1,507 research outputs found
Qubit-oscillator system under ultrastrong coupling and extreme driving
We introduce an approach to studying a driven qubit-oscillator system in the
ultrastrong coupling regime, where the ratio between coupling
strength and oscillator frequency approaches unity or goes beyond, and
simultaneously for driving strengths much bigger than the qubit energy
splitting (extreme driving). Both qubit-oscillator coupling and external
driving lead to a dressing of the qubit tunneling matrix element of different
nature: the former can be used to suppress selectively certain oscillator modes
in the spectrum, while the latter can bring the qubit's dynamics to a
standstill at short times (coherent destruction of tunneling) even in the case
of ultrastrong coupling.Comment: 4+ pages, 5 figures (published version
Decoherence in quantum dots due to real and virtual transitions: a non-perturbative calculation
We investigate theoretically acoustic phonon induced decoherence in quantum
dots. We calculate the dephasing of fundamental (interband or intraband)
optical transitions due to real and virtual transitions with higher energy
levels. Up to two acoustic phonon processes (absorption and/or emission) are
taken into account simultaneously in a non-perturbative manner. An analytic
expression of acoustic phonon induced broadening is given as a function of the
electron-phonon matrix elements and is physically interpreted. The theory is
applied to the dephasing of intersublevel transitions in self-assembled quantum
dots.Comment: 8 pages, 4 figure
Coherent pumping of a Mott insulator: Fermi golden rule versus Rabi oscillations
Cold atoms provide a unique arena to study many-body systems far from
equilibrium. Furthermore, novel phases in cold atom systems are conveniently
investigated by dynamical probes pushing the system out of equilibrium. Here,
we discuss the pumping of doubly-occupied sites in a fermionic Mott insulator
by a periodic modulation of the hopping amplitude. We show that deep in the
insulating phase the many-body system can be mapped onto an effective two-level
system which performs coherent Rabi oscillations due to the driving. Coupling
the two-level system to the remaining degrees of freedom renders the Rabi
oscillations damped. We compare this scheme to an alternative description where
the particles are incoherently pumped into a broad continuum.Comment: 4 pages, 3 figure
Direct observation of quantum phonon fluctuations in a one dimensional Bose gas
We report the first direct observation of collective quantum fluctuations in
a continuous field. Shot-to-shot atom number fluctuations in small sub-volumes
of a weakly interacting ultracold atomic 1D cloud are studied using \textit{in
situ} absorption imaging and statistical analysis of the density profiles. In
the cloud centers, well in the \textit{quantum quasicondensate} regime, the
ratio of chemical potential to thermal energy is , and,
owing to high resolution, up to 20% of the microscopically observed
fluctuations are quantum phonons. Within a non-local analysis at variable
observation length, we observe a clear deviation from a classical field
prediction, which reveals the emergence of dominant quantum fluctuations at
short length scales, as the thermodynamic limit breaks down.Comment: 4 pages, 3 figures (Supplementary material 3 pages, 3 figures
Noncovariant gauge fixing in the quantum Dirac field theory of atoms and molecules
Starting from the Weyl gauge formulation of quantum electrodynamics (QED),
the formalism of quantum-mechanical gauge fixing is extended using techniques
from nonrelativistic QED. This involves expressing the redundant gauge degrees
of freedom through an arbitrary functional of the gauge-invariant transverse
degrees of freedom. Particular choices of functional can be made to yield the
Coulomb gauge and Poincar\'{e} gauge representations. The Hamiltonian we derive
therefore serves as a good starting point for the description of atoms and
molecules by means of a relativistic Dirac field. We discuss important
implications for the ontology of noncovariant canonical QED due to the gauge
freedom that remains present in our formulation.Comment: 8 pages, 0 figure
Low-decoherence flux qubit
A flux qubit can have a relatively long decoherence time at the degeneracy
point, but away from this point the decoherence time is greatly reduced by
dephasing. This limits the practical applications of flux qubits. Here we
propose a new qubit design modified from the commonly used flux qubit by
introducing an additional capacitor shunted in parallel to the smaller
Josephson junction (JJ) in the loop. Our results show that the effects of noise
can be considerably suppressed, particularly away from the degeneracy point, by
both reducing the coupling energy of the JJ and increasing the shunt
capacitance. This shunt capacitance provides a novel way to improve the qubit.Comment: 4 pages, 4 figure
Effect of random interactions in spin baths on decoherence
We study the decoherence of a central spin 1/2 induced by a spin bath with
intrabath interactions. Since we are interested in the cumulative effect of
interaction and disorder, we study baths comprising Ising spins with random
ferro- and antiferromagnetic interactions between the spins. Using the
resolvent operator method which goes beyond the standard Born-Markov master
equation approach, we show that, in the weak coupling regime, the decoherence
of the central spin at all times is entirely determined by the local-field
distribution or equivalently, the dynamical structure factor of the Ising bath.
We present analytic results for the Ising spin chain bath at arbitrary
temperature for different distributions of the intrabath interaction strengths.
We find clear evidence of non-Markovian behavior in the low temperature regime.
We also consider baths described by Ising models on higher-dimensional
lattices. We find that interactions lead to a significant reduction of the
decoherence. An important feature of interacting spinbaths is the saturation of
the asymptotic Markovian decay rate at high temperatures, as opposed to the
conventional Ohmic boson bath.Comment: 13 page
Emission spectrum of a dressed exciton-biexciton complex in a semiconductor quantum dot
The photoluminescence spectrum of a single quantum dot was recorded as a
secondary resonant laser optically dressed either the vacuum-to-exciton or the
exciton-to-biexciton transitions. High-resolution polarization-resolved
measurements using a scanning Fabry-Perot interferometer reveal splittings of
the linearly-polarized fine-structure states that are non-degenerate in an
asymmetric quantum dot. These splittings manifest as either triplets or
doublets and depend sensitively on laser intensity and detuning. Our approach
realizes complete resonant control of a multi-excitonic system in emission,
which can be either pulsed or continuous-wave, and offers direct access to the
emitted photons.Comment: 4 pages, 4 figure
Temperature can enhance coherent oscillations at a Landau-Zener transition
We consider sweeping a system through a Landau-Zener avoided-crossing, when
that system is also coupled to an environment or noise. Unsurprisingly, we find
that decoherence suppresses the coherent oscillations of quantum superpositions
of system states, as superpositions decohere into mixed states. However, we
also find an effect we call "Lamb-assisted coherent oscillations", in which a
Lamb shift exponentially enhances the coherent oscillation amplitude. This
dominates for high-frequency environments such as super-Ohmic environments,
where the coherent oscillations can grow exponentially as either the
environment coupling or temperature are increased. The effect could be used as
an experimental probe for high-frequency environments in such systems as
molecular magnets, solid-state qubits, spin-polarized gases (neutrons or He3)
or Bose-condensates.Comment: 4 Pages & 4 Figs - New version: introduction extended & citations
adde
State-dependent rotations of spins by weak measurements
IIt is shown that a weak measurement of a quantum system produces a new state
of the quantum system which depends on the prior state, as well as the
(uncontrollable) measured position of the pointer variable of the weak
measurement apparatus. The result imposes a constraint on hidden-variable
theories which assign a different state to a quantum system than standard
quantum mechanics. The constraint means that a crypto-nonlocal hidden-variable
theory can be ruled out in a more direct way than previously.Comment: 10 pages, 2 figures. Substantially revised to concentrate on weak
measurement transformation of states and application to crypto-nonlocal
hidden-variable theor
- …