265 research outputs found
The theory of coherent dynamic nuclear polarization in quantum dots
We consider the dynamic nuclear spin polarization (DNP) using two electrons
in a double quantum dot in presence of external magnetic field and spin-orbit
interaction, in various schemes of periodically repeated sweeps through the
S-T+ avoided crossing. By treating the problem semi-classically, we find that
generally the DNP have two distinct contributions - a geometrical polarization
and a dynamic polarization, which have different dependence on the control
parameters such as the sweep rates and waiting times in each period. Both terms
show non-trivial dependence on those control parameter. We find that even for
small spin-orbit term, the dynamical polarization dominates the DNP in presence
of a long waiting period near the S-T+ avoided crossing, of the order of the
nuclear Larmor precession periods. A detailed numerical analysis of a specific
control regime can explain the oscillations observed by Foletti et.~al.~in
arXiv:0801.3613.Comment: 22 pages, 6 figure
Entanglement, Dephasing, and Phase Recovery via Cross-Correlation Measurements of Electrons
Determination of the path taken by a quantum particle leads to a suppression
of interference and to a classical behavior. We employ here a quantum 'which
path' detector to perform accurate path determination in a
two-path-electron-interferometer; leading to full suppression of the
interference. Following the dephasing process we recover the interference by
measuring the cross-correlation between the interferometer and detector
currents. Under our measurement conditions every interfering electron is
dephased by approximately a single electron in the detector - leading to mutual
entanglement of approximately single pairs of electrons.Comment: 13 Pages, 5 Figure
Controlled Dephasing of Electrons by Non-Gaussian Shot Noise
In a 'controlled dephasing' experiment [1-3], an interferometer loses its
coherence due to entanglement with a controlled quantum system ('which path'
detector). In experiments that were conducted thus far in mesoscopic systems
only partial dephasing was achieved. This was due to weak interactions between
many detector electrons and the interfering electron, resulting in a Gaussian
phase randomizing process [4-10]. Here, we report the opposite extreme: a
complete destruction of the interference via strong phase randomization only by
a few electrons in the detector. The realization was based on interfering edge
channels (in the integer quantum Hall effect regime, filling factor 2) in a
Mach-Zehnder electronic interferometer, with an inner edge channel serving as a
detector. Unexpectedly, the visibility quenched in a periodic lobe-type form as
the detector current increased; namely, it periodically decreased as the
detector current, and thus the detector's efficiency, increased. Moreover, the
visibility had a V-shape dependence on the partitioning of the detector
current, and not the expected dependence on the second moment of the shot
noise, T(1-T), with T the partitioning. We ascribe these unexpected features to
the strong detector-interferometer coupling, allowing only 1-3 electrons in the
detector to fully dephase the interfering electron. Consequently, in this work
we explored the non-Gaussian nature of noise [11], namely, the direct effect of
the shot noise full counting statistics [12-15].Comment: 14 pages, 4 figure
Entanglement at finite temperatures in the electronic two-particle interferometer
In this work we discuss a theory for entanglement generation,
characterization and detection in fermionic two-particle interferometers at
finite temperature. The motivation for our work is provided by the recent
experiment by the Heiblum group, Neder et al, Nature 448, 333 (2007), realizing
the two particle interferometer proposed by Samuelsson, Sukhorukov, and
Buttiker, Phys. Rev. Lett. 92, 026805 (2004). The experiment displayed a clear
two-particle Aharonov-Bohm effect, however with an amplitude suppressed due to
finite temperature and dephasing. This raised qualitative as well quantitative
questions about entanglement production and detection in mesoscopic conductors
at finite temperature. As a response to these questions, in our recent work,
Samuelsson, Neder, and Buttiker, Phys. Rev. Lett. 102, 106804 (2009) we
presented a general theory for finite temperature entanglement in mesoscopic
conductors. Applied to the two-particle interferometer we showed that the
emitted two-particle state in the experiment was clearly entangled. Moreover,
we demonstrated that the entanglement of the reduced two-particle state,
reconstructed from measurements of average currents and current cross
correlations, constitutes a lower bound to the entanglement of the emitted
state. The present work provides an extended and more detailed discussion of
these findings.Comment: Proceedings of the Nobel Symposium 2009, Qubits for future quantum
computers, May 2009 in Goteborg, Swede
Semi-classical model for the dephasing of a two-electron spin qubit coupled to a coherently evolving nuclear spin bath
We study electron spin decoherence in a two-electron double quantum dot due
to the hyperfine interaction, under spin-echo conditions as studied in recent
experiments. We develop a semi-classical model for the interaction between the
electron and nuclear spins, in which the time-dependent Overhauser fields
induced by the nuclear spins are treated as classical vector variables.
Comparison of the model with experimentally-obtained echo signals allows us to
quantify the contributions of various processes such as coherent Larmor
precession and spin diffusion to the nuclear spin evolution.Comment: 14 Pages, some equations were corrected; Published July 27, 201
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Detecting Non-Abelian Anyons by Charging Spectroscopy
Observation of non-Abelian statistics for the quasiparticles in the fractional quantum Hall state remains an outstanding experimental problem. The non-Abelian statistics are linked to the presence of additional low energy states in a system with localized quasiparticles, and, hence, an additional low temperature entropy. Recent experiments, which detect changes in the number of quasiparticles trapped in a local potential well as a function of an applied gate voltage, VG, provide a possibility for measuring this entropy, if carried out over a suitable range of temperatures, T. We present a microscopic model for quasiparticles in a potential well and study the effects of non-Abelian statistics on the charge stability diagram in the VG−T plane, including broadening at finite temperature. We predict a measurable slope for the first quasiparticle charging line and an even-odd effect in the diagram, which is a signature of non-Abelian statistics.Engineering and Applied SciencesPhysic
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