perturbation with respect to the e-ph interaction. With optical phonons of
energy , only the elastic process takes place when , in
which electrons emit and absorb phonons virtually. The process suppresses the
resonant amplitude. When , the inelastic process is possible which
is accompanied by real emission of phonons. It results in the dephasing and
broadens the resonant width. The bias-voltage dependence of the decoherence
cannot be obtained by the canonical transformation method to consider the e-ph
interaction if its effect on the tunnel coupling is neglected. With acoustic
phonons, the asymmetric shape of the Fano resonance grows like a symmetric one
as the bias voltage increases, in qualitative accordance with experimental
results.Comment: 28 pages, 11 figure
Hermitian conjugate measurement
We propose a new class of probabilistic reversing operations on the state of
a system that was disturbed by a weak measurement. It can approximately recover
the original state from the disturbed state especially with an additional
information gain using the Hermitian conjugate of the measurement operator. We
illustrate the general scheme by considering a quantum measurement consisting
of spin systems with an experimentally feasible interaction and show that the
reversing operation simultaneously increases both the fidelity to the original
state and the information gain with such a high probability of success that
their average values increase simultaneously.Comment: 26 pages, 4 figures; a paragraph is added in the introductio
Topological Winding and Unwinding in Metastable Bose-Einstein Condensates
Topological winding and unwinding in a quasi-one-dimensional metastable
Bose-Einstein condensate are shown to be manipulated by changing the strength
of interaction or the frequency of rotation. Exact diagonalization analysis
reveals that quasidegenerate states emerge spontaneously near the transition
point, allowing a smooth crossover between topologically distinct states. On a
mean-field level, the transition is accompanied by formation of grey solitons,
or density notches, which serve as an experimental signature of this
phenomenon.Comment: 4 pages, 3 figure
Reversible quantum measurement with arbitrary spins
We propose a physically reversible quantum measurement of an arbitrary spin-s
system using a spin-j probe via an Ising interaction. In the case of a spin-1/2
system (s=1/2), we explicitly construct a reversing measurement and evaluate
the degree of reversibility in terms of fidelity. The recovery of the measured
state is pronounced when the probe has a high spin (j>1/2), because the
fidelity changes drastically during the reversible measurement and the
reversing measurement. We also show that the reversing measurement scheme for a
spin-1/2 system can serve as an experimentally feasible approximate reversing
measurement for a high-spin system (s>1/2). If the interaction is sufficiently
weak, the reversing measurement can recover a cat state almost
deterministically in spite of there being a large fidelity change.Comment: 35 pages, 11 figures, Sec. 3.2 is adde
Many body effects in finite metallic carbon nanotubes
The non homogeneity of the charge distribution in a carbon nanotube leads to
the formation of an excitonic resonance, in a similar way to the one observed
in X-ray absorption in metals. As a result, a positive anomaly at low bias
appears in the tunnelling density of states. This effect depends on the
screening of the electron--electron interactions by metallic gates, and it
modifies the coupling of the nanotube to normal and superconducting electrodes.Comment: 5 page
Phonon Spectroscopy by Electric Measurements of Coupled Quantum Dots
We propose phonon spectroscopy by electric measurements of the
low-temperature conductance of coupled-quantum dots, specifically employing
dephasing of the quantum electronic transport by the phonons. The setup we
consider consists of a T-shaped double-quantum-dot (DQD) system in which only
one of the dots (dot 1) is connected to external leads and the other (dot 2) is
coupled solely to the first one. For noninteracting electrons, the differential
conductance of such a system vanishes at a voltage located in-between the
energies of the bonding and the anti-bonding states, due to destructive
interference. When electron-phonon (e-ph) on the DQD is invoked, we find that,
at low temperatures, phonon emission taking place on dot 1 does not affect the
interference, while phonon emission from dot 2 suppresses it. The amount of
this suppression, as a function of the bias voltage, follows the effective e-ph
coupling reflecting the phonon density of states and can be used for phonon
spectroscopy.Comment: 9 pages, 6 figure
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