960 research outputs found
Upper bounds on fault tolerance thresholds of noisy Clifford-based quantum computers
We consider the possibility of adding noise to a quantum circuit to make it efficiently simulatable classically. In previous works, this approach has been used to derive upper bounds to fault tolerance thresholds-usually by identifying a privileged resource, such as an entangling gate or a non-Clifford operation, and then deriving the noise levels required to make it 'unprivileged'. In this work, we consider extensions of this approach where noise is added to Clifford gates too and then 'commuted' around until it concentrates on attacking the non-Clifford resource. While commuting noise around is not always straightforward, we find that easy instances can be identified in popular fault tolerance proposals, thereby enabling sharper upper bounds to be derived in these cases. For instance we find that if we take Knill's (2005 Nature 434 39) fault tolerance proposal together with the ability to prepare any possible state in the XY plane of the Bloch sphere, then not more than 3.69% error-per-gate noise is sufficient to make it classical, and 13.71% of Knill's noise model is sufficient. These bounds have been derived without noise being added to the decoding parts of the circuits. Introducing such noise in a toy example suggests that the present approach can be optimized further to yield tighter bounds
Critical and noncritical long range entanglement in the Klein-Gordon field
We investigate the entanglement between two separated segments in the vacuum
state of a free 1D Klein-Gordon field, where explicit computations are
performed in the continuum limit of the linear harmonic chain. We show that the
entanglement, which we measure by the logarithmic negativity, is finite with no
further need for renormalization. We find that the quantum correlations decay
much faster than the classical correlations as in the critical limit long range
entanglement decays exponentially for separations larger than the size of the
segments. As the segments become closer to each other the entanglement diverges
as a power law. The noncritical regime manifests richer behavior, as the
entanglement depends both on the size of the segments and on their separation.
In correspondence with the von Neumann entropy long-range entanglement also
distinguishes critical from noncritical systems
Quantum dynamics of bio-molecular systems in noisy environments
We discuss three different aspects of the quantum dynamics of bio-molecular
systems and more generally complex networks in the presence of strongly coupled
environments. Firstly, we make a case for the systematic study of fundamental
structural elements underlying the quantum dynamics of these systems, identify
such elements and explore the resulting interplay of quantum dynamics and
environmental decoherence. Secondly, we critically examine some existing
approaches to the numerical description of system-environment interaction in
the non-perturbative regime and present a promising new method that can
overcome some limitations of existing methods. Thirdly, we present an approach
towards deciding and quantifying the non-classicality of the action of the
environment and the observed system-dynamics. We stress the relevance of these
tools for strengthening the interplay between theoretical and experimental
research in this field.Comment: Proceedings of the 22nd Solvay Conference in Chemistry on "Quantum
Effects in Chemistry and Biology
Environment-Mediated Quantum State Transfer
We propose a scheme for quantum state transfer(QST) between two qubits which
is based on their individual interaction with a common boson environment. The
corresponding single mode spin-boson Hamiltonian is solved by mapping it onto a
wave propagation problem in a semi-infinite ladder and the fidelity is
obtained. High fidelity occurs when the qubits are equally coupled to the boson
while the fidelity becomes smaller for nonsymmetric couplings. The complete
phase diagram for such an arbitrary QST mediated by bosons is discussed.Comment: 6 pages and 5 figure
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