1,110 research outputs found
Quantum Computation and Spin Electronics
In this chapter we explore the connection between mesoscopic physics and
quantum computing. After giving a bibliography providing a general introduction
to the subject of quantum information processing, we review the various
approaches that are being considered for the experimental implementation of
quantum computing and quantum communication in atomic physics, quantum optics,
nuclear magnetic resonance, superconductivity, and, especially, normal-electron
solid state physics. We discuss five criteria for the realization of a quantum
computer and consider the implications that these criteria have for quantum
computation using the spin states of single-electron quantum dots. Finally, we
consider the transport of quantum information via the motion of individual
electrons in mesoscopic structures; specific transport and noise measurements
in coupled quantum dot geometries for detecting and characterizing
electron-state entanglement are analyzed.Comment: 28 pages RevTeX, 4 figures. To be published in "Quantum Mesoscopic
Phenomena and Mesoscopic Devices in Microelectronics," eds. I. O. Kulik and
R. Ellialtioglu (NATO Advanced Study Institute, Turkey, June 13-25, 1999
Direct access to quantum fluctuations through cross-correlation measurements
Detection of the quantum fluctuations by conventional methods meets certain
obstacles, since it requires high frequency measurements. Moreover, quantum
fluctuations are normally dominated by classical noise, and are usually further
obstructed by various accompanying effects such as a detector backaction. In
present work, we demonstrate that these difficulties can be bypassed by
performing the cross-correlation measurements. We propose to use a pair of
two-level detectors, weakly coupled to a collective mode of an electric
circuit. Fluctuations of the current source accumulated in the collective mode
induce stochastic transitions in the detectors. These transitions are then read
off by quantum point contact (QPC) electrometers and translated into two
telegraph processes in the QPC currents. Since both detectors interact with the
same collective mode, this leads to a certain fraction of the correlated
transitions. These correlated transitions are fingerprinted in the
cross-correlations of the telegraph processes, which can be detected at zero
frequency, i.e., with a long time measurements. Concerning the dependance of
the cross-correlator on the detectors' energy splittings, the most interesting
region is at the degeneracy points, where it exhibits a sharp non-local
resonance, that stems from higher order processes. We find that at certain
conditions the main contribution to this resonance comes from the quantum
noise. Namely, while the resonance line shape is weakly broadened by the
classical noise, the height of the peak is directly proportional to the square
of the quantum component of the noise spectral function.Comment: Added discussion of the time scales in the introduction and one
figure. 14 pages, 8 figure
Detection of non-Gaussian Fluctuations in a Quantum Point Contact
An experimental study of current fluctuations through a tunable transmission
barrier, a quantum point contact, are reported. We measure the probability
distribution function of transmitted charge with precision sufficient to
extract the first three cumulants. To obtain the intrinsic quantities,
corresponding to voltage-biased barrier, we employ a procedure that accounts
for the response of the external circuit and the amplifier. The third cumulant,
obtained with a high precision, is found to agree with the prediction for the
statistics of transport in the non-Poissonian regime.Comment: 4 pages, 4 figures; published versio
Young's modulus of polyelectrolyte multilayers from microcapsule swelling
We measure Young's modulus of a free polyelectrolyte multilayer film by
studying osmotically induced swelling of polyelectrolyte multilayer
microcapsules filled with the polyelectrolyte solution. Different filling
techniques and core templates were used for the capsule preparation. Varying
the concentration of the polyelectrolyte inside the capsule, its radius and the
shell thickness yielded an estimate of an upper limit for Young's modulus of
the order of 100 MPa. This corresponds to an elastomer and reflects strong
interactions between polyanions and polycations in the multilayer
Mesoscopic threshold detectors: Telegraphing the size of a fluctuation
We propose a two-terminal method to measure shot noise in mesoscopic systems
based on an instability in the current-voltage characteristic of an on-chip
detector. The microscopic noise drives the instability, which leads to random
switching of the current between two values, the telegraph process. In the
Gaussian regime, the shot noise power driving the instability may be extracted
from the I-V curve, with the noise power as a fitting parameter. In the
threshold regime, the extreme value statistics of the mesoscopic conductor can
be extracted from the switching rates, which reorganize the complete
information about the current statistics in an indirect way, "telegraphing" the
size of a fluctuation. We propose the use of a quantum double dot as a
mesoscopic threshold detector.Comment: 9 pages, 7 figures, published versio
Landau-Zener Tunnelling in Waveguide Arrays
Landau-Zener tunnelling is discussed in connection with optical waveguide
arrays. Light injected in a specific band of the Bloch spectrum in the
propagation constant can be transmitted to another band, changing its physical
properties. This is achieved using two waveguide arrays with different
refractive indices, which amounts to consider a Schr\"odinger equation in a
periodic potential with a step. The step causes wave "acceleration" and thus
induces Landau-Zener tunnelling. The region of physical parameters where this
phenomenon can occur is analytically determined and a realistic experimental
setup is suggested. Its application could allow the realization of light
filters.Comment: 4 pages, 6 figure
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