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 ${\xi}_{th}$, 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