1,409 research outputs found
Analytic Solution for the Ground State Energy of the Extensive Many-Body Problem
A closed form expression for the ground state energy density of the general
extensive many-body problem is given in terms of the Lanczos tri-diagonal form
of the Hamiltonian. Given the general expressions of the diagonal and
off-diagonal elements of the Hamiltonian Lanczos matrix, and
, asymptotic forms and can be defined in
terms of a new parameter ( is the Lanczos iteration and is
the size of the system). By application of theorems on the zeros of orthogonal
polynomials we find the ground-state energy density in the bulk limit to be
given in general by .Comment: 10 pages REVTex3.0, 3 PS figure
Towards visualisation of central-cell-effects in scanning-tunnelling-microscope images of subsurface dopant qubits in silicon
Atomic-scale understanding of phosphorous donor wave functions underpins the
design and optimisation of silicon based quantum devices. The accuracy of
large-scale theoretical methods to compute donor wave functions is dependent on
descriptions of central-cell-corrections, which are empirically fitted to match
experimental binding energies, or other quantities associated with the global
properties of the wave function. Direct approaches to understanding such
effects in donor wave functions are of great interest. Here, we apply a
comprehensive atomistic theoretical framework to compute scanning tunnelling
microscopy (STM) images of subsurface donor wave functions with two
central-cell-correction formalisms previously employed in the literature. The
comparison between central-cell models based on real-space image features and
the Fourier transform profiles indicate that the central-cell effects are
visible in the simulated STM images up to ten monolayers below the silicon
surface. Our study motivates a future experimental investigation of the
central-cell effects via STM imaging technique with potential of fine tuning
theoretical models, which could play a vital role in the design of donor-based
quantum systems in scalable quantum computer architectures.Comment: Nanoscale 201
Measurable quantum geometric phase from a rotating single spin
We demonstrate that the internal magnetic states of a single nitrogen-vacancy
defect, within a rotating diamond crystal, acquire geometric phases. The
geometric phase shift is manifest as a relative phase between components of a
superposition of magnetic substates. We demonstrate that under reasonable
experimental conditions a phase shift of up to four radians could be measured.
Such a measurement of the accumulation of a geometric phase, due to macroscopic
rotation, would be the first for a single atom-scale quantum system.Comment: 5 pages, 2 figures: Accepted for publication in Physical Review
Letter
Effects of J-gate potential and interfaces on donor exchange coupling in the Kane quantum computer architecture
We calculate the electron exchange coupling for a phosphorus donor pair in
silicon perturbed by a J-gate potential and the boundary effects of the silicon
host geometry. In addition to the electron-electron exchange interaction we
also calculate the contact hyperfine interaction between the donor nucleus and
electron as a function of the varying experimental conditions. Donor
separation, depth of the P nuclei below the silicon oxide layer and J-gate
voltage become decisive factors in determining the strength of both the
exchange coupling and the hyperfine interaction - both crucial components for
qubit operations in the Kane quantum computer. These calculations were
performed using an anisotropic effective-mass Hamiltonian approach. The
behaviour of the donor exchange coupling as a function of the device parameters
varied provides relevant information for the experimental design of these
devices.Comment: 15 pages, 15 figures. Accepted for Journal of Physics: Condensed
Matte
Valley filtering and spatial maps of coupling between silicon donors and quantum dots
Exchange coupling is a key ingredient for spin-based quantum technologies
since it can be used to entangle spin qubits and create logical spin qubits.
However, the influence of the electronic valley degree of freedom in silicon on
exchange interactions is presently the subject of important open questions.
Here we investigate the influence of valleys on exchange in a coupled
donor/quantum dot system, a basic building block of recently proposed schemes
for robust quantum information processing. Using a scanning tunneling
microscope tip to position the quantum dot with sub-nm precision, we find a
near monotonic exchange characteristic where lattice-aperiodic modulations
associated with valley degrees of freedom comprise less than 2~\% of exchange.
From this we conclude that intravalley tunneling processes that preserve the
donor's and valley index are filtered out of the interaction
with the valley quantum dot, and that the and
intervalley processes where the electron valley index changes are weak.
Complemented by tight-binding calculations of exchange versus donor depth, the
demonstrated electrostatic tunability of donor/QD exchange can be used to
compensate the remaining intravalley oscillations to realise uniform
interactions in an array of highly coherent donor spins.Comment: 6 pages, 4 figures, 6 pages Supplemental Materia
A highly efficient two level diamond based single photon source
An unexplored diamond defect centre which is found to emit stable single
photons at a measured rate of 1.6 MHz at room temperature is reported. The
novel centre, identified in chemical vapour deposition grown diamond crystals,
exhibits a sharp zero phonon line at 734 nm with a full width at half maximum
of ~ 4 nm. The photon statistics confirm the center is a single emitter and
provides direct evidence of the first true two-level single quantum system in
diamond.Comment: 3 pages, 4 figure
Voltage Control of Exchange Coupling in Phosphorus Doped Silicon
Motivated by applications to quantum computer architectures we study the
change in the exchange interaction between neighbouring phosphorus donor
electrons in silicon due to the application of voltage biases to surface
control electrodes. These voltage biases create electro-static fields within
the crystal substrate, perturbing the states of the donor electrons and thus
altering the strength of the exchange interaction between them. We find that
control gates of this kind can be used to either enhance, or reduce the
strength of the interaction, by an amount that depends both on the magnitude
and orientation of the donor separation.Comment: 5 Pages, 5 Figure
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