771 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
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
Highly efficient energy excitation transfer in light-harvesting complexes: The fundamental role of noise-assisted transport
Excitation transfer through interacting systems plays an important role in
many areas of physics, chemistry, and biology. The uncontrollable interaction
of the transmission network with a noisy environment is usually assumed to
deteriorate its transport capacity, especially so when the system is
fundamentally quantum mechanical. Here we identify key mechanisms through which
noise such as dephasing, perhaps counter intuitively, may actually aid
transport through a dissipative network by opening up additional pathways for
excitation transfer. We show that these are processes that lead to the
inhibition of destructive interference and exploitation of line broadening
effects. We illustrate how these mechanisms operate on a fully connected
network by developing a powerful analytical technique that identifies the
invariant (excitation trapping) subspaces of a given Hamiltonian. Finally, we
show how these principles can explain the remarkable efficiency and robustness
of excitation energy transfer from the light-harvesting chlorosomes to the
bacterial reaction center in photosynthetic complexes and present a numerical
analysis of excitation transport across the Fenna-Matthew-Olson (FMO) complex
together with a brief analysis of its entanglement properties. Our results show
that, in general, it is the careful interplay of quantum mechanical features
and the unavoidable environmental noise that will lead to an optimal system
performance.Comment: 16 pages, 9 figures; See Video Abstract at
http://www.quantiki.org/video_abstracts/09014454 . New revised version;
discussion of entanglement properties enhance
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