2,510 research outputs found
Quantum-state tomography for spin-l systems
We show that the density matrix of a spin-l system can be described entirely
in terms of the measurement statistics of projective spin measurements along a
minimum of 4l+1 different spin directions. It is thus possible to represent the
complete quantum statistics of any N-level system within the spherically
symmetric three dimensional space defined by the spin vector. An explicit
method for reconstructing the density matrix of a spin-1 system from the
measurement statistics of five non-orthogonal spin directions is presented and
the generalization to spin-l systems is discussed.Comment: 10 pages, including 2 tables, minor modifications in section II,
final version for publication in Phys. Rev.
Local effective dynamics of quantum systems: A generalized approach to work and heat
By computing the local energy expectation values with respect to some local
measurement basis we show that for any quantum system there are two
fundamentally different contributions: changes in energy that do not alter the
local von Neumann entropy and changes that do. We identify the former as work
and the latter as heat. Since our derivation makes no assumptions on the system
Hamiltonian or its state, the result is valid even for states arbitrarily far
from equilibrium. Examples are discussed ranging from the classical limit to
purely quantum mechanical scenarios, i.e. where the Hamiltonian and the density
operator do not commute.Comment: 5 pages, 1 figure, published versio
Complex joint probabilities as expressions of determinism in quantum mechanics
The density operator of a quantum state can be represented as a complex joint
probability of any two observables whose eigenstates have non-zero mutual
overlap. Transformations to a new basis set are then expressed in terms of
complex conditional probabilities that describe the fundamental relation
between precise statements about the three different observables. Since such
transformations merely change the representation of the quantum state, these
conditional probabilities provide a state-independent definition of the
deterministic relation between the outcomes of different quantum measurements.
In this paper, it is shown how classical reality emerges as an approximation to
the fundamental laws of quantum determinism expressed by complex conditional
probabilities. The quantum mechanical origin of phase spaces and trajectories
is identified and implications for the interpretation of quantum measurements
are considered. It is argued that the transformation laws of quantum
determinism provide a fundamental description of the measurement dependence of
empirical reality.Comment: 12 pages, including 1 figure, updated introduction includes
references to the historical background of complex joint probabilities and to
related work by Lars M. Johanse
Out of plane analysis for composite structures
Simple two dimensional analysis techniques were developed to aid in the design of strong joints for integrally stiffened/bonded composite structures subjected to out of plane loads. It was found that most out of plane failures were due to induced stresses arising from rapid changes in load path direction or geometry, induced stresses due to changes in geometry caused by buckling, or direct stresses produced by fuel pressure or bearing loads. While the analysis techniques were developed to address a great variety of out of plane loading conditions, they were primarily derived to address the conditions described above. The methods were developed and verified using existing element test data. The methods were demonstrated using the data from a test failure of a high strain wingbox that was designed, built, and tested under a previous program. Subsequently, a set of design guidelines were assembled to assist in the design of safe, strong integral composite structures using the analysis techniques developed
Violation of Heisenberg's Measurement-Disturbance Relationship by Weak Measurements
While there is a rigorously proven relationship about uncertainties intrinsic
to any quantum system, often referred to as "Heisenberg's Uncertainty
Principle," Heisenberg originally formulated his ideas in terms of a
relationship between the precision of a measurement and the disturbance it must
create. Although this latter relationship is not rigorously proven, it is
commonly believed (and taught) as an aspect of the broader uncertainty
principle. Here, we experimentally observe a violation of Heisenberg's
"measurement-disturbance relationship", using weak measurements to characterize
a quantum system before and after it interacts with a measurement apparatus.
Our experiment implements a 2010 proposal of Lund and Wiseman to confirm a
revised measurement-disturbance relationship derived by Ozawa in 2003. Its
results have broad implications for the foundations of quantum mechanics and
for practical issues in quantum mechanics.Comment: 5 pages, 4 figure
Passive, broadband and low-frequency suppression of laser amplitude noise to the shot-noise limit using hollow-core fibre
We use hollow-core fibre to preserve the spectrum and temporal profile of
picosecond laser pulses in CBD to suppress 2.6 dB of amplitude noise at MHz
noise frequencies, to within 0.01 dB of the shot-noise limit. We provide an
enhanced version of the CBD scheme that concatenates circuits to suppress over
multiple frequencies and over broad frequency ranges --- we perform a first
demonstration that reduces total excess amplitude noise, between 2 - 6 MHz, by
85%. These demonstrations enable passive, broad-band, all-guided fibre laser
technology operating at the shot-noise limit.Comment: 8 pages, 8 figure
Spectral densities and partition functions of modular quantum systems as derived from a central limit theorem
Using a central limit theorem for arrays of interacting quantum systems, we
give analytical expressions for the density of states and the partition
function at finite temperature of such a system, which are valid in the limit
of infinite number of subsystems. Even for only small numbers of subsystems we
find good accordance with some known, exact results.Comment: 6 pages, 4 figures, some steps added to derivation, accepted for
publication in J. Stat. Phy
Single-mode operation of terahertz quantum cascade lasers with distributed feedback resonators
Distributed feedback terahertz quantum-cascade lasers emitting at 4.34 and 4.43 THz are presented. Mode selection is based on a complex-coupling scheme implemented into the top-contact layer by a combination of wet chemical etching and ohmic-contact deposition. Single-mode emission stable at all injection currents and operating temperatures is shown, with a side-mode suppression ratio exceeding 20 dB. Peak output powers of up to 1.8 mW are obtained at low temperatures. (C) 2004 American Institute of Physics
Entanglement in the interaction between two quantum oscillator systems
The fundamental quantum dynamics of two interacting oscillator systems are
studied in two different scenarios. In one case, both oscillators are assumed
to be linear, whereas in the second case, one oscillator is linear and the
other is a non-linear, angular-momentum oscillator; the second case is, of
course, more complex in terms of energy transfer and dynamics. These two
scenarios have been the subject of much interest over the years, especially in
developing an understanding of modern concepts in quantum optics and quantum
electronics. In this work, however, these two scenarios are utilized to
consider and discuss the salient features of quantum behaviors resulting from
the interactive nature of the two oscillators, i.e., coherence, entanglement,
spontaneous emission, etc., and to apply a measure of entanglement in analyzing
the nature of the interacting systems. ... For the coupled linear and
angular-momentum oscillator system in the fully quantum-mechanical description,
we consider special examples of two, three, four-level angular momentum
systems, demonstrating the explicit appearances of entanglement. We also show
that this entanglement persists even as the coupled angular momentum oscillator
is taken to the limit of a large number of levels, a limit which would go over
to the classical picture for an uncoupled angular momentum oscillator
High-performance operation of single-mode terahertz quantum cascade lasers with metallic gratings
A periodic array of thin slits opened on a metallic surface can act as a one-dimensional photonic crystal for the propagation of surface-plasmon waves. We have used such structure for the implementation of distributed feedback resonators in quantum cascade lasers emitting near 2.5 THz. Single-mode emission, stable at all injection currents and operating temperatures, was achieved both in pulsed and continuous wave. The devices exhibited output powers of several milliwatts with low threshold current densities of ∼100 A cm2
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