5,089 research outputs found
Time-Reversal Symmetry and Universal Conductance Fluctuations in a Driven Two-Level System
In the presence of time-reversal symmetry, quantum interference gives strong
corrections to the electric conductivity of disordered systems. The
self-interference of an electron wavefunction traveling time-reversed paths
leads to effects such as weak localization and universal conductance
fluctuations. Here, we investigate the effects of broken time-reversal symmetry
in a driven artificial two-level system. Using a superconducting flux qubit, we
implement scattering events as multiple Landau-Zener transitions by driving the
qubit periodically back and forth through an avoided crossing. Interference
between different qubit trajectories give rise to a speckle pattern in the
qubit transition rate, similar to the interference patterns created when
coherent light is scattered off a disordered potential. Since the scattering
events are imposed by the driving protocol, we can control the time-reversal
symmetry of the system by making the drive waveform symmetric or asymmetric in
time. We find that the fluctuations of the transition rate exhibit a sharp peak
when the drive is time-symmetric, similar to universal conductance fluctuations
in electronic transport through mesoscopic systems
No approximate complex fermion coherent states
Whereas boson coherent states with complex parametrization provide an
elegant, and intuitive representation, there is no counterpart for fermions
using complex parametrization. However, a complex parametrization provides a
valuable way to describe amplitude and phase of a coherent beam. Thus we pose
the question of whether a fermionic beam can be described, even approximately,
by a complex-parametrized coherent state and define, in a natural way,
approximate complex-parametrized fermion coherent states. Then we identify four
appealing properties of boson coherent states (eigenstate of annihilation
operator, displaced vacuum state, preservation of product states under linear
coupling, and factorization of correlators) and show that these approximate
complex fermion coherent states fail all four criteria. The inapplicability of
complex parametrization supports the use of Grassman algebras as an appropriate
alternative.Comment: Argumentation made cleare
Reinsurance and the Law of Aggregation
"In excess of loss reinsurance, the reinsurer covers the amount of a loss exceeding the policy’s deductible but not piercing its cover limit. Accordingly, a policy’s quantitative scope of cover is significantly affected by the parties’ agreement of a deductible and a cover limit. Yet, the examination of whether a loss has exceeded deductible or cover limit necessitates an educated understanding of what constitutes one loss. In so-called aggregation clauses, the parties to (re-)insurance contracts regularly provide that multiple individual losses are to be added together for presenting one loss to the reinsurer when they arise from the same event, occurrence, catastrophe, cause or accident. Aggregation mechanisms are one of the core instruments for structuring reinsurance contracts.
This book systematically examines each element of an aggregation mechanism, tracing the inconsistent usage of aggregation language in the markets and scrutinizing the tests developed by courts and arbitral tribunals. In doing so, it seeks to support insurers, reinsurers, brokers and lawyers in drafting aggregation clauses and in settling claims.
Focusing on an analysis of primary sources, particularly judicial decisions, the book interprets each judicial decision to describe a system of inter-related rules, collating, organising and describing the English law of aggregation as applied by the courts and arbitral tribunals. It further draws a comparison between the English position and the corresponding rules in the Principles of Reinsurance Contract Law (PRICL).
Heisenberg-limited qubit readout with two-mode squeezed light
We show how to use two-mode squeezed light to exponentially enhance
cavity-based dispersive qubit measurement. Our scheme enables true
Heisenberg-limited scaling of the measurement, and crucially, is not restricted
to small dispersive couplings or unrealistically long measurement times. It
involves coupling a qubit dispersively to two cavities, and making use of a
symmetry in the dynamics of joint cavity quadratures (a so-called
quantum-mechanics-free subsystem). We discuss the basic scaling of the scheme
and its robustness against imperfections, as well as a realistic implementation
in circuit quantum electrodynamics.Comment: 5 pages, 4 figures, Supplemental Materia
Noise correlations in a flux qubit with tunable tunnel coupling
We have measured flux-noise correlations in a tunable superconducting flux
qubit. The device consists of two loops that independently control the qubit's
energy splitting and tunnel coupling. Low frequency flux noise in the loops
causes fluctuations of the qubit frequency and leads to dephasing. Since the
noises in the two loops couple to different terms of the qubit Hamiltonian, a
measurement of the dephasing rate at different bias points provides a way to
extract both the amplitude and the sign of the noise correlations. We find that
the flux fluctuations in the two loops are anti-correlated, consistent with a
model where the flux noise is generated by randomly oriented unpaired spins on
the metal surface.Comment: 7 pages, including supplementary materia
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