282 research outputs found
Correlated transport through junction arrays in the small Josephson energy limit: incoherent Cooper-pairs and hot electrons
We study correlated transport in a Josephson junction array for small
Josephson energies. In this regime transport is dominated by Cooper-pair
hopping, although we observe that quasiparticles can not be neglected. We
assume that the energy dissipated by a Cooper-pair is absorbed by the intrinsic
impedance of the array. This allows us to formulate explicit Cooper-pair
hopping rates without adding any parameters to the system. We show that the
current is correlated and crucially, these correlations rely fundamentally on
the interplay between the Cooper-pairs and equilibrium quasiparticles.Comment: 11 pages, 9 figures - Published Versio
The parity effect in Josephson junction arrays
We study the parity effect and transport due to quasiparticles in circuits
comprised of many superconducting islands. We develop a general approach and
show that it is equivalent to previous methods for describing the parity effect
in their more limited regimes of validity. As an example we study transport
through linear arrays of Josephson junctions in the limit of negligible
Josephson energy and observe the emergence of the parity effect with decreasing
number of non-equilibrium quasiparticles. Due to the exponential increase in
the number of relevant charge states with increasing length, in multi-junction
arrays the parity effect manifests in qualitatively different ways to the two
junction case. The role of charge disorder is also studied as this hides much
of the parity physics which would otherwise be observed. Nonetheless, we see
that the current through a multi-junction array at low bias is limited by the
formation of meta-stable even-parity states.Comment: 8 pages, 5 figure
Influence of two-level fluctuators on adiabatic passage techniques
We study the process of Stimulated Raman Adiabatic Passage (STIRAP) under the
influence of a non-trivial solid-state environment, particularly the effect of
two-level fluctuators (TLFs) as they are frequently present in solid-state
devices. When the amplitudes of the driving-pulses used in STIRAP are in
resonance with the level spacing of the fluctuators the quality of the
protocol, i.e., the transferred population decreases sharply. In general the
effect can not be reduced by speeding up the STIRAP process. We also discuss
the effect of a structured noise environment on the process of Coherent
Tunneling by Adiabatic Passage (CTAP). The effect of a weakly structured
environment or TLFs with short coherence times on STIRAP and CTAP can be
described by the Bloch-Redfield theory. For a strongly structured environment a
higher-dimensional approach must be used, where the TLFs are treated as part of
the system.Comment: 8 pages, 8 figure
The role of damping for the driven anharmonic quantum oscillator
For the model of a linearly driven quantum anharmonic oscillator, the role of
damping is investigated. We compare the position of the stable points in phase
space obtained from a classical analysis to the result of a quantum mechanical
analysis. The solution of the full master equation shows that the stable points
behave qualitatively similar to the classical solution but with small
modifications. Both the quantum effects and additional effects of temperature
can be described by renormalizing the damping.Comment: 4 pages, 2 figures; submitted to "Journal of Physics: Conference
Series
Few-Qubit lasing in circuit QED
Motivated by recent experiments, which demonstrated lasing and cooling of the
electromagnetic modes in a resonator coupled to a superconducting qubit, we
describe the specific mechanisms creating the population inversion, and we
study the spectral properties of these systems in the lasing state. Different
levels of the theoretical description, i.e., the semi-classical and the
semi-quantum approximation, as well as an analysis based on the full Liouville
equation are compared. We extend the usual quantum optics description to
account for strong qubit-resonator coupling and include the effects of
low-frequency noise. Beyond the lasing transition we find for a single- or
few-qubit system the phase diffusion strength to grow with the coupling
strength, which in turn deteriorates the lasing state.Comment: Prepared for the proceedings of the Nobel Symposium 2009, Qubits for
future quantum computers, May 2009 in Goeteborg, Sweden. Published versio
Dual-probe decoherence microscopy: Probing pockets of coherence in a decohering environment
We study the use of a pair of qubits as a decoherence probe of a non-trivial
environment. This dual-probe configuration is modelled by three
two-level-systems which are coupled in a chain in which the middle system
represents an environmental two-level-system (TLS). This TLS resides within the
environment of the qubits and therefore its coupling to perturbing fluctuations
(i.e. its decoherence) is assumed much stronger than the decoherence acting on
the probe qubits. We study the evolution of such a tripartite system including
the appearance of a decoherence-free state (dark state) and non-Markovian
behaviour. We find that all parameters of this TLS can be obtained from
measurements of one of the probe qubits. Furthermore we show the advantages of
two qubits in probing environments and the new dynamics imposed by a TLS which
couples to two qubits at once.Comment: 29 pages, 10 figure
Reframing Resilience: Equitable Access to Essential Services
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/156467/2/risa13492.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/156467/1/risa13492_am.pd
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