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Coupling a single electron on superfluid helium to a superconducting resonator.
Electrons on helium form a unique two-dimensional system on the interface of liquid helium and vacuum. A small number of trapped electrons on helium exhibits strong interactions in the absence of disorder, and can be used as a qubit. Trapped electrons typically have orbital frequencies in the microwave regime and can therefore be integrated with circuit quantum electrodynamics (cQED), which studies light-matter interactions using microwave photons. Here, we experimentally realize a cQED platform with the orbitals of single electrons on helium. We deterministically trap one to four electrons in a dot integrated with a microwave resonator, allowing us to study the electrons' response to microwaves. Furthermore, we find a single-electron-photon coupling strength of [Formula: see text] MHz, greatly exceeding the resonator linewidth [Formula: see text] MHz. These results pave the way towards microwave studies of Wigner molecules and coherent control of the orbital and spin state of a single electron on helium
Phase switching in a voltage-biased Aharonov-Bohm interferometer
Recent experiment [Sigrist et al., Phys. Rev. Lett. {\bf 98}, 036805 (2007)]
reported switches between 0 and in the phase of Aharonov-Bohm
oscillations of the two-terminal differential conductance through a two-dot
ring with increasing voltage bias. Using a simple model, where one of the dots
contains multiple interacting levels, these findings are explained as a result
of transport through the interferometer being dominated at different biases by
quantum dot levels of different "parity" (i.e. the sign of the overlap integral
between the dot state and the states in the leads). The redistribution of
electron population between different levels with bias leads to the fact that
the number of switching events is not necessarily equal to the number of dot
levels, in agreement with experiment. For the same reason switching does not
always imply that the parity of levels is strictly alternating. Lastly, it is
demonstrated that the correlation between the first switching of the phase and
the onset of the inelastic cotunneling, as well as the sharp (rather than
gradual) change of phase when switching occurs, give reason to think that the
present interpretation of the experiment is preferable to the one based on
electrostatic AB effect.Comment: 12 pages, 9 figure
One-dimensional Hubbard model at quarter filling on periodic potentials
Using the Hubbard chain at quarter filling as a model system, we study the
ground state properties of highly doped antiferromagnets. In particular, the
Hubbard chain at quarter filling is unstable against 2k_F- and 4k_F-periodic
potentials, leading to a large variety of charge and spin ordered ground
states. Employing the density matrix renormalization group method, we compare
the energy gain of the ground state induced by different periodic potentials.
For interacting systems the lowest energy is found for a 2k_F-periodic magnetic
field, resulting in a band insulator with spin gap. For strong interaction, the
4k_F-periodic potential leads to a half-filled Heisenberg chain and thus to a
Mott insulating state without spin gap. This ground state is more stable than
the band insulating state caused by any non-magnetic 2k_F-periodic potential.
Adding more electrons, a cluster-like ordering is preferred.Comment: 8 pages, 5 figures, accepted by Phys. Rev.
Stability borders of feedback control of delayed measured systems
When stabilization of unstable periodic orbits or fixed points by the method
given by Ott, Grebogi and Yorke (OGY) has to be based on a measurement delayed
by orbit lengths, the performance of unmodified OGY method is expected
to decline. For experimental considerations, it is desired to know the range of
stability with minimal knowledge of the system. We find that unmodified OGY
control fails beyond a maximal Ljapunov number of
. In this paper the area of stability is
investigated both for OGY control of known fixed points and for difference
control of unknown or inaccurately known fixed points. An estimated value of
the control gain is given. Finally we outline what extensions have to be
considered if one wants to stabilize fixed points with Ljapunov numbers above
.Comment: 5 pages LaTeX using revtex and epsfig (4 figs included). Revised
versio
Tunable multi-photon Rabi oscillations in an electronic spin system
We report on multi-photon Rabi oscillations and controlled tuning of a
multi-level system at room temperature (S=5/2 for Mn2+:MgO) in and out of a
quasi-harmonic level configuration. The anisotropy is much smaller than the
Zeeman splittings, such as the six level scheme shows only a small deviation
from an equidistant diagram. This allows us to tune the spin dynamics by either
compensating the cubic anisotropy with a precise static field orientation, or
by microwave field intensity. Using the rotating frame approximation, the
experiments are very well explained by both an analytical model and a
generalized numerical model. The calculated multi-photon Rabi frequencies are
in excellent agreement with the experimental data
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