1,271 research outputs found
Lasing in circuit quantum electrodynamics with strong noise
We study a model which can describe a superconducting single electron
transistor (SSET) or a double quantum dot coupled to transmission-line
oscillator. In both cases the degree of freedom is given by a charged particle,
which couples strongly to the electromagnetic environment or phonons. We
consider the case where a lasing condition is established and study the
dependence of the average photon number in the resonator on the spectral
function of the electromagnetic environment. We focus on three important cases:
a strongly coupled environment with a small cut-off frequency, a structured
environment peaked at a specific frequency and 1/f-noise. We find that the
electromagnetic environment can have a substantial impact on the photon
creation. Resonance peaks are in general broadened and additional resonances
can appear
Full counting statistics for transport through a molecular quantum dot magnet
Full counting statistics (FCS) for the transport through a molecular quantum
dot magnet is studied theoretically in the incoherent tunneling regime. We
consider a model describing a single-level quantum dot, magnetically coupled to
an additional local spin, the latter representing the total molecular spin s.
We also assume that the system is in the strong Coulomb blockade regime, i.e.,
double occupancy on the dot is forbidden. The master equation approach to FCS
introduced in Ref. [12] is applied to derive a generating function yielding the
FCS of charge and current. In the master equation approach, Clebsch-Gordan
coefficients appear in the transition probabilities, whereas the derivation of
generating function reduces to solving the eigenvalue problem of a modified
master equation with counting fields. To be more specific, one needs only the
eigenstate which collapses smoothly to the zero-eigenvalue stationary state in
the limit of vanishing counting fields. We discovered that in our problem with
arbitrary spin s, some quartic relations among Clebsch-Gordan coefficients
allow us to identify the desired eigenspace without solving the whole problem.
Thus we find analytically the FCS generating function in the following two
cases: i) both spin sectors lying in the bias window, ii) only one of such spin
sectors lying in the bias window. Based on the obtained analytic expressions,
we also developed a numerical analysis in order to perform a similar
contour-plot of the joint charge-current distribution function, which have
recently been introduced in Ref. [13], here in the case of molecular quantum
dot magnet problem.Comment: 17 pages, 5 figure
Kondo effect in quantum dots coupled to ferromagnetic leads
We study the Kondo effect in a quantum dot which is coupled to ferromagnetic
leads and analyse its properties as a function of the spin polarization of the
leads. Based on a scaling approach we predict that for parallel alignment of
the magnetizations in the leads the strong-coupling limit of the Kondo effect
is reached at a finite value of the magnetic field. Using an equation-of-motion
technique we study nonlinear transport through the dot. For parallel alignment
the zero-bias anomaly may be split even in the absence of an external magnetic
field. For antiparallel spin alignment and symmetric coupling, the peak is
split only in the presence of a magnetic field, but shows a characteristic
asymmetry in amplitude and position.Comment: 5 pages, 2 figure
Fluctuation Theorem in a Quantum-Dot Aharonov-Bohm Interferometer
In the present study, we investigate the full counting statistics in a
two-terminal Aharonov-Bohm interferometer embedded with an interacting quantum
dot. We introduce a novel saddle-point solution for a cumulant-generating
function, which satisfies the fluctuation theorem and accounts for the
interaction in the mean-field level approximation. Nonlinear transport
coefficients satisfy universal relations imposed by microscopic reversibility,
though the scattering matrix itself is not reversible. The skewness can be
finite even in equilibrium, owing to the interaction and is proportional to the
asymmetric component of nonlinear conductance.Comment: 5 pages, 2 figure
On-line Learning of an Unlearnable True Teacher through Mobile Ensemble Teachers
On-line learning of a hierarchical learning model is studied by a method from
statistical mechanics. In our model a student of a simple perceptron learns
from not a true teacher directly, but ensemble teachers who learn from the true
teacher with a perceptron learning rule. Since the true teacher and the
ensemble teachers are expressed as non-monotonic perceptron and simple ones,
respectively, the ensemble teachers go around the unlearnable true teacher with
the distance between them fixed in an asymptotic steady state. The
generalization performance of the student is shown to exceed that of the
ensemble teachers in a transient state, as was shown in similar
ensemble-teachers models. Further, it is found that moving the ensemble
teachers even in the steady state, in contrast to the fixed ensemble teachers,
is efficient for the performance of the student.Comment: 18 pages, 8 figure
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