לוח שנה משולב

Current of discrete particles driven through a system with quenched randomness gives rise to fluctuations whose second moment is known as shot noise. The “full counting statistics” is the generating function for the entire spectrum of current cumulants. I will present a general approach to obtaining these quantities, and discuss to what extent an entirely classical calculation can provide this information.

We have shown how to implement protected qubits using some
particular Josephson junction networks. The low energy physics
of these systems is well described by the Kitaev toric code model,
with proper boundary conditions ensuring the two-fold degeneracy of
the ground-state. Using a simple model for the system environment, I
will show that the decoherence times of such qubits are expected
to grow exponentially with the system length, provided the spectral
density of the noise is contained in a frequency interval smaller than
the energy gap of the circuit. I will also describe how to implement
single qubit rotations. A rather good surprise is that, in spite of
the perturbation induced by the manipulation, some features of the
topological protection remain effective. For instance, the rotation
axis is itself protected with high accuracy against the effect of the
environmental noise during the manipulation. A key role in these
analyses is played by the non-local symmetries of such systems.

Recently, 60Fe was found in the Earth crust, which is believed
to have formed in a recent nearby supernova. If the time, distance,
and mass of the progenitor of that supernova would be known,
then one can test and constrain supernova ejecta models.

Knowing the positions, proper motions, and distances of dozens of
young nearby neutron stars (within a few kpc), we can determine
their past flight path and possible kinematic origin. For such
calculations, we have to assume the otherwise unknown radial velocity
through Monte-Carlo simulations. By tracing back its motion, we can
then find the stellar association, in which the neutron star may have
been formed by a recent supernova. If a neutron star seems to have flown
through a nearby young stellar association, where at least one supernova
may have taken place given its current mass function, it may have formed
there. We search for additional indications for such events, like
run-away stars ejected in supernovae in binaries, 26Al emission, etc.
Once the birth place of a neutron star in a supernova is found,
we would have determined the distance of the supernova
and the age of the neutron star (flight time as kinematic age).
If all stars in such an association have formed roughly at the
same time, as assumed by star formation theories and often observed,
we also know the life-time and, hence, mass of the supernova
progenitor star.

In this way, we also try to find the neutron star, which was born
in the nearby recent supernova, which may have ejected the 60Fe
found in the Earth crust. We can then test and calibrate supernova
ejecta models. If 244Pu can be found in the Earth crust with the same
age, too, this would be evidence for the r-process to form 244Pu.
Any identification of a known neutron star with its birth
association (and/or run-away star) would be interesting also to
compare kinematic ages with characteristic ages, to study the
formation and re-heating of the Local Bubble by supernovae,
to constrain neutron star cooling models, etc.

We will present our method and first results.
We will also present briefly other neutron stars projects
at University Jena including X-ray and optical observations
of young nearby isolated neutron stars in order to determine
masses and radii to constrain the equation-of-state.