1,627 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
Scheme for direct measurement of a general two-qubit Hamiltonian
The construction of two-qubit gates appropriate for universal quantum
computation is of enormous importance to quantum information processing.
Building such gates is dependent on accurate knowledge of the interaction
dynamics between two qubit systems. This letter will present a systematic
method for reconstructing the full two-qubit interaction Hamiltonian through
experimental measures of concurrence. This not only gives a convenient method
for constructing two qubit quantum gates, but can also be used to
experimentally determine various Hamiltonian parameters in physical systems. We
show explicitly how this method can be employed to determine the first and
second order spin-orbit corrections to the exchange coupling in quantum dots.Comment: 4 Pages, 1 Figur
Sensing of Fluctuating Nanoscale Magnetic Fields Using NV Centres in Diamond
New magnetometry techniques based on Nitrogen-Vacancy (NV) defects in diamond
allow for the imaging of static (DC) and oscillatory (AC) nanoscopic magnetic
systems. However, these techniques require accurate knowledge and control of
the sample dynamics, and are thus limited in their ability to image fields
arising from rapidly fluctuating (FC) environments. We show here that FC fields
place restrictions on the DC field sensitivity of an NV qubit magnetometer, and
that by probing the dephasing rate of the qubit in a magnetic FC environment,
we are able to measure fluctuation rates and RMS field strengths that would be
otherwise inaccessible with the use of DC and AC magnetometry techniques. FC
sensitivities are shown to be comparable to those of AC fields, whilst
requiring no additional experimental overheads or control over the sample.Comment: 5 pages, 4 figure
Single atom-scale diamond defect allows large Aharonov-Casher phase
We propose an experiment that would produce and measure a large
Aharonov-Casher (A-C) phase in a solid-state system under macroscopic motion. A
diamond crystal is mounted on a spinning disk in the presence of a uniform
electric field. Internal magnetic states of a single NV defect, replacing
interferometer trajectories, are coherently controlled by microwave pulses. The
A-C phase shift is manifested as a relative phase, of up to 17 radians, between
components of a superposition of magnetic substates, which is two orders of
magnitude larger than that measured in any other atom-scale quantum system.Comment: 5 pages, 2 figure
Subspace confinement : how good is your qubit?
The basic operating element of standard quantum computation is the qubit, an isolated two-level system that can be accurately controlled, initialized and measured. However, the majority of proposed physical architectures for quantum computation are built from systems that contain much more complicated Hilbert space structures. Hence, defining a qubit requires the identification of an appropriate controllable two-dimensional sub-system. This prompts the obvious question of how well a qubit, thus defined, is confined to this subspace, and whether we can experimentally quantify the potential leakage into states outside the qubit subspace. We demonstrate how subspace leakage can be characterized using minimal theoretical assumptions by examining the Fourier spectrum of the oscillation experiment
Atomistic simulations of adiabatic coherent electron transport in triple donor systems
A solid-state analogue of Stimulated Raman Adiabatic Passage can be
implemented in a triple well solid-state system to coherently transport an
electron across the wells with exponentially suppressed occupation in the
central well at any point of time. Termed coherent tunneling adiabatic passage
(CTAP), this method provides a robust way to transfer quantum information
encoded in the electronic spin across a chain of quantum dots or donors. Using
large scale atomistic tight-binding simulations involving over 3.5 million
atoms, we verify the existence of a CTAP pathway in a realistic solid-state
system: gated triple donors in silicon. Realistic gate profiles from commercial
tools were combined with tight-binding methods to simulate gate control of the
donor to donor tunnel barriers in the presence of cross-talk. As CTAP is an
adiabatic protocol, it can be analyzed by solving the time independent problem
at various stages of the pulse - justifying the use of time-independent
tight-binding methods to this problem. Our results show that a three donor CTAP
transfer, with inter-donor spacing of 15 nm can occur on timescales greater
than 23 ps, well within experimentally accessible regimes. The method not only
provides a tool to guide future CTAP experiments, but also illuminates the
possibility of system engineering to enhance control and transfer times.Comment: 8 pages, 5 figure
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
Gate induced g-factor control and dimensional transition for donors in multi-valley semiconductors
The dependence of the g-factors of semiconductor donors on applied electric
and magnetic fields is of immense importance in spin based quantum computation
and in semiconductor spintronics. The donor g-factor Stark shift is sensitive
to the orientation of the electric and magnetic fields and strongly influenced
by the band-structure and spin-orbit interactions of the host. Using a
multimillion atom tight-binding framework the spin-orbit Stark parameters are
computed for donors in multi-valley semiconductors, silicon and germanium.
Comparison with limited experimental data shows good agreement for a donor in
silicon. Results for gate induced transition from 3D to 2D wave function
confinement show that the corresponding g-factor shift in Si is experimentally
observable.Comment: 4 pages, 4 figure
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