48 research outputs found
Noise and decoherence in quantum two-level systems
Motivated by recent experiments with Josephson-junction circuits we
reconsider decoherence effects in quantum two-level systems (TLS). On one hand,
the experiments demonstrate the importance of 1/f noise, on the other hand, by
operating at symmetry points one can suppress noise effects in linear order.
We, therefore, analyze noise sources with a variety of power spectra, with
linear or quadratic coupling, which are longitudinal or transverse relative to
the eigenbasis of the unperturbed Hamiltonian. To evaluate the dephasing time
for transverse 1/f noise second-order contributions have to be taken into
account. Manipulations of the quantum state of the TLS define characteristic
time scales. We discuss the consequences for relaxation and dephasing
processes.Comment: To appear in Proceedings of the Nobel Jubilee Symposium on
Condensation and Coherence in Condensed Systems (Physica Scripta
Jordan-Wigner transformations for tree structures
The celebrated Jordan--Wigner transformation provides an efficient mapping
between spin chains and fermionic systems in one dimension. Here we extend this
spin-fermion mapping to arbitrary tree structures, which enables mapping
between fermionic and spin systems with nearest-neighbor coupling. The mapping
is achieved with the help of additional spins at the junctions between
one-dimensional chains. This property allows for straightforward simulation of
Majorana braiding in spin or qubit systems
Topological Josephson junction in transverse magnetic field
We consider Majorana zero modes in a Josephson junction on top of a
topological insulator in transverse magnetic field. Majorana zero modes reside
at periodically located nodes of Josephson vortices. We find that hybridization
of these modes is prohibited by symmetries of the problem at vanishing chemical
potential, which ensures better protection of zero modes and yields methods to
control the tunnel coupling between Majorana modes for quantum information
processing applications
Dephasing of solid-state qubits at optimal points
Motivated by recent experiments with Josephson-junction circuits, we analyze
the influence of various noise sources on the dynamics of two-level systems at
optimal operation points where the linear coupling to low-frequency
fluctuations is suppressed. We study the decoherence due to nonlinear
(quadratic) coupling, focusing on the experimentally relevant 1/f and Ohmic
noise power spectra. For 1/f noise strong higher-order effects influence the
evolution.Comment: minor corrections and clarification