4,488 research outputs found
Photon position measure
The positive operator valued measure (POVM) for a photon counting array
detector is derived and found to equal photon flux density integrated over
pixel area and measurement time. Since photon flux density equals number
density multiplied by the speed of light, this justifies theoretically the
observation that a photon counting array provides a coarse grained measurement
of photon position. The POVM obtained here can be written as a set of
projectors onto a basis of localized states, consistent with the description of
photon position in a recent quantum imaging proposal [M. Tsang, Phys. Rev.
Lett. \textbf{102}, 253601 (2009)]. The wave function that describes a photon
counting experiment is the projection of the photon state vector onto this
localized basis. Collapse is to the electromagnetic vacuum and not to a
localized state, thus violating the text book rules of quantum mechanics but
compatible with the theory of generalized observables and the nonlocalizability
of an incoming photon
Giant anisotropy of Zeeman splitting of quantum confined acceptors in Si/Ge
Shallow acceptor levels in Si/Ge/Si quantum well heterostructures are
characterized by resonant tunneling spectroscopy in the presence of high
magnetic fields. In a perpendicular magnetic field we observe a linear Zeeman
splitting of the acceptor levels. In an in-plane field, on the other hand, the
Zeeman splitting is strongly suppressed. This anisotropic Zeeman splitting is
shown to be a consequence of the huge light hole-heavy hole splitting caused by
a large biaxial strain and a strong quantum confinement in the Ge quantum well.Comment: 5 figures, 4 page
Three-terminal thermoelectric transport through a molecule placed on an Aharonov-Bohm ring
The thermoelectric transport through a ring threaded by an Aharonov-Bohm
flux, with a molecular bridge on one of its arms, is analyzed. The transport
electrons also interact with the vibrational excitations of that molecule. This
nano-system is connected to three terminals: two are electronic reservoirs,
which supply the transport electrons, and the third is the phonon bath which
thermalizes the molecular vibrations. Expressions for the transport
coefficients, relating all charge and heat currents to the temperature and
chemical potential differences between the terminals, are derived to second
order in the electron-vibration coupling. At linear response, all these
coefficients obey the full Onsager-Casimir relations. When the phonon bath is
held at a temperature different from those of the electronic reservoirs, a heat
current exchanged between the molecular vibrations and the transport electrons
can be converted into electric and/or heat electronic currents. The related
transport coefficients, which exist only due to the electron-vibration
coupling, change sign under the interchange between the electronic terminals
and the sign change of the magnetic flux. It is also demonstrated that the
Aharonov-Bohm flux can enhance this type of conversion.Comment: Added clearer kists of the new result
Ultrafast Magnetization Dynamics in Diluted Magnetic Semiconductors
We present a dynamical model that successfully explains the observed time
evolution of the magnetization in diluted magnetic semiconductor quantum wells
after weak laser excitation. Based on the pseudo-fermion formalism and a second
order many-particle expansion of the exact p-d exchange interaction, our
approach goes beyond the usual mean-field approximation. It includes both the
sub-picosecond demagnetization dynamics and the slower relaxation processes
which restore the initial ferromagnetic order in a nanosecond time scale. In
agreement with experimental results, our numerical simulations show that,
depending on the value of the initial lattice temperature, a subsequent
enhancement of the total magnetization may be observed within a time scale of
few hundreds of picoseconds.Comment: Submitted to PR
Observation of inter-edge magnetoplasmon mode in a degenerate two-dimensional electron gas
We study the propagation of edge magnetoplasmons by time-resolved current
measurements in a sample which allows for selective detection of edge states in
the quantum Hall regime. We observe two decoupled modes of edge and inter-edge
magnetoplasmons at filling factors close to 3. From the analysis of the
propagation velocities of each mode the internal spatial parameters of the edge
structure are derived.Comment: 4 pages, 4 figures, submitte
Thermal Effects on Photon-Induced Quantum Transport
We theoretically investigate laser induced quantum transport in a two-level
quantum dot attached to electric contacts. Our approach, based on
nonequilibrium Green function technique, allows to include thermal effects on
the photon-induced quantum transport and excitonic coherent dynamics. By
solving a set of coupled integrodifferential equations, involving correlation
and propagator functions, we obtain the photocurrent and the dot occupations as
a function of time. The characteristic coherent Rabi oscillations are found in
both occupations and photocurrent, with two distinct sources of decoherence:
incoherent tunneling and thermal fluctuations. In particular, for increasing
temperature the dot becomes more thermally occupied which shrinks the amplitude
of the Rabi oscillations, due to Pauli blockade. Finally, due to the interplay
between photon and thermal induced electron populations, the photocurrent can
switch sign as time evolves and its stationary value can be maximized by
tunning the laser intensity.Comment: 5 pages, 4 figure
Fermi Edge Singularities in Transport through Quantum Dots
We study the Fermi-edge singularity appearing in the current-voltage
characteristics for resonant tunneling through a localized level at finite
temperature. An explicit expression for the current at low temperature and near
the threshold for the tunneling process is presented which allows to coalesce
data taken at different temperatures to a single curve. Based on this scaling
function for the current we analyze experimental data from a GaAs-AlAs-GaAs
tunneling device with embedded InAs quantum dots obtained at low temperatures
in high magnetic fields.Comment: 12 pages, 5 figure
Polariton Lasing in a Multilevel Quantum Dot Strongly Coupled To a Single Photon Mode
We present an approximate analytic expression for the photoluminescence
spectral function of a model polariton system, which describes a quantum dot,
with a finite number of fermionic levels, strongly interacting with the lowest
photon mode of a pillar microcavity. Energy eigenvalues and wavefunctions of
the electron-hole-photon system are obtained by numerically diagonalizing the
Hamiltonian. Pumping and photon losses through the cavity mirrors are described
with a master equation, which is solved in order to determine the stationary
density matrix. The photon first-order correlation function, from which the
spectral function is found, is computed with the help of the Quantum Regression
Theorem. The spectral function qualitatively describes the polariton lasing
regime in the model, corresponding to pumping rates two orders of magnitude
lower than those needed for ordinary (photon) lasing. The second-order
coherence functions for the photon and the electron-hole subsystems are
computed as functions of the pumping rate.Comment: version accepted in Phys. Rev.
Numerical time propagation of quantum systems in radiation fields
Atoms, molecules or excitonic quasiparticles, for which excitations are
induced by external radiation fields and energy is dissipated through radiative
decay, are examples of driven open quantum systems. We explain the use of
commutator-free exponential time-propagators for the numerical solution of the
associated Schr\"odinger or master equations with a time-dependent Hamilton
operator. These time-propagators are based on the Magnus series but avoid the
computation of commutators, which makes them suitable for the efficient
propagation of systems with a large number of degrees of freedom. We present an
optimized fourth order propagator and demonstrate its efficiency in comparison
to the direct Runge-Kutta computation. As an illustrative example we consider
the parametrically driven dissipative Dicke model, for which we calculate the
periodic steady state and the optical emission spectrum.Comment: 23 pages, 11 figure
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