36 research outputs found
Spatial quantum error correction threshold
We consider a spatial analogue of the quantum error correction threshold.
Given individual time-independent subsystems in which quantum information is
coherent over sufficiently long lengths, we show how the information can be
kept coherent for arbitrarily long lengths by forming time-independent
composite systems. The subsystem coherence length exhibits threshold behavior.
When it exceeds a length , meaningful information can be extracted
from the ground state of the composite system. Otherwise, the information is
garbled. The threshold transition implies that the parent Hamiltonian of the
ground state has gone from gapped to gapless. Ramifications of the construction
for PEPS and for adiabatic quantum computation are noted.Comment: Comments welcome. Complete revision with corrected claims. Replaces
earlier attempt arXiv:1002.0846, which was never submitted for publicatio
Theory of superconducting qubits beyond the lumped element approximation
In the design and investigation of superconducting qubits and related
devices, a lumped element circuit model is the standard theoretical approach.
However, many important physical questions lie beyond the scope of this
approach, such as the consequences of very strong or otherwise unconventional
Josephson junctions, the properties of small qubit devices, and the number of
entangled electrons in superconducting Schrodinger cats. By performing gauge
transformations on self-consistent solutions of the Bogoliubov-de Gennes
equations, we develop here a formalism that is capable of addressing these
questions. We then apply the formalism to a charge qubit and to an RF squid
qubit. This theory provides a promising tool to accompany the remarkable
experimental achievements driving superconducting qubits forward.Comment: 11 pages, 4 figure
Could light harvesting complexes exhibit non-classical effects at room temperature?
Mounting experimental and theoretical evidence suggests that coherent quantum
effects play a role in the efficient transfer of an excitation from a
chlorosome antenna to a reaction center in the Fenna-Matthews-Olson protein
complex. However, it is conceivable that a satisfying alternate interpretation
of the results is possible in terms of a classical theory. To address this
possibility, we consider a class of classical theories satisfying the minimal
postulates of macrorealism and frame Leggett-Garg-type tests that could rule
them out. Our numerical simulations indicate that even in the presence of
decoherence, several tests could exhibit the required violations of the
Leggett-Garg inequality. Remarkably, some violations persist even at room
temperature for our decoherence model.Comment: 10 pages, 4 figures, 2 tables, submitted to the Proceedings of the
Royal Society