27 research outputs found
BASEL - The base language for an extensible language facility
Basic language for extensible language facilit
Noisy One-Way Quantum Computations: The Role of Correlations
A scheme to evaluate computation fidelities within the one-way model is
developed and explored to understand the role of correlations in the quality of
noisy quantum computations. The formalism is promptly applied to many
computation instances, and unveils that a higher amount of entanglement in the
noisy resource state does not necessarily imply a better computation.Comment: 10 pages, 6 figures, extension of a previous versio
A relational quantum computer using only two-qubit total spin measurement and an initial supply of highly mixed single qubit states
We prove that universal quantum computation is possible using only (i) the
physically natural measurement on two qubits which distinguishes the singlet
from the triplet subspace, and (ii) qubits prepared in almost any three
different (potentially highly mixed) states. In some sense this measurement is
a `more universal' dynamical element than a universal 2-qubit unitary gate,
since the latter must be supplemented by measurement. Because of the rotational
invariance of the measurement used, our scheme is robust to collective
decoherence in a manner very different to previous proposals - in effect it is
only ever sensitive to the relational properties of the qubits.Comment: TR apologises for yet again finding a coauthor with a ridiculous
middle name [12
Quantum entanglement analysis based on abstract interpretation
Entanglement is a non local property of quantum states which has no classical
counterpart and plays a decisive role in quantum information theory. Several
protocols, like the teleportation, are based on quantum entangled states.
Moreover, any quantum algorithm which does not create entanglement can be
efficiently simulated on a classical computer. The exact role of the
entanglement is nevertheless not well understood. Since an exact analysis of
entanglement evolution induces an exponential slowdown, we consider
approximative analysis based on the framework of abstract interpretation. In
this paper, a concrete quantum semantics based on superoperators is associated
with a simple quantum programming language. The representation of entanglement,
i.e. the design of the abstract domain is a key issue. A representation of
entanglement as a partition of the memory is chosen. An abstract semantics is
introduced, and the soundness of the approximation is proven.Comment: 13 page
Measurement-based quantum computation beyond the one-way model
We introduce novel schemes for quantum computing based on local measurements
on entangled resource states. This work elaborates on the framework established
in [Phys. Rev. Lett. 98, 220503 (2007), quant-ph/0609149]. Our method makes use
of tools from many-body physics - matrix product states, finitely correlated
states or projected entangled pairs states - to show how measurements on
entangled states can be viewed as processing quantum information. This work
hence constitutes an instance where a quantum information problem - how to
realize quantum computation - was approached using tools from many-body theory
and not vice versa. We give a more detailed description of the setting, and
present a large number of new examples. We find novel computational schemes,
which differ from the original one-way computer for example in the way the
randomness of measurement outcomes is handled. Also, schemes are presented
where the logical qubits are no longer strictly localized on the resource
state. Notably, we find a great flexibility in the properties of the universal
resource states: They may for example exhibit non-vanishing long-range
correlation functions or be locally arbitrarily close to a pure state. We
discuss variants of Kitaev's toric code states as universal resources, and
contrast this with situations where they can be efficiently classically
simulated. This framework opens up a way of thinking of tailoring resource
states to specific physical systems, such as cold atoms in optical lattices or
linear optical systems.Comment: 21 pages, 7 figure
Generalized Flow and Determinism in Measurement-based Quantum Computation
We extend the notion of quantum information flow defined by Danos and Kashefi
for the one-way model and present a necessary and sufficient condition for the
deterministic computation in this model. The generalized flow also applied in
the extended model with measurements in the X-Y, X-Z and Y-Z planes. We apply
both measurement calculus and the stabiliser formalism to derive our main
theorem which for the first time gives a full characterization of the
deterministic computation in the one-way model. We present several examples to
show how our result improves over the traditional notion of flow, such as
geometries (entanglement graph with input and output) with no flow but having
generalized flow and we discuss how they lead to an optimal implementation of
the unitaries. More importantly one can also obtain a better quantum
computation depth with the generalized flow rather than with flow. We believe
our characterization result is particularly essential for the study of the
algorithms and complexity in the one-way model.Comment: 16 pages, 10 figure
Valence bond solid formalism for d-level one-way quantum computation
The d-level or qudit one-way quantum computer (d1WQC) is described using the
valence bond solid formalism and the generalised Pauli group. This formalism
provides a transparent means of deriving measurement patterns for the
implementation of quantum gates in the computational model. We introduce a new
universal set of qudit gates and use it to give a constructive proof of the
universality of d1WQC. We characterise the set of gates that can be performed
in one parallel time step in this model.Comment: 26 pages, 9 figures. Published in Journal of Physics A: Mathematical
and Genera
Generalized Flow and Determinism in Measurement-based Quantum Computation
We extend the notion of quantum information flow defined by Danos and Kashefi
for the one-way model and present a necessary and sufficient condition for the
deterministic computation in this model. The generalized flow also applied in
the extended model with measurements in the X-Y, X-Z and Y-Z planes. We apply
both measurement calculus and the stabiliser formalism to derive our main
theorem which for the first time gives a full characterization of the
deterministic computation in the one-way model. We present several examples to
show how our result improves over the traditional notion of flow, such as
geometries (entanglement graph with input and output) with no flow but having
generalized flow and we discuss how they lead to an optimal implementation of
the unitaries. More importantly one can also obtain a better quantum
computation depth with the generalized flow rather than with flow. We believe
our characterization result is particularly essential for the study of the
algorithms and complexity in the one-way model.Comment: 16 pages, 10 figure
A Successful Broad-band Survey for Giant Lya Nebulae I: Survey Design and Candidate Selection
Giant Lya nebulae (or Lya "blobs") are likely sites of ongoing massive galaxy
formation, but the rarity of these powerful sources has made it difficult to
form a coherent picture of their properties, ionization mechanisms, and space
density. Systematic narrow-band Lya nebula surveys are ongoing, but the small
redshift range covered and the observational expense limit the comoving volume
that can be probed by even the largest of these surveys and pose a significant
problem when searching for such rare sources. We have developed a systematic
search technique designed to find large Lya nebulae at 2<z<3 within deep
broad-band imaging and have carried out a survey of the 9.4 square degree NOAO
Deep Wide-Field Survey (NDWFS) Bootes field. With a total survey comoving
volume of ~10^8 h^-3_70 Mpc^3, this is the largest volume survey for Lya
nebulae ever undertaken. In this first paper in the series, we present the
details of the survey design and a systematically-selected sample of 79
candidates, which includes one previously discovered Lya nebula.Comment: Accepted to ApJ after minor revision; 25 pages in emulateapj format;
18 figures, 3 table
Formal Analysis of Quantum Systems using Process Calculus
Quantum communication and cryptographic protocols are well on the way to
becoming an important practical technology. Although a large amount of
successful research has been done on proving their correctness, most of this
work does not make use of familiar techniques from formal methods, such as
formal logics for specification, formal modelling languages, separation of
levels of abstraction, and compositional analysis. We argue that these
techniques will be necessary for the analysis of large-scale systems that
combine quantum and classical components, and summarize the results of initial
investigation using behavioural equivalence in process calculus. This paper is
a summary of Simon Gay's invited talk at ICE'11.Comment: In Proceedings ICE 2011, arXiv:1108.014