7,952 research outputs found
Analysis and optimization of an omnidirectional direction-finding system
System determines the direction of arrival of an electromagnetic wave with the direction information in a readily usable form. It presents a relatively small physical structure and does not require mechanical positioning
A study of antenna and radio frequency tracking system Final summary report, 1 Feb. 1963 - 30 Nov. 1965
Phase stability of very low frequency radio signal propagation, electronically scanned tracking antenna array, and inverted rf tracking syste
Transport and Dissipation in Quantum Pumps
This paper is about adiabatic transport in quantum pumps. The notion of
``energy shift'', a self-adjoint operator dual to the Wigner time delay, plays
a role in our approach: It determines the current, the dissipation, the noise
and the entropy currents in quantum pumps. We discuss the geometric and
topological content of adiabatic transport and show that the mechanism of
Thouless and Niu for quantized transport via Chern numbers cannot be realized
in quantum pumps where Chern numbers necessarily vanish.Comment: 31 pages, 10 figure
Quantum response of dephasing open systems
We develop a theory of adiabatic response for open systems governed by
Lindblad evolutions. The theory determines the dependence of the response
coefficients on the dephasing rates and allows for residual dissipation even
when the ground state is protected by a spectral gap. We give quantum response
a geometric interpretation in terms of Hilbert space projections: For a two
level system and, more generally, for systems with suitable functional form of
the dephasing, the dissipative and non-dissipative parts of the response are
linked to a metric and to a symplectic form. The metric is the Fubini-Study
metric and the symplectic form is the adiabatic curvature. When the metric and
symplectic structures are compatible the non-dissipative part of the inverse
matrix of response coefficients turns out to be immune to dephasing. We give
three examples of physical systems whose quantum states induce compatible
metric and symplectic structures on control space: The qubit, coherent states
and a model of the integer quantum Hall effect.Comment: Article rewritten, two appendices added. 16 pages, 2 figure
Optimal parametrizations of adiabatic paths
The parametrization of adiabatic paths is optimal when tunneling is
minimized. Hamiltonian evolutions do not have unique optimizers. However,
dephasing Lindblad evolutions do. The optimizers are simply characterized by an
Euler-Lagrange equation and have a constant tunneling rate along the path
irrespective of the gap. Application to quantum search algorithms recovers the
Grover result for appropriate scaling of the dephasing. Dephasing rates that
beat Grover imply hidden resources in Lindblad operators.Comment: 4 pages, 2 figures; To prevent from misunderstanding, we clarified
the discussion of an apparent speedup in the Grover algorithm; figures
improved + minor change
Are there nodes in LaFePO, BaFe(AsP), and KFeAs ?
We reexamined the experimental evidences for the possible existence of the
superconducting (SC) gap nodes in the three most suspected Fe-pnictide SC
compounds: LaFePO, BaFe(AsP), and KFeAs. We
showed that while the -linear temperature dependence of the penetration
depth of these three compounds indicate extremely clean nodal gap
superconductors, the thermal conductivity data unambiguously showed that LaFePO and
BaFe(AsP) are extremely dirty, while KFeAs
can be clean. This apparently conflicting experimental data casts a serious
doubt on the nodal gap possibility on LaFePO and
BaFe(AsP).Comment: 11 pages, 5 figures A new section "4. Remark on the quantum
oscillation (QO) experiments" is adde
Time-Energy coherent states and adiabatic scattering
Coherent states in the time-energy plane provide a natural basis to study
adiabatic scattering. We relate the (diagonal) matrix elements of the
scattering matrix in this basis with the frozen on-shell scattering data. We
describe an exactly solvable model, and show that the error in the frozen data
cannot be estimated by the Wigner time delay alone. We introduce the notion of
energy shift, a conjugate of Wigner time delay, and show that for incoming
state the energy shift determines the outgoing state.Comment: 11 pages, 1 figur
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