2,586 research outputs found
Field-dependent dynamics of the Anderson impurity model
Single-particle dynamics of the Anderson impurity model in the presence of a
magnetic field are considered, using a recently developed local moment
approach that encompasses all energy scales, field and interaction strengths.
For strong coupling in particular, the Kondo scaling regime is recovered. Here
the frequency () and field ()
dependence of the resultant universal scaling spectrum is obtained in large
part analytically, and the field-induced destruction of the Kondo resonance
investigated. The scaling spectrum is found to exhibit the slow logarithmic
tails recently shown to dominate the zero-field scaling spectrum. At the
opposite extreme of the Fermi level, it gives asymptotically exact agreement
with results for statics known from the Bethe ansatz. Good agreement is also
found with the frequency and field-dependence of recent numerical
renormalization group calculations. Differential conductance experiments on
quantum dots in the presence of a magnetic field are likewise considered; and
appear to be well accounted for by the theory. Some new exact results for the
problem are also established
Modeling Quantum Optical Components, Pulses and Fiber Channels Using OMNeT++
Quantum Key Distribution (QKD) is an innovative technology which exploits the
laws of quantum mechanics to generate and distribute unconditionally secure
cryptographic keys. While QKD offers the promise of unconditionally secure key
distribution, real world systems are built from non-ideal components which
necessitates the need to model and understand the impact these non-idealities
have on system performance and security. OMNeT++ has been used as a basis to
develop a simulation framework to support this endeavor. This framework,
referred to as "qkdX" extends OMNeT++'s module and message abstractions to
efficiently model optical components, optical pulses, operating protocols and
processes. This paper presents the design of this framework including how
OMNeT++'s abstractions have been utilized to model quantum optical components,
optical pulses, fiber and free space channels. Furthermore, from our toolbox of
created components, we present various notional and real QKD systems, which
have been studied and analyzed.Comment: Published in: A. F\"orster, C. Minkenberg, G. R. Herrera, M. Kirsche
(Eds.), Proc. of the 2nd OMNeT++ Community Summit, IBM Research - Zurich,
Switzerland, September 3-4, 201
A pilot study from The Gambia to improve access to water, energy, and mobile phones
Across Sub-Saharan Africa many communities lack reliable access to mains electricity, and therefore depend upon unconventional power sources to recharge their mobile phones. Many of these informal recharging centres are powered by a diesel generator or solar panel. Furthermore, many of these same communities are frequently served by broken water pumps. Previous reports indicate that many individuals are prepared to pay a small regular fee to recharge their mobile phone, whilst their local water point committee lacks sufficient funds to keep their water infrastructure maintained. This paper presents a novel funding strategy aimed at helping communities cover the maintenance costs of their local water supply. This premise was demonstrated using a pilot system in Gambia which combined a solar powered community water point with an off-grid smart battery rental hub for everyday electricity needs such as mobile phone charging. The paper presents preliminary field data from this site and explores the wider context surrounding the pilot system
Mean-field results on the Anderson impurity model out of equilibrium
We investigate the mean-field phase diagram of the Anderson impurity model
out of equilibrium. Generalising the unrestricted Hartree-Fock approach to the
non-equilibrium situation we derive and analyse the system of equations
defining the critical surface separating the magnetic regime from the
non-magnetic one. An exact analytic solution for the phase boundary as a
function of the applied voltage is found in the symmetric case. Surprisingly,
we find that as soon as there is an asymmetry, even small, between the
contacts, no finite voltage is able to destroy the magnetic regime which
persists at arbitrary high voltages.Comment: 4 pages, 2 figures (eps files); to appear in PRB Brief Report
Spectral properties of a narrow-band Anderson model
We consider single-particle spectra of a symmetric narrow-band Anderson
impurity model, where the host bandwidth is small compared to the
hybridization strength . Simple 2nd order perturbation theory (2PT)
in is found to produce a rich spectral structure, that leads to rather good
agreement with extant Lanczos results and offers a transparent picture of the
underlying physics. It also leads naturally to two distinct regimes of spectral
behaviour, and (with the quasi-particle
weight), whose existence and essential characteristics are discussed and shown
to be independent of 2PT itself. The self-energy is also
examined beyond the confines of PT. It is argued that on frequency scales of
order , the self-energy in {\em strong} coupling
is given precisely by the 2PT result, and we point out that the resultant poles
in connect continuously to that characteristic of the atomic
limit. This in turn offers a natural rationale for the known inability of the
skeleton expansion to capture such behaviour, and points to the intrinsic
dangers of partial infinite-order summations that are based on PT in .Comment: 10 pages, 2 Postscript figures, uses RevTex 3.1; accepted for
publication in Phys. Rev. B1
Quantum Key Distribution: Boon or Bust
Quantum Key Distribution (QKD) is an emerging cybersecurity technology which provides the means for two geographically separated parties to grow “unconditionally secure” symmetric cryptographic keying material. Unlike traditional key distribution techniques, the security of QKD rests on the laws of quantum mechanics and not computational complexity. This unique aspect of QKD is due to the fact that any unauthorized eavesdropping on the key distribution channel necessarily introduces detectable errors (Gisin, Ribordy, Tittel, & Zbinden, 2002). This attribute makes QKD desirable for high-security environments such as banking, government, and military applications. However, QKD is a nascent technology where implementation non-idealities can negatively impact system performance and security (Mailloux, Grimaila, Hodson, Baumgartner, & McLaughlin, 2015). While the QKD community is making progress towards the viability of QKD solutions, it is clear that more work is required to quantify the impact of such non-idealities in real-world QKD systems (Scarani & Kurtsiefer, 2009)
Modeling, Simulation, and Performance Analysis of Decoy State Enabled Quantum Key Distribution Systems
Quantum Key Distribution (QKD) systems exploit the laws of quantum mechanics to generate secure keying material for cryptographic purposes. To date, several commercially viable decoy state enabled QKD systems have been successfully demonstrated and show promise for high-security applications such as banking, government, and military environments. In this work, a detailed performance analysis of decoy state enabled QKD systems is conducted through model and simulation of several common decoy state configurations. The results of this study uniquely demonstrate that the decoy state protocol can ensure Photon Number Splitting (PNS) attacks are detected with high confidence, while maximizing the system’s quantum throughput at no additional cost. Additionally, implementation security guidance is provided for QKD system developers and users
System Safety Engineering for Social and Ethical ML Risks: A Case Study
Governments, industry, and academia have undertaken efforts to identify and
mitigate harms in ML-driven systems, with a particular focus on social and
ethical risks of ML components in complex sociotechnical systems. However,
existing approaches are largely disjointed, ad-hoc and of unknown
effectiveness. Systems safety engineering is a well established discipline with
a track record of identifying and managing risks in many complex sociotechnical
domains. We adopt the natural hypothesis that tools from this domain could
serve to enhance risk analyses of ML in its context of use. To test this
hypothesis, we apply a "best of breed" systems safety analysis, Systems
Theoretic Process Analysis (STPA), to a specific high-consequence system with
an important ML-driven component, namely the Prescription Drug Monitoring
Programs (PDMPs) operated by many US States, several of which rely on an
ML-derived risk score. We focus in particular on how this analysis can extend
to identifying social and ethical risks and developing concrete design-level
controls to mitigate them.Comment: 14 pages, 5 figures, 3 tables. Accepted to 36th Conference on Neural
Information Processing Systems, Workshop on ML Safety (NeurIPS 2022
Optimizing Decoy State Enabled Quantum Key Distribution Systems to Maximize Quantum Throughput and Detect Photon Number Splitting Attacks with High Confidence
Quantum Key Distribution (QKD) is an innovative quantum communications
protocol which exploits the laws of quantum mechanics to generate
unconditionally secure cryptographic keying material between two geographically
separated parties. The unique nature of QKD shows promise for high-security
applications such as those found in banking, government, and military
environments. However, QKD systems contain implementation non-idealities which
can negatively impact their performance and security.In particular, QKD systems
often employ the decoy state protocol to improve system throughput and mitigate
the threat of Photon Number Splitting (PNS) attacks. In this work, a detailed
analysis of the decoy state protocol is conducted which optimizes both
performance in terms of quantum throughput and security with respect to
detecting PNS attacks. The results of this study uniquely demonstrate that the
decoy state protocol can ensure PNS attacks are detected with high confidence,
while maximizing the secure key generation rate at no additional cost.
Additionally, implementation security guidance is provided for QKD system
developers and users
Multichannel pseudogap Kondo model: Large-N solution and quantum-critical dynamics
We discuss a multichannel SU(N) Kondo model which displays non-trivial
zero-temperature phase transitions due to a conduction electron density of
states vanishing with a power law at the Fermi level. In a particular large-N
limit, the system is described by coupled integral equations corresponding to a
dynamic saddle point. We exactly determine the universal low-energy behavior of
spectral densities at the scale-invariant fixed points, obtain anomalous
exponents, and compute scaling functions describing the crossover near the
quantum-critical points. We argue that our findings are relevant to recent
experiments on impurity-doped d-wave superconductors.Comment: 4 pages, 3 figs; extended discussion of large-N spin representations,
added references; accepted for publication in PR
- …