1,654 research outputs found
Cherenkov radiation from fluxon in a stack of coupled long Josephson junctions
We present a systematic study of the Cherenkov radiation of Josephson plasma
waves by fast moving fluxon in a stack of coupled long Josephson junctions for
different fluxon modes. It is found that at some values of parameters
current-voltage characteristic may exhibit a region of the back-bending on the
fluxon step. In the opposite limit the emission of the Cherenkov radiation
takes place. In the annular junctions of moderate length the interaction of the
emitted waves with fluxon results in the novel resonances which emerge on the
top of the fluxon step. We present more exact formulas which describe the
position of such resonances taking into account difference between junction and
non-linear corrections. The possibility of direct detection of the Cherenkov
radiation in junctions of linear geometry is discussed.Comment: 10 pages, 12 figures, accepted to JLT
High resolution measurements of the switching current in a Josephson tunnel junction: Thermal activation and macroscopic quantum tunneling
We have developed a scheme for a high resolution measurement of the switching
current distribution of a current biased Josephson tunnel junction using a
timing technique. The measurement setup is implemented such that the digital
control and read-out electronics are optically decoupled from the analog bias
electronics attached to the sample. We have successfully used this technique to
measure the thermal activation and the macroscopic quantum tunneling of the
phase in a small Josephson tunnel junction with a high experimental resolution.
This technique may be employed to characterize current-biased Josephson tunnel
junctions for applications in quantum information processing.Comment: 10 pages, 8 figures, 1 tabl
Exact solution for the dynamical decoupling of a qubit with telegraph noise
We study the dissipative dynamics of a qubit that is afflicted by classical
random telegraph noise and it is subject to dynamical decoupling. We derive
exact formulas for the qubit dynamics at arbitrary working points in the limit
of infinitely strong control pulses (bang-bang control) and we investigate in
great detail the efficiency of the dynamical decoupling techniques both for
Gaussian and non-Gaussian (slow) noise at qubit pure dephasing and at optimal
point. We demonstrate that control sequences can be successfully implemented as
diagnostic tools to infer spectral proprieties of a few fluctuators interacting
with the qubit. The analysis is extended in order to include the effect of
noise in the pulses and we give upper bounds on the noise levels that can be
tolerated in the pulses while still achieving efficient dynamical decoupling
performance
Maximum velocity of a fluxon in a stack of coupled Josephson junctions
Dynamics of a fluxon in a stack of inductively coupled long Josephson
junctions is studied analytically and numerically. We demonstrate that the
fluxon has a maximum velocity, which does not necessarily coincide with any of
the characteristic Josephson plasma wave velocities. The maximum fluxon
velocity is found by means of numerical simulations of the quasi-infinite
system. Using the variational approximation, we propose a simple analytical
formula for the dependence of the fluxon's maximum velocity on the coupling
constant and on the distribution of critical currents in different layers. This
analysis yields rather precise results in the limit of small dissipation. The
simulations also show that nonzero dissipation additionally stabilizes the
fluxon.Comment: 8 pages, 5 figures, 1 table. submitted to Phys. Lett. A. Suggestions
and criticism are welcom
Digital quantum simulation of spin models with circuit quantum electrodynamics
Systems of interacting quantum spins show a rich spectrum of quantum phases
and display interesting many-body dynamics. Computing characteristics of even
small systems on conventional computers poses significant challenges. A quantum
simulator has the potential to outperform standard computers in calculating the
evolution of complex quantum systems. Here, we perform a digital quantum
simulation of the paradigmatic Heisenberg and Ising interacting spin models
using a two transmon-qubit circuit quantum electrodynamics setup. We make use
of the exchange interaction naturally present in the simulator to construct a
digital decomposition of the model-specific evolution and extract its full
dynamics. This approach is universal and efficient, employing only resources
which are polynomial in the number of spins and indicates a path towards the
controlled simulation of general spin dynamics in superconducting qubit
platforms.Comment: 12 pages, 9 figure
Superconducting quantum simulator for topological order and the toric code
Topological order is now being established as a central criterion for
characterizing and classifying ground states of condensed matter systems and
complements categorizations based on symmetries. Fractional quantum Hall
systems and quantum spin liquids are receiving substantial interest because of
their intriguing quantum correlations, their exotic excitations and prospects
for protecting stored quantum information against errors. Here we show that the
Hamiltonian of the central model of this class of systems, the Toric Code, can
be directly implemented as an analog quantum simulator in lattices of
superconducting circuits. The four-body interactions, which lie at its heart,
are in our concept realized via Superconducting Quantum Interference Devices
(SQUIDs) that are driven by a suitably oscillating flux bias. All physical
qubits and coupling SQUIDs can be individually controlled with high precision.
Topologically ordered states can be prepared via an adiabatic ramp of the
stabilizer interactions. Strings of qubit operators, including the stabilizers
and correlations along non-contractible loops, can be read out via a capacitive
coupling to read-out resonators. Moreover, the available single qubit
operations allow to create and propagate elementary excitations of the Toric
Code and to verify their fractional statistics. The architecture we propose
allows to implement a large variety of many-body interactions and thus provides
a versatile analog quantum simulator for topological order and lattice gauge
theories
Studying Light-Harvesting Models with Superconducting Circuits
The process of photosynthesis, the main source of energy in the animate
world, converts sunlight into chemical energy. The surprisingly high efficiency
of this process is believed to be enabled by an intricate interplay between the
quantum nature of molecular structures in photosynthetic complexes and their
interaction with the environment. Investigating these effects in biological
samples is challenging due to their complex and disordered structure. Here we
experimentally demonstrate a new approach for studying photosynthetic models
based on superconducting quantum circuits. In particular, we demonstrate the
unprecedented versatility and control of our method in an engineered three-site
model of a pigment protein complex with realistic parameters scaled down in
energy by a factor of . With this system we show that the excitation
transport between quantum coherent sites disordered in energy can be enabled
through the interaction with environmental noise. We also show that the
efficiency of the process is maximized for structured noise resembling
intramolecular phononic environments found in photosynthetic complexes.Comment: 8+12 pages, 4+12 figure
Multi-photon transitions between energy levels in a current-biased Josephson tunnel junction
The escape of a small current-biased Josephson tunnel junction from the zero
voltage state in the presence of weak microwave radiation is investigated
experimentally at low temperatures. The measurements of the junction switching
current distribution indicate the macroscopic quantum tunneling of the phase
below a cross-over temperature of . At
temperatures below we observe both single-photon and
\emph{multi-photon} transitions between the junction energy levels by applying
microwave radiation in the frequency range between and to the junction. These observations reflect the anharmonicity of the
junction potential containing only a small number of levels.Comment: 4 pages, 5 figure
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