4,977 research outputs found
Quantum Computing with Electron Spins in Quantum Dots
Several topics on the implementation of spin qubits in quantum dots are
reviewed. We first provide an introduction to the standard model of quantum
computing and the basic criteria for its realization. Other alternative
formulations such as measurement-based and adiabatic quantum computing are
briefly discussed. We then focus on spin qubits in single and double GaAs
electron quantum dots and review recent experimental achievements with respect
to initialization, coherent manipulation and readout of the spin states. We
extensively discuss the problem of decoherence in this system, with particular
emphasis on its theoretical treatment and possible ways to overcome it.Comment: Lecture notes for Course CLXXI "Quantum Coherence in Solid State
Systems" Int. School of Physics "Enrico Fermi", Varenna, July 2008, 61 pages,
20 figure
Optimizing power, delay and reliability for digital logic circuits with CMOS and single-electron technologies.
In this thesis, we present two low power approaches with consideration of delay and/or reliability. The first approach is based on CMOS (Complementary Metal-Oxide Semiconductor) technology. Given a gate level topology of digital circuits and a target library, we propose a greedy algorithm for delay budgeting in order to optimize power dissipation. The algorithm is implemented with JAVA SDK. The developed software tool estimates how much power dissipation (percentage) can be saved without increasing the circuit delay, and the potential of power savings by relaxing the circuit\u27s timing constraints. The second low power approach is proposed with SET (Single Electron Tunneling) technology. We focus on an elementary logic structure called threshold gate, and present a standard procedure of logic implementation, with analysis of delay, power and reliability due to background charge effect. As an application example, an FSM (Finite State Machine) for RFID (Radio Frequency Identification) system is designed and simulated successfully.Dept. of Electrical and Computer Engineering. Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis2006 .M5. Source: Masters Abstracts International, Volume: 45-01, page: 0418. Thesis (M.A.Sc.)--University of Windsor (Canada), 2006
Microscopic mechanisms of dephasing due to electron-electron interactions
We develop a non-perturbative numerical method to study tunneling of a single
electron through an Aharonov-Bohm ring where several strongly interacting
electrons are bound. Inelastic processes and spin-flip scattering are taken
into account. The method is applied to study microscopic mechanisms of
dephasing in a non-trivial model. We show that electron-electron interactions
described by the Hubbard Hamiltonian lead to strong dephasing: the transmission
probability at flux is high even at small interaction strength. In
addition to inelastic scattering, we identify two energy conserving mechanisms
of dephasing: symmetry-changing and spin-flip scattering. The many-electron
state on the ring determines which of these mechanisms will be at play:
transmitted current can occur either in elastic or inelastic channels, with or
without changing the spin of the scattering electron.Comment: 11 pages, 16 figures Submitted to Phys. Rev.
Classical information driven quantum dot thermal machines
We analyze the transient response of quantum dot thermal machines that can be
driven by hyperfine interaction acting as a source of classical information.
Our setup comprises a quantum dot coupled to two contacts that drive heat flow
while coupled to a nuclear spin bath. The quantum dot thermal machines operate
both as batteries and as engines, depending on the parameter range. The
electrons in the quantum dot interact with the nuclear spins via hyperfine
spin-flip processes as typically seen in solid state systems such as GaAs
quantum dots. The hyperfine interaction in such systems, which is often treated
as a deterrent for quantum information processing, can favorably be regarded as
a driving agent for classical information flow into a heat engine setup. We
relate this information flow to Landauer's erasure of the nuclear spin bath,
leading to a battery operation. We further demonstrate that the setup can
perform as a transient power source even under a voltage bias across the dot.
Focusing on the transient thermoelectric operation, our analysis clearly
indicates the role of Landauer's erasure to deliver a higher output power than
a conventional quantum dot thermoelectric setup and an efficiency greater than
that of an identical Carnot cycle in steady state, which is consistent with
recently proposed bounds on efficiency for systems subject to a feedback
controller. The role of nuclear spin relaxation processes on these aspects is
also studied. Finally, we introduce the Coulomb interaction in the dot and
analyze the transient thermoelectric response of the system. Our results
elaborate on the effective use of somewhat undesirable scattering processes as
a non-equilibrium source of Shannon information flow in thermal machines and
the possibilities that may arise from the use of a quantum information source.Comment: 10 pages, 7 figure
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
An ab initio path integral Monte Carlo simulation method for molecules and clusters: application to Li_4 and Li_5^+
A novel method for simulating the statistical mechanics of molecular systems
in which both nuclear and electronic degrees of freedom are treated quantum
mechanically is presented. The scheme combines a path integral description of
the nuclear variables with a first-principles adiabatic description of the
electronic structure. The electronic problem is solved for the ground state
within a density functional approach, with the electronic orbitals expanded in
a localized (Gaussian) basis set. The discretized path integral is computed by
a Metropolis Monte Carlo sampling technique on the normal modes of the
isomorphic ring-polymer. An effective short-time action correct to order
is used. The validity and performance of the method are tested in two
small Lithium clusters, namely Li and Li. Structural and electronic
properties computed within this fully quantum-mechanical scheme are presented
and compared to those obtained within the classical nuclei approximation.
Quantum delocalization effects are significant but tunneling turns out to be
irrelevant at low temperatures.Comment: 11 text pages, 7 figures, to be published in J. Chem. Phy
Fast Hole Tunneling Times in Germanium Hut Wires Probed by Single-Shot Reflectometry
Heavy holes confined in quantum dots are predicted to be promising candidates
for the realization of spin qubits with long coherence times. Here we focus on
such heavy-hole states confined in Germanium hut wires. By tuning the growth
density of the latter we can realize a T-like structure between two neighboring
wires. Such a structure allows the realization of a charge sensor, which is
electrostatically and tunnel coupled to a quantum dot, with charge-transfer
signals as high as 0.3e. By integrating the T-like structure into a
radio-frequency reflectometry setup, single-shot measurements allowing the
extraction of hole tunneling times are performed. The extracted tunneling times
of less than 10s are attributed to the small effective mass of Ge
heavy-hole states and pave the way towards projective spin readout
measurements
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