3,413 research outputs found
Almost Euclidean sections of the N-dimensional cross-polytope using O(N) random bits
It is well known that R^N has subspaces of dimension proportional to N on
which the \ell_1 norm is equivalent to the \ell_2 norm; however, no explicit
constructions are known. Extending earlier work by Artstein--Avidan and Milman,
we prove that such a subspace can be generated using O(N) random bits.Comment: 16 pages; minor changes in the introduction to make it more
accessible to both Math and CS reader
Exchange coupling between silicon donors: the crucial role of the central cell and mass anisotropy
Donors in silicon are now demonstrated as one of the leading candidates for
implementing qubits and quantum information processing. Single qubit
operations, measurements and long coherence times are firmly established, but
progress on controlling two qubit interactions has been slower. One reason for
this is that the inter donor exchange coupling has been predicted to oscillate
with separation, making it hard to estimate in device designs. We present a
multivalley effective mass theory of a donor pair in silicon, including both a
central cell potential and the effective mass anisotropy intrinsic in the Si
conduction band. We are able to accurately describe the single donor properties
of valley-orbit coupling and the spatial extent of donor wave functions,
highlighting the importance of fitting measured values of hyperfine coupling
and the orbital energy of the levels. Ours is a simple framework that can
be applied flexibly to a range of experimental scenarios, but it is nonetheless
able to provide fast and reliable predictions. We use it to estimate the
exchange coupling between two donor electrons and we find a smoothing of its
expected oscillations, and predict a monotonic dependence on separation if two
donors are spaced precisely along the [100] direction.Comment: Published version. Corrected b and B values from previous versio
A general approach to quantum dynamics using a variational master equation: Application to phonon-damped Rabi rotations in quantum dots
We develop a versatile master equation approach to describe the
non-equilibrium dynamics of a two-level system in contact with a bosonic
environment, which allows for the exploration of a wide range of parameter
regimes within a single formalism. As an experimentally relevant example, we
apply this technique to the study of excitonic Rabi rotations in a driven
quantum dot, and compare its predictions to the numerical Feynman integral
approach. We find excellent agreement between the two methods across a
generally difficult range of parameters. In particular, the variational master
equation technique captures effects usually considered to be non-perturbative,
such as multi-phonon processes and bath-induced driving renormalisation, and
can give reliable results even in regimes in which previous master equation
approaches fail.Comment: 5 pages, 2 figures. Published version, revised title, minor changes
to the tex
Virtual money, virtual control?: electronic money, electronic cash and governance
The modern monetary system is comprised of a number of different types of money, many of which are in forms connected with developing information and communications technologies. This category of money is generally referred to as electronic money. This thesis explores whether these new forms of money are in part responsible for the apparently changing abilities of central banks to govern monetary policy. Lastly, I seek to determine whether theorized trends of money toward electronic cash are likely, and if so, what sort of impact they will have on central banks\u27 monetary policy efficacy
Global nonlinear optimization of spacecraft protective structures design
The global optimization of protective structural designs for spacecraft subject to hypervelocity meteoroid and space debris impacts is presented. This nonlinear problem is first formulated for weight minimization of the space station core module configuration using the Nysmith impact predictor. Next, the equivalence and uniqueness of local and global optima is shown using properties of convexity. This analysis results in a new feasibility condition for this problem. The solution existence is then shown, followed by a comparison of optimization techniques. Finally, a sensitivity analysis is presented to determine the effects of variations in the systemic parameters on optimal design. The results show that global optimization of this problem is unique and may be achieved by a number of methods, provided the feasibility condition is satisfied. Furthermore, module structural design thicknesses and weight increase with increasing projectile velocity and diameter and decrease with increasing separation between bumper and wall for the Nysmith predictor
Surface code architecture for donors and dots in silicon with imprecise and nonuniform qubit couplings
A scaled quantum computer with donor spins in silicon would benefit from a
viable semiconductor framework and a strong inherent decoupling of the qubits
from the noisy environment. Coupling neighbouring spins via the natural
exchange interaction according to current designs requires gate control
structures with extremely small length scales. We present a silicon
architecture where bismuth donors with long coherence times are coupled to
electrons that can shuttle between adjacent quantum dots, thus relaxing the
pitch requirements and allowing space between donors for classical control
devices. An adiabatic SWAP operation within each donor/dot pair solves the
scalability issues intrinsic to exchange-based two-qubit gates, as it does not
rely on sub-nanometer precision in donor placement and is robust against noise
in the control fields. We use this SWAP together with well established global
microwave Rabi pulses and parallel electron shuttling to construct a surface
code that needs minimal, feasible local control.Comment: Published version - more detailed discussions, robustness to
dephasing pointed out additionall
Entanglement distribution for a practical quantum-dot-based quantum processor architecture
We propose a quantum dot (QD) architecture for enabling universal quantum information processing. Quantum registers, consisting of arrays of vertically stacked self-assembled semiconductor QDs, are connected by chains of in-plane self-assembled dots. We propose an entanglement distributor, a device for producing and distributing maximally entangled qubits on demand, communicated through in-plane dot chains. This enables the transmission of entanglement to spatially separated register stacks, providing a resource for the realization of a sizeable quantum processor built from coupled register stacks of practical size. Our entanglement distributor could be integrated into many of the present proposals for self-assembled QD-based quantum computation (QC). Our device exploits the properties of simple, relatively short, spin-chains and does not require microcavities. Utilizing the properties of self-assembled QDs, after distribution the entanglement can be mapped into relatively long-lived spin qubits and purified, providing a flexible, distributed, off-line resource. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft
Freezing distributed entanglement in spin chains
We show how to freeze distributed entanglement that has been created from the
natural dynamics of spin chain systems. The technique that we propose simply
requires single-qubit operations and isolates the entanglement in specific
qubits at the ends of branches. Such frozen entanglement provides a useful
resource, for example for teleportation or distributed quantum processing. The
scheme can be applied to a wide range of systems -- including actual spin
systems and alternative qubit embodiments in strings of quantum dots, molecules
or atoms.Comment: 5 pages, to appear in Phys. Rev. A (Rapid Communication
Creating excitonic entanglement in quantum dots through the optical Stark effect
We show that two initially non-resonant quantum dots may be brought into
resonance by the application of a single detuned laser. This allows for control
of the inter-dot interactions and the generation of highly entangled excitonic
states on the picosecond timescale. Along with arbitrary single qubit
manipulations, this system would be sufficient for the demonstration of a
prototype excitonic quantum computer.Comment: 4 pages, 3 figures; published version, figure 3 improved, corrections
to RWA derive
Optical Quantum Computation with Perpetually Coupled Spins
The possibility of using strongly and continuously interacting spins for
quantum computation has recently been discussed. Here we present a simple
optical scheme that achieves this goal while avoiding the drawbacks of earlier
proposals. We employ a third state, accessed by a classical laser field, to
create an effective barrier to information transfer. The mechanism proves to be
highly efficient both for continuous and pulsed laser modes; moreover it is
very robust, tolerating high decay rates for the excited states. The approach
is applicable to a broad range of systems, in particular dense structures such
as solid state self-assembled (e.g., molecular) devices. Importantly, there are
existing structures upon which `first step' experiments could be immediately
performed.Comment: 5 pages including 3 figures. Updated to published versio
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