11,805 research outputs found
Measurement-induced entanglement of two superconducting qubits
We study the problem of two superconducting quantum qubits coupled via a
resonator. If only one quanta is present in the system and the number of
photons in the resonator is measured with a null result, the qubits end up in
an entangled Bell state. Here we look at one source of errors in this quantum
nondemolition scheme due to the presence of more than one quanta in the
resonator, previous to the measurement. By analyzing the structure of the
conditional Hamiltonian with arbitrary number of quanta, we show that the
scheme is remarkably robust against these type of errors.Comment: 4 pages, 2 figure
Robust signatures of quantum radiation reaction in focused ultrashort laser pulses
Radiation reaction effects in the interaction of an electron bunch with a
superstrong focused ultrashort laser pulse are investigated in the quantum
radiation dominated regime. The angle-resolved Compton scattering spectra are
calculated in laser pulses of variable duration using a semi-classical
description for the radiation dominated dynamics and a full quantum treatment
for the emitted radiation. In dependence of the laser pulse duration we find
signatures of quantum radiation reaction in the radiation spectra, which are
characteristic for the focused laser beam and visible in the qualitative
behaviour of both the angular spread and the spectral bandwidth of the
radiation spectra. The signatures are robust with respect to the variation of
the electron and laser beam parameters in a large range. They fully differ
qualitatively from those in the classical radiation reaction regime and are
measurable with presently available laser technology
Ion Acceleration by Short Chirped Laser Pulses
Direct laser acceleration of ions by short frequency-chirped laser pulses is
investigated theoretically. We demonstrate that intense beams of ions with a
kinetic energy broadening of about 1 % can be generated. The chirping of the
laser pulse allows the particles to gain kinetic energies of hundreds of MeVs,
which is required for hadron cancer therapy, from pulses of energies of the
order of 100 J. It is shown that few-cycle chirped pulses can accelerate ions
more efficiently than long ones, i.e. higher ion kinetic energies are reached
with the same amount of total electromagnetic pulse energy
Godel Metrics with Chronology Protection in Horndeski Gravities
G\"odel universe, one of the most interesting exact solutions predicted by
General Relativity, describes a homogeneous rotating universe containing naked
closed time-like curves (CTCs). It was shown that such CTCs are the consequence
of the null energy condition in General Relativity. In this paper, we show that
the G\"odel-type metrics with chronology protection can emerge in
Einstein-Horndeski gravity. We construct such exact solutions also in
Einstein-Horndeski-Maxwell and Einstein-Horndeski-Proca theories.Comment: Latex, 11 pages, references adde
Averaged null energy condition in Loop Quantum Cosmology
Wormhole and time machine are very interesting objects in general relativity.
However, they need exotic matters which are impossible in classical level to
support them. But if we introduce the quantum effects of gravity into the
stress-energy tensor, these peculiar objects can be constructed
self-consistently. Fortunately, loop quantum cosmology (LQC) has the potential
to serve as a bridge connecting the classical theory and quantum gravity.
Therefore it provides a simple way for the study of quantum effect in the
semiclassical case. As is well known, loop quantum cosmology is very successful
to deal with the behavior of early universe. In the early stage, if taken the
quantum effect into consideration, inflation is natural because of the
violation of every kind of local energy conditions. Similar to the inflationary
universe, the violation of the averaged null energy condition is the necessary
condition for the traversable wormholes. In this paper, we investigate the
averaged null energy condition in LQC in the framework of effective
Hamiltonian, and find out that LQC do violate the averaged null energy
condition in the massless scalar field coupled model.Comment: 5 page
Electron-Angular-Distribution Reshaping in Quantum Radiation-Dominated Regime
Dynamics of an electron beam head-on colliding with an ultraintense focused
ultrashort circularly-polarized laser pulse are investigated in the quantum
radiation-dominated regime. Generally, the ponderomotive force of the laser
fields may deflect the electrons transversely, to form a ring structure on the
cross-section of the electron beam. However, we find that when the Lorentz
factor of the electron is approximately one order of magnitude larger
than the invariant laser field parameter , the stochastic nature of the
photon emission leads to electron aggregation abnormally inwards to the
propagation axis of the laser pulse. Consequently, the electron angular
distribution after the interaction exhibits a peak structure in the beam
propagation direction, which is apparently distinguished from the
"ring"-structure of the distribution in the classical regime, and therefore,
can be recognized as a proof of the fundamental quantum stochastic nature of
radiation. The stochasticity signature is robust with respect to the laser and
electron parameters and observable with current experimental techniques
Feasibility of electron cyclotron autoresonance acceleration by a short terahertz pulse
A vacuum autoresonance accelerator scheme for electrons, which employs
terahertz radiation and currently available magnetic fields, is suggested.
Based on numerical simulations, parameter values, which could make the scheme
experimentally feasible, are identified and discussed
Attosecond gamma-ray pulses via nonlinear Compton scattering in the radiation dominated regime
The feasibility of generation of bright ultrashort gamma-ray pulses is
demonstrated in the interaction of a relativistic electron bunch with a
counterpropagating tightly-focused superstrong laser beam in the radiation
dominated regime. The Compton scattering spectra of gamma-radiation are
investigated using a semiclassical description for the electron dynamics in the
laser field and a quantum electrodynamical description for the photon emission.
We demonstrate the feasibility of ultrashort gamma-ray bursts of hundreds of
attoseconds and of dozens of megaelectronvolt photon energies in the
near-backwards direction of the initial electron motion. The tightly focused
laser field structure and radiation reaction are shown to be responsible for
such short gamma-ray bursts which are independent of the durations of the
electron bunch and of the laser pulse. The results are measurable with the
laser technology available in a near-future
Ultrarelativistic polarized positron jets via collision of electron and ultraintense laser beams
Relativistic spin-polarized positron beams are indispensable for future
electron-positron colliders to test modern high-energy physics theory with high
precision. However, present techniques require very large scale facilities for
those experiments.
We put forward a novel efficient way for generating ultrarelativistic
polarized positron beams employing currently available laser fields. For this
purpose the generation of polarized positrons via multiphoton Breit-Wheeler
pair production and the associated spin dynamics in single-shot interaction of
an ultraintense laser pulse with an ultrarelativistic electron beam is
investigated in the quantum radiation-dominated regime. A specifically tailored
small ellipticity of the laser field is shown to promote splitting of the
polarized particles along the minor axis of laser polarization into two
oppositely polarized beams. In spite of radiative de-polarization, a dense
positron beam with up to about 90\% polarization can be generated in tens of
femtoseconds. The method may eventually usher high-energy physics studies into
smaller-scale laser laboratories
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