31,738 research outputs found
Rapid purification of quantum systems by measuring in a feedback-controlled unbiased basis
Rapid-purification by feedback --- specifically, reducing the mean impurity
faster than by measurement alone --- can be achieved by making the eigenbasis
of the density matrix to be unbiased relative to the measurement basis. Here we
further examine the protocol introduced by Combes and Jacobs [Phys.Rev.Lett.
{\bf 96}, 010504 (2006)] involving continuous measurement of the observable
for a -dimensional system. We rigorously re-derive the lower bound
on the achievable speed-up factor, and also an upper bound, namely
, for all feedback protocols that use measurements in unbiased bases.
Finally we extend our results to independent measurements on a register of
qubits, and derive an upper bound on the achievable speed-up factor that
scales linearly with .Comment: v2: published versio
New camera tube improves ultrasonic inspection system
Electron multiplier, incorporated into the camera tube of an ultrasonic imaging system, improves resolution, effectively shields low level circuits, and provides a high level signal input to the television camera. It is effective for inspection of metallic materials for bonds, voids, and homogeneity
Spray combustion under oscillatory pressure conditions
The performance and stability of liquid rocket engines is often argued to be significantly impacted by atomization and droplet vaporization processes. In particular, combustion instability phenomena may result from the interactions between the oscillating pressure field present in the rocket combustor and the fuel and oxidizer injection process. Few studies have been conducted to examine the effects of oscillating pressure fields on spray formation and its evolution under rocket engine conditions. The pressure study is intended to address the need for such studies. In particular, two potentially important phenomena are addressed in the present effort. The first involves the enhancement of the atomization process for a liquid jet subjected to an oscillating pressure field of known frequency and amplitude. The objective of this part of the study is to examine the coupling between the pressure field and or the resulting periodically perturbed velocity field on the breakup of the liquid jet. In particular, transverse mode oscillations are of interest since such modes are considered of primary importance in combustion instability phenomena. The second aspect of the project involves the effects of an oscillating pressure on droplet coagulation and secondary atomization. The objective of this study is to examine the conditions under which phenomena following the atomization process are affected by perturbations to the pressure or velocity fields. Both coagulation and represent a coupling mechanism between the pressure field and the energy release process in rocket combustors. It is precisely this coupling which drives combustion instability phenomena. Consequently, the present effort is intended to provide the fundamental insights needed to evaluate these processes as important mechanisms in liquid rocket instability phenomena
Classical Robustness of Quantum Unravellings
We introduce three measures which quantify the degree to which quantum
systems possess the robustness exhibited by classical systems when subjected to
continuous observation. Using these we show that for a fixed environmental
interaction the level of robustness depends on the measurement strategy, or
unravelling, and that no single strategy is maximally robust in all ways.Comment: 8 Pages, 2 figures, Version 2. Minor changes to wording for
clarification and some references added. Accepted for publication in
Europhysics Letter
Shuttle on-orbit contamination and environmental effects
Ensuring the compatibility of the space shuttle system with payloads and payload measurements is discussed. An extensive set of quantitative requirements and goals was developed and implemented by the space shuttle program management. The performance of the Shuttle system as measured by these requirements and goals was assessed partly through the use of the induced environment contamination monitor on Shuttle flights 2, 3, and 4. Contamination levels are low and generally within the requirements and goals established. Additional data from near-term payloads and already planned contamination measurements will complete the environment definition and allow for the development of contamination avoidance procedures as necessary for any payload
Tradeoff between extractable mechanical work, accessible entanglement, and ability to act as a reference system, under arbitrary superselection rules
Superselection rules (SSRs) limit the mechanical and quantum processing
resources represented by quantum states. However SSRs can be violated using
reference systems to break the underlying symmetry. We show that there is a
duality between the ability of a system to do mechanical work and to act as a
reference system. Further, for a bipartite system in a globally symmetric pure
state, we find a triality between the system's ability to do local mechanical
work, its ability to do ``logical work'' due to its accessible entanglement,
and its ability to act as a shared reference system.Comment: 5 pages, no figures. Extended resubmitted version. Slightly modified
title. Transferred to PR
Ultrafast charge transfer and vibronic coupling in a laser-excited hybrid inorganic/organic interface
Hybrid interfaces formed by inorganic semiconductors and organic molecules are intriguing materials for opto-electronics. Interfacial charge transfer is primarily responsible for their peculiar electronic structure and optical response. Hence, it is essential to gain insight into this fundamental process also beyond the static picture. Ab initio methods based on real-time time-dependent density-functional theory coupled to the Ehrenfest molecular dynamics scheme are ideally suited for this problem. We investigate a laser-excited hybrid inorganic/organic interface formed by the electron acceptor molecule 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4TCNQ) physisorbed on a hydrogenated silicon cluster, and we discuss the fundamental mechanisms of charge transfer in the ultrashort time window following the impulsive excitation. The considered interface is p-doped and exhibits charge transfer in the ground state. When it is excited by a resonant laser pulse, the charge transfer across the interface is additionally increased, but contrary to previous observations in all-organic donor/acceptor complexes, it is not further promoted by vibronic coupling. In the considered time window of 100 fs, the molecular vibrations are coupled to the electron dynamics and enhance intramolecular charge transfer. Our results highlight the complexity of the physics involved and demonstrate the ability of the adopted formalism to achieve a comprehensive understanding of ultrafast charge transfer in hybrid materials
Absolute Dynamical Limit to Cooling Weakly-Coupled Quantum Systems
Cooling of a quantum system is limited by the size of the control forces that
are available (the "speed" of control). We consider the most general cooling
process, albeit restricted to the regime in which the thermodynamics of the
system is preserved (weak coupling). Within this regime, we further focus on
the most useful control regime, in which a large cooling factor, and good
ground-state cooling can be achieved. We present a control protocol for
cooling, and give clear structural arguments, as well as strong numerical
evidence, that this protocol is globally optimal. From this we obtain simple
expressions for the limit to cooling that is imposed by the speed of control.Comment: 4 pages, Revetex4-1, 2 png figure
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