1,567 research outputs found
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
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
Rapid and robust spin state amplification
Electron and nuclear spins have been employed in many of the early
demonstrations of quantum technology (QT). However applications in real world
QT are limited by the difficulty of measuring single spins. Here we show that
it is possible to rapidly and robustly amplify a spin state using a lattice of
ancillary spins. The model we employ corresponds to an extremely simple
experimental system: a homogenous Ising-coupled spin lattice in one, two or
three dimensions, driven by a continuous microwave field. We establish that the
process can operate at finite temperature (imperfect initial polarisation) and
under the effects of various forms of decoherence.Comment: 5 pages, 2 figure
From Goldilocks to Twin Peaks: multiple optimal regimes for quantum transport in disordered networks
Understanding energy transport in quantum systems is crucial for an
understanding of light-harvesting in nature, and for the creation of new
quantum technologies. Open quantum systems theory has been successfully applied
to predict the existence of environmental noise-assisted quantum transport
(ENAQT) as a widespread phenomenon occurring in biological and artificial
systems. That work has been primarily focused on several `canonical'
structures, from simple chains, rings and crystals of varying dimensions, to
well-studied light-harvesting complexes. Studying those particular systems has
produced specific assumptions about ENAQT, including the notion of a single,
ideal, range of environmental coupling rates that improve energy transport. In
this paper we show that a consistent subset of physically modelled transport
networks can have at least two ENAQT peaks in their steady state transport
efficiency.Comment: Accepted Manuscript, 11 pages, 16 figure
Localisation determines the optimal noise rate for quantum transport
This work was supported by EPSRC Grant No. EP/L015110/1.Environmental noise plays a key role in determining the efficiency of transport in quantum systems. However, disorder and localisation alter the impact of such noise on energy transport. To provide a deeper understanding of this relationship we perform a systematic study of the connection between eigenstate localisation and the optimal dephasing rate in 1D chains. The effects of energy gradients and disorder on chains of various lengths are evaluated and we demonstrate how optimal transport efficiency is determined by both size-independent, as well as size-dependent factors. By discussing how size-dependent influences emerge from finite size effects we establish when these effects are suppressed, and show that a simple power law captures the interplay between size-dependent and size-independent responses. Moving beyond phenomenological pure dephasing, we implement a finite temperature Bloch–Redfield model that captures detailed balance. We show that the relationship between localisation and optimal environmental coupling strength continues to apply at intermediate and high temperature but breaks down in the low temperature limit.Publisher PDFPeer reviewe
Polarization--universal rejection filtering by ambichiral structures made of indefinite dielectric--magnetic materials
An ambichiral structure comprising sheets of an anisotropic dielectric
material rejects normally incident plane waves of one circular polarization
(CP) state but not of the other CP state, in its fundamental Bragg regime.
However, if the same structure is made of an dielectric--magnetic material with
indefinite permittivity and permeability dyadics, it may function as a
polarization--universal rejection filter because two of the four planewave
components of the electromagnetic field phasors in each sheet are of the
positive--phase--velocity type and two are of the negative--phase--velocity
type.Comment: Cleaned citations in the tex
Bath induced coherence and the secular approximation
Finding efficient descriptions of how an environment affects a collection of discrete quantum systems would lead to new insights into many areas of modern physics. Markovian, or time-local, methods work well for individual systems, but for groups a question arises: does system-bath or inter-system coupling dominate the dissipative dynamics? The answer has profound consequences for the long-time quantum correlations within the system. We consider two bosonic modes coupled to a bath. By comparing an exact solution to different Markovian master equations, we find that a smooth crossover of the equations-of-motion between dominant inter-system and system-bath coupling exists - but requires a non-secular master equation. We predict a singular behaviour of the dynamics, and show that the ultimate failure of non-secular equations of motion is essentially a failure of the Markov approximation. Our findings justify the use of time-local theories throughout the crossover between system-bath dominated and inter-system-coupling dominated dynamics.PostprintPeer reviewe
Equilibrium fluctuation theorems compatible with anomalous response
Previously, we have derived a generalization of the canonical fluctuation
relation between heat capacity and energy fluctuations , which is able to describe the existence of macrostates with negative
heat capacities . In this work, we extend our previous results for an
equilibrium situation with several control parameters to account for the
existence of states with anomalous values in other response functions. Our
analysis leads to the derivation of three different equilibrium fluctuation
theorems: the \textit{fundamental and the complementary fluctuation theorems},
which represent the generalization of two fluctuation identities already
obtained in previous works, and the \textit{associated fluctuation theorem}, a
result that has no counterpart in the framework of Boltzmann-Gibbs
distributions. These results are applied to study the anomalous susceptibility
of a ferromagnetic system, in particular, the case of 2D Ising model.Comment: Extended version of the paper published in JSTA
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