85 research outputs found
Simplified Quantum Process Tomography
We propose and evaluate experimentally an approach to quantum process
tomography that completely removes the scaling problem plaguing the standard
approach. The key to this simplification is the incorporation of prior
knowledge of the class of physical interactions involved in generating the
dynamics, which reduces the problem to one of parameter estimation. This allows
part of the problem to be tackled using efficient convex methods, which, when
coupled with a constraint on some parameters allows globally optimal estimates
for the Kraus operators to be determined from experimental data. Parameterising
the maps provides further advantages: it allows the incorporation of mixed
states of the environment as well as some initial correlation between the
system and environment, both of which are common physical situations following
excitation of the system away from thermal equilibrium. Although the approach
is not universal, in cases where it is valid it returns a complete set of
positive maps for the dynamical evolution of a quantum system at all times.Comment: Added references to interesting related work by Bendersky et a
Van bijen en eendenkroos tot parasiet
Om de groeiende wereldbevolking duurzaam te voeden, werft de campagne Food for Thought, Thought for Food geld voor baanbrekend onderzoek. Daarvoor zijn twaalf nieuwe projecten geselecteerd
Ontwerpen aan klimaatadaptatie : plannen voor het ondenkbare
In Nederland zal het natter worden ten gevolge van de klimaatverandering. Meer regens, meer smeltwater in rivieren en een stijging van de zeespiegel zorgen voor een groter risico op overstromingen. Daarmee wordt klimaatverandering een niet weg te denken element in waterbeheer, kustverdediging, stedelijke ontwikkeling en natuurbeheer. Ieder ontwerp voor stedelijke of gebiedsontwikkeling zal rekening moeten houden met de mogelijke gevolgen van de opwarming van de aarde, of met een mooi woord âklimaatbestendigâ moeten zijn. De vraag hoe je klimaatbestendige landschappen kunt ontwerpen, speelt een grote rol binnen Europese projecten waar de groene hogeschool aan heeft deelgenomen en deelneemt
Use of tetravalent galabiose for inhibition of Streptococcus suis serotype 2 infection in a mouse model
Peer reviewe
Encoding a qubit into multilevel subspaces
We present a formalism for encoding the logical basis of a qubit into
subspaces of multiple physical levels. The need for this multilevel encoding
arises naturally in situations where the speed of quantum operations exceeds
the limits imposed by the addressability of individual energy levels of the
qubit physical system. A basic feature of the multilevel encoding formalism is
the logical equivalence of different physical states and correspondingly, of
different physical transformations. This logical equivalence is a source of a
significant flexibility in designing logical operations, while the multilevel
structure inherently accommodates fast and intense broadband controls thereby
facilitating faster quantum operations. Another important practical advantage
of multilevel encoding is the ability to maintain full quantum-computational
fidelity in the presence of mixing and decoherence within encoding subspaces.
The formalism is developed in detail for single-qubit operations and
generalized for multiple qubits. As an illustrative example, we perform a
simulation of closed-loop optimal control of single-qubit operations for a
model multilevel system, and subsequently apply these operations at finite
temperatures to investigate the effect of decoherence on operational fidelity.Comment: IOPart LaTeX, 2 figures, 31 pages; addition of a numerical simulatio
Optimal quantum multi-parameter estimation and application to dipole- and exchange-coupled qubits
We consider the problem of quantum multi-parameter estimation with
experimental constraints and formulate the solution in terms of a convex
optimization. Specifically, we outline an efficient method to identify the
optimal strategy for estimating multiple unknown parameters of a quantum
process and apply this method to a realistic example. The example is two
electron spin qubits coupled through the dipole and exchange interactions with
unknown coupling parameters -- explicitly, the position vector relating the two
qubits and the magnitude of the exchange interaction are unknown. This coupling
Hamiltonian generates a unitary evolution which, when combined with arbitrary
single-qubit operations, produces a universal set of quantum gates. However,
the unknown parameters must be known precisely to generate high-fidelity gates.
We use the Cram\'er-Rao bound on the variance of a point estimator to construct
the optimal series of experiments to estimate these free parameters, and
present a complete analysis of the optimal experimental configuration. Our
method of transforming the constrained optimal parameter estimation problem
into a convex optimization is powerful and widely applicable to other systems.Comment: 13 pages. Published versio
Robust Online Hamiltonian Learning
In this work we combine two distinct machine learning methodologies,
sequential Monte Carlo and Bayesian experimental design, and apply them to the
problem of inferring the dynamical parameters of a quantum system. We design
the algorithm with practicality in mind by including parameters that control
trade-offs between the requirements on computational and experimental
resources. The algorithm can be implemented online (during experimental data
collection), avoiding the need for storage and post-processing. Most
importantly, our algorithm is capable of learning Hamiltonian parameters even
when the parameters change from experiment-to-experiment, and also when
additional noise processes are present and unknown. The algorithm also
numerically estimates the Cramer-Rao lower bound, certifying its own
performance.Comment: 24 pages, 12 figures; to appear in New Journal of Physic
Entanglement quantification from incomplete measurements: Applications using photon-number-resolving weak homodyne detectors
The certificate of success for a number of important quantum information
processing protocols, such as entanglement distillation, is based on the
difference in the entanglement content of the quantum states before and after
the protocol. In such cases, effective bounds need to be placed on the
entanglement of non-local states consistent with statistics obtained from local
measurements. In this work, we study numerically the ability of a novel type of
homodyne detector which combines phase sensitivity and photon-number resolution
to set accurate bounds on the entanglement content of two-mode quadrature
squeezed states without the need for full state tomography. We show that it is
possible to set tight lower bounds on the entanglement of a family of two-mode
degaussified states using only a few measurements. This presents a significant
improvement over the resource requirements for the experimental demonstration
of continuous-variable entanglement distillation, which traditionally relies on
full quantum state tomography.Comment: 18 pages, 6 figure
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