2,548 research outputs found
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
How cities prepare for climate change: Comparing Hamburg and Rotterdam
This paper compares the different ways in which the cities of Hamburg and Rotterdam are taking preemptive action to adapt to climate change. Literature, interviews, secondary data, official statistics, project reports and policy briefs were used to identify institutional arrangements used by the city governments to encourage innovations in climate adaptation strategies and involve the private sector in climate change policy implementation. We focus on cases that create positive opportunities; exploring how innovations are facilitated within the theoretical frameworks of the Porter hypothesis and eco-innovation. We examine two possible pathways of climate change governance, firstly strict regulation and formal enforcement, and secondly institutional eco-innovation and voluntary measures. We found that different emphasis is placed on the preferred pathway in each of the case studies. Hamburg focuses on formal enforcements while the Rotterdam city government encourages institutional eco-innovation by acting as a platform and also providing incentives. Our findings suggest that a well-designed institutional framework can enhance innovation and increase environmental and business performance. The framework could vary in instruments and patterns, using both formal constraints and incentives to increase voluntary actions to shape policy. The formal rules could be stringent or incentivising to shape the climate change measures. The research aims to contribute to both practice and science by providing examples that might motivate and inspire other cities to design appropriate institutions for climate change policy implementation
Superabsorption of light via quantum engineering
Almost 60 years ago Dicke introduced the term superradiance to describe a
signature quantum effect: N atoms can collectively emit light at a rate
proportional to N^2. Even for moderate N this represents a significant increase
over the prediction of classical physics, and the effect has found applications
ranging from probing exciton delocalisation in biological systems, to
developing a new class of laser, and even in astrophysics. Structures that
super-radiate must also have enhanced absorption, but the former always
dominates in natural systems. Here we show that modern quantum control
techniques can overcome this restriction. Our theory establishes that
superabsorption can be achieved and sustained in certain simple nanostructures,
by trapping the system in a highly excited state while extracting energy into a
non-radiative channel. The effect offers the prospect of a new class of quantum
nanotechnology, capable of absorbing light many times faster than is currently
possible; potential applications of this effect include light harvesting and
photon detection. An array of quantum dots or a porphyrin ring could provide an
implementation to demonstrate this effect
Selective spin coupling through a single exciton
We present a novel scheme for performing a conditional phase gate between two
spin qubits in adjacent semiconductor quantum dots through delocalized single
exciton states, formed through the inter-dot Foerster interaction. We consider
two resonant quantum dots, each containing a single excess conduction band
electron whose spin embodies the qubit. We demonstrate that both the two-qubit
gate, and arbitrary single-qubit rotations, may be realized to a high fidelity
with current semiconductor and laser technology.Comment: 5 pages, 3 figures; published version, equation formatting improved,
references adde
Phonon-Induced Rabi-Frequency Renormalization of Optically Driven Single InGaAs/GaAs Quantum Dots
The authors thank the EPSRC (U.K.) EP/G001642, and the QIPIRC U.K. for financial support. A. N. is supported by the EPSRC and B.W. L. by the Royal Society.We study optically driven Rabi rotations of a quantum dot exciton transition between 5 and 50 K, and for pulse areas of up to 14 pi. In a high driving field regime, the decay of the Rabi rotations is nonmonotonic, and the period decreases with pulse area and increases with temperature. By comparing the experiments to a weak-coupling model of the exciton-phonon interaction, we demonstrate that the observed renormalization of the Rabi frequency is induced by fluctuations in the bath of longitudinal acoustic phonons, an effect that is a phonon analogy of the Lamb shift.Peer reviewe
Renormalon Singularities of the QCD Vacuum Polarization Function to Leading Order in
We explicitly determine the residues and orders of all the ultra-violet (UV)
and infra-red (IR) renormalon poles in the Borel plane for the QCD vacuum
polarization function (Adler D-function), to leading order in an expansion in
the number of quark flavours, . The singularity structure is precisely
as anticipated on general grounds. In particular, the leading IR renormalon is
absent, in agreement with operator product expansion ideas. There is a curious
and unexplained symmetry between the third and higher UV and IR renormalon
residues. We are able to sum up separately UV and IR contributions to obtain
closed form results involving -functions. We argue that the leading UV
renormalon should have a more complicated structure than conventionally
assumed. The disappearance of IR renormalons in flavour-saturated SU() QCD
is shown to occur for or 9.Comment: 22 pages of LaTeX, revisions to this paper are mainly typographica
Comparison of Mycobacterium tuberculosis drug susceptibility using solid and liquid culture in Nigeria.
BACKGROUND: This study compares Mycobacterium tuberculosis culture isolation and drug sensitivity testing (DST) using solid (LJ) and liquid (BACTEC-MGIT-960) media in Nigeria. METHODS: This was a cross sectional survey of adults attending reference centres in Abuja, Ibadan and Nnewi with a new diagnosis of pulmonary tuberculosis (TB) or having failed the first-line TB treatment. Patients were requested to provide three sputum specimens for smear-microscopy and culture on LJ and BACTEC-MGIT-960. Positive cultures underwent DST for streptomycin, isoniazid, rifampicin and ethambutol. RESULTS: 527 specimens were cultured. 428 (81%) were positive with BACTEC-MGIT-960, 59 (11%) negative, 36 (7%) contaminated and 4 (1%) had non-tuberculosis mycobacteria (NTM). 411 (78%) LJ cultures were positive, 89 (17%) negative, 22 (4%) contaminated and 5 (1%) had NTM. The mean (SD) detection time was 11 (6) and 30 (11) days for BACTEC-MGIT-960 and LJ. DST patterns were compared in the 389 concordant positive BACTEC-MGIT-960 and LJ cultures. Rifampicin and isoniazid DST patterns were similar. Streptomycin resistance was detected more frequently with LJ than BACTEC-MGIT-960 and ethambutol resistance was detected more frequently with BACTEC-MGIT-960 than LJ, but differences were not statistically significant. MDR-TB was detected in 27 cases by LJ and 25 by BACTEC-MGIT-960 and using both methods detected 29 cases. CONCLUSIONS: There was a substantial degree of agreement between the two methods. However using the two in tandem increased the number of culture-positive patients and those with MDR-TB. The choice of culture method should depend on local availability, cost and test performance characteristics
Process monitoring and fault detection on a hot-melt extrusion process using in-line Raman spectroscopy and a hybrid soft sensor
We propose a real-time process monitoring and fault detection scheme for a pharmaceutical hot-melt extrusion process producing Paracetamol-Affinisol extrudate. The scheme involves prediction of Paracetamol concentration from two independent sources: a hybrid soft sensor and a Raman-based Partial Least Squares (PLS) calibration model. Both these predictions are used by the developed PCA (Principal Component Analysis) and SPC (Statistical Process Control) monitors to detect process faults and raise alarms. Through real-time extrusion results, it is shown that this two-sensor approach enables the detection of various common process faults which would otherwise remain undetected with a single-sensor monitoring scheme
RS-invariant all-orders renormalon resummations for some QCD observables
We propose a renormalon-inspired resummation of QCD perturbation theory based
on approximating the renormalization scheme (RS) invariant effective charge
beta-function coefficients by the portion containing the highest power of
=--, for SU() QCD with quark flavours.
This can be accomplished using exact large- all-orders results. The
resulting resummation is RS-invariant and the exact next-to-leading order (NLO)
and next-to-NLO (NNLO) coefficients in any RS are included. This improves on a
previously employed naive resummation of the leading- piece of the
perturbative coefficients which is RS-dependent, making its comparison with
fixed-order perturbative results ambiguous. The RS-invariant resummation is
used to assess the reliability of fixed-order perturbation theory for the
-ratio, the analogous -lepton decay ratio , and
Deep Inelastic Scattering (DIS) sum rules, by comparing it with the exact NNLO
results in the effective charge RS. For the -ratio and , where
large-order perturbative behaviour is dominated by a leading ultra-violet
renormalon singularity, the comparison indicates fixed-order perturbation
theory to be very reliable. For DIS sum rules, which have a leading infra-red
renormalon singularity, the performance is rather poor. In this way we estimate
that at LEP/SLD energies ideal data on the -ratio could determine
to three-significant figures, and for the we
estimate a theoretical uncertainty
corresponding to . This encouragingly small
uncertainty is much less than has recently been deduced from comparison with
the ambiguous naive resummation.Comment: 25 pages, uses LaTeX, 12 Postscript figures, epsfig.sty 'elsart.sty'
and 'elsart12.sty' are available via anonymous-ftp at
ftp://ftp.tex.ac.uk/tex-archive/macros/latex/contrib/supported/elsevie
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