38,293 research outputs found
Fault tolerance for holonomic quantum computation
We review an approach to fault-tolerant holonomic quantum computation on
stabilizer codes. We explain its workings as based on adiabatic dragging of the
subsystem containing the logical information around suitable loops along which
the information remains protected.Comment: 16 pages, this is a chapter in the book "Quantum Error Correction",
edited by Daniel A. Lidar and Todd A. Brun, (Cambridge University Press,
2013), at
http://www.cambridge.org/us/academic/subjects/physics/quantum-physics-quantum-information-and-quantum-computation/quantum-error-correctio
Treating Time Travel Quantum Mechanically
The fact that closed timelike curves (CTCs) are permitted by general
relativity raises the question as to how quantum systems behave when time
travel to the past occurs. Research into answering this question by utilising
the quantum circuit formalism has given rise to two theories: Deutschian-CTCs
(D-CTCs) and "postselected" CTCs (P-CTCs). In this paper the quantum circuit
approach is thoroughly reviewed, and the strengths and shortcomings of D-CTCs
and P-CTCs are presented in view of their non-linearity and time travel
paradoxes. In particular, the "equivalent circuit model"---which aims to make
equivalent predictions to D-CTCs, while avoiding some of the difficulties of
the original theory---is shown to contain errors. The discussion of D-CTCs and
P-CTCs is used to motivate an analysis of the features one might require of a
theory of quantum time travel, following which two overlapping classes of new
theories are identified. One such theory, the theory of "transition
probability" CTCs (T-CTCs), is fully developed. The theory of T-CTCs is shown
not to have certain undesirable features---such as time travel paradoxes, the
ability to distinguish non-orthogonal states with certainty, and the ability to
clone or delete arbitrary pure states---that are present with D-CTCs and
P-CTCs. The problems with non-linear extensions to quantum mechanics are
discussed in relation to the interpretation of these theories, and the physical
motivations of all three theories are discussed and compared.Comment: 20 pages, 4 figures. Edited in response to peer revie
Quantum Experiments and Graphs III: High-Dimensional and Multi-Particle Entanglement
Quantum entanglement plays an important role in quantum information
processes, such as quantum computation and quantum communication. Experiments
in laboratories are unquestionably crucial to increase our understanding of
quantum systems and inspire new insights into future applications. However,
there are no general recipes for the creation of arbitrary quantum states with
many particles entangled in high dimensions. Here, we exploit a recent
connection between quantum experiments and graph theory and answer this
question for a plethora of classes of entangled states. We find experimental
setups for Greenberger-Horne-Zeilinger states, W states, general Dicke states,
and asymmetrically high-dimensional multipartite entangled states. This result
sheds light on the producibility of arbitrary quantum states using photonic
technology with probabilistic pair sources and allows us to understand the
underlying technological and fundamental properties of entanglement.Comment: 7 pages, 7 figures; Appendix 3 pages, 5 figure
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