216 research outputs found
An Electronic Mach-Zehnder Interferometer
Double-slit electron interferometers, fabricated in high mobility
two-dimensional electron gas (2DEG), proved to be very powerful tools in
studying coherent wave-like phenomena in mesoscopic systems. However, they
suffer from small fringe visibility due to the many channels in each slit and
poor sensitivity to small currents due to their open geometry. Moreover, the
interferometers do not function in a high magnetic field, namely, in the
quantum Hall effect (QHE) regime, since it destroys the symmetry between left
and right slits. Here, we report on the fabrication and operation of a novel,
single channel, two-path electron interferometer that functions in a high
magnetic field. It is the first electronic analog of the well-known optical
Mach-Zehnder (MZ) interferometer. Based on single edge state and closed
geometry transport in the QHE regime the interferometer is highly sensitive and
exhibits very high visibility (62%). However, the interference pattern decays
precipitously with increasing electron temperature or energy. While we do not
understand the reason for the dephasing we show, via shot noise measurement,
that it is not a decoherence process that results from inelastic scattering
events.Comment: to appear in Natur
Quantum-Limited Measurement and Information in Mesoscopic Detectors
We formulate general conditions necessary for a linear-response detector to
reach the quantum limit of measurement efficiency, where the
measurement-induced dephasing rate takes on its minimum possible value. These
conditions are applicable to both non-interacting and interacting systems. We
assess the status of these requirements in an arbitrary non-interacting
scattering based detector, identifying the symmetries of the scattering matrix
needed to reach the quantum limit. We show that these conditions are necessary
to prevent the existence of information in the detector which is not extracted
in the measurement process.Comment: 13 pages, 1 figur
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
Continuous weak measurement of quantum coherent oscillations
We consider the problem of continuous quantum measurement of coherent
oscillations between two quantum states of an individual two-state system. It
is shown that the interplay between the information acquisition and the
backaction dephasing of the oscillations by the detector imposes a fundamental
limit, equal to 4, on the signal-to-noise ratio of the measurement. The limit
is universal, e.g., independent of the coupling strength between the detector
and system, and results from the tendency of quantum measurement to localize
the system in one of the measured eigenstates
Effect of quantum entanglement on Aharonov-Bohm oscillations, spin-polarized transport and current magnification effect
We present a simple model of transmission across a metallic mesoscopic ring.
In one of its arm an electron interacts with a single magnetic impurity via an
exchange coupling. We show that entanglement between electron and spin impurity
states leads to reduction of Aharonov-Bohm oscillations in the transmission
coefficient. The spin-conductance is asymmetric in the flux reversal as opposed
to the two probe electrical conductance which is symmetric. In the same model
in contradiction to the naive expectation of a current magnification effect, we
observe enhancement as well as the suppression of this effect depending on the
system parameters. The limitations of this model to the general notion of
dephasing or decoherence in quantum systems are pointed out.Comment: Talk presented at the International Discussion Meeting on Mesoscopic
and Disordered systems, December, 2000, at IISc Bangalore 17 pages, 8figure
Crossover from mesoscopic to universal phase for electron transmission in quantum dots
Measuring phase in coherent electron systems (mesoscopic systems) provides
ample information not easily revealed by conductance measurements. Phase
measurements in relatively large quantum dots (QDs) recently demonstrated a
universal like phase evolution independent of dot size, shape, and occupancy.
Explicitly, in Coulomb blockaded QDs the transmission phase increased
monotonically by pi throughout each conductance peak, thereafter, in the
conductance valleys the phase returned sharply to its base value. Expected
mesoscopic features in the phase, related to spin degeneracy or to exchange
effects, were never observed. Presently, there is no satisfactory full
explanation for the observed phase universality. Unfortunately, the phase in a
few-electron QDs, where it can be better understood was never measured. Here we
report on such measurements on a small QD that occupy only 1-20 electrons. Such
dot was embedded in one arm of a two path electron interferometer, with an
electron counter near the dot. Unlike the repetitive behavior found in larger
dots we found now mesoscopic features for dot occupation of less than some 10
electrons. An unexpected feature in this regime is a clear observation of the
occupation of two different orbital states by the first two electrons -
contrary to the recent publications. As the occupation increased the phase
evolved and turned universal like for some 14 electrons and higher. The present
measurements allowed us to determine level occupancy and parity. More
importantly, they suggest that QDs go through a phase transition, from
mesoscopic to universal like behavior, as the occupancy increases. These
measurements help in singling out potential few theoretical models among the
many proposed.Comment: 12 pages, 6 figure
Noise and Measurement Efficiency of a Partially Coherent Mesoscopic Detector
We study the noise properties and efficiency of a mesoscopic resonant-level
conductor which is used as a quantum detector, in the regime where transport
through the level is only partially phase coherent. We contrast models in which
detector incoherence arises from escape to a voltage probe, versus those in
which it arises from a random time-dependent potential. Particular attention is
paid to the back-action charge noise of the system. While the average detector
current is similar in all models, we find that its noise properties and
measurement efficiency are sensitive both to the degree of coherence and to the
nature of the dephasing source. Detector incoherence prevents quantum limited
detection, except in the non-generic case where the source of dephasing is not
associated with extra unobserved information. This latter case can be realized
in a version of the voltage probe model.Comment: 15 pages, 5 figures; revised dicussion of voltage probe model
Storage Qubits and Their Potential Implementation Through a Semiconductor Double Quantum Dot
In the context of a semiconductor based implementation of a quantum computer
the idea of a quantum storage bit is presented and a possible implementation
using a double quantum dot structure is considered. A measurement scheme using
a stimulated Raman adiabatic passage is discussed.Comment: Revised version accepted for publication in Phys.Rev. B. 19 pages, 4
eps figure
Dephasing and Measurement Efficiency via a Quantum Dot Detector
We study charge detection and controlled dephasing of a mesoscopic system via
a quantum dot detector (QDD), where the mesoscopic system and the QDD are
capacitively coupled. The QDD is considered to have coherent resonant
tunnelling via a single level. It is found that the dephasing rate is
proportional to the square of the conductance of the QDD for the Breit-Wigner
model, showing that the dephasing is completely different from the shot noise
of the detector. The measurement rate, on the other hand, shows a dip near the
resonance. Our findings are peculiar especially for a symmetric detector in the
following aspect: The dephasing rate is maximum at resonance of the QDD where
the detector conductance is insensitive to the charge state of the mesoscopic
system. As a result, the efficiency of the detector shows a dip and vanishes at
resonance, in contrast to the single-channel symmetric non-resonant detector
that has always a maximum efficiency. We find that this difference originates
from a very general property of the scattering matrix: The abrupt phase change
exists in the scattering amplitudes in the presence of the symmetry, which is
insensitive to the detector current but {\em stores} the information of the
quantum state of the mesoscopic system.Comment: 7 pages, 3 figure
Spectrum of qubit oscillations from Bloch equations
We have developed a formalism suitable for calculation of the output spectrum
of a detector continuously measuring quantum coherent oscillations in a
solid-state qubit, starting from microscopic Bloch equations. The results
coincide with that obtained using Bayesian and master equation approaches. The
previous results are generalized to the cases of arbitrary detector response
and finite detector temperature.Comment: 8 page
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