1,765 research outputs found
Linear response theory of Josephson junction arrays in a microwave cavity
Recent experiments on Josephson junction arrays (JJAs) in microwave cavities
have opened up a new avenue for investigating the properties of these devices
while minimising the amount of external noise coming from the measurement
apparatus itself. These experiments have already shown promise for probing
many-body quantum effects in JJAs. In this work, we develop a general
theoretical description of such experiments by deriving a quantum phase model
for planar JJAs containing quantized vortices. The dynamical susceptibility of
this model is calculated for some simple circuits, and signatures of the
injection of additional vortices are identified. The effects of decoherence are
considered via a Lindblad master equation.Comment: 15 pages, 10 figure
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
Depinning of disordered bosonic chains
We consider one-dimensional bosonic chains with a repulsive boson-boson
interaction that decays exponentially on large length-scales. This model
describes transport of Cooper-pairs in a Josepshon junction array, or transport
of magnetic flux quanta in quantum-phase-slip ladders, i.e. arrays of
superconducting wires in a ladder-configuration that allow for the coherent
tunnelling of flux quanta. In the low-frequency, long wave-length regime these
chains can be mapped to an effective model of a one-dimensional elastic field
in a disordered potential. The onset of transport in these systems, when biased
by external voltage, is described by the standard depinning theory of elastic
media in disordered pinning potentials. We numerically study the regimes that
are of relevance for quantum-phase-slip ladders. These are (i) very short
chains and (ii) the regime of weak disorder. For chains shorter than the
typical pinning length, i.e., the Larkin length, the chains reach a saturation
regime where the depinning voltage does not depend on the decay length of the
repulsive interaction. In the regime of weak disorder we find an emergent
correlation length-scale that depends on the disorder strength. For arrays
shorter than this length the onset of transport is similar to the clean arrays,
i.e., is due to the penetration of solitons into the array. We discuss the
depinning scenarios for longer arrays in this regime.Comment: 11 pages, 6 figure
Towards understanding two-level-systems in amorphous solids -- Insights from quantum circuits
Amorphous solids show surprisingly universal behaviour at low temperatures.
The prevailing wisdom is that this can be explained by the existence of
two-state defects within the material. The so-called standard tunneling model
has become the established framework to explain these results, yet it still
leaves the central question essentially unanswered -- what are these two-level
defects? This question has recently taken on a new urgency with the rise of
superconducting circuits in quantum computing, circuit quantum electrodynamics,
magnetometry, electrometry and metrology. Superconducting circuits made from
aluminium or niobium are fundamentally limited by losses due to two-level
defects within the amorphous oxide layers encasing them. On the other hand,
these circuits also provide a novel and effective method for studying the very
defects which limit their operation. We can now go beyond ensemble measurements
and probe individual defects -- observing the quantum nature of their dynamics
and studying their formation, their behaviour as a function of applied field,
strain, temperature and other properties. This article reviews the plethora of
recent experimental results in this area and discusses the various theoretical
models which have been used to describe the observations. In doing so, it
summarises the current approaches to solving this fundamentally important
problem in solid-state physics.Comment: 34 pages, 7 figures, 1 tabl
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
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