15,851 research outputs found
Magnetic Collective Mode Dispersion in High Temperature Superconductors
Recent neutron scattering experiments in the superconducting state of YBCO
have been interpreted in terms of a magnetic collective mode whose dispersion
relative to the commensurate wavevector has a curvature opposite in sign to a
conventional magnon dispersion. The purpose of this article is to demonstrate
that simple linear response calculations are in support of a collective mode
interpretation, and to explain why the dispersion has the curvature it does.Comment: 3 pages, revtex, 4 encapsulated postscript figure
Randomized Benchmarking as Convolution: Fourier Analysis of Gate Dependent Errors
We provide an alternative proof of Wallman's [Quantum 2, 47 (2018)] and
Proctor's [Phys. Rev. Lett. 119, 130502 (2017)] bounds on the effect of
gate-dependent noise on randomized benchmarking (RB). Our primary insight is
that a RB sequence is a convolution amenable to Fourier space analysis, and we
adopt the mathematical framework of Fourier transforms of matrix-valued
functions on groups established in recent work from Gowers and Hatami [Sbornik:
Mathematics 208, 1784 (2017)]. We show explicitly that as long as our faulty
gate-set is close to some representation of the Clifford group, an RB sequence
is described by the exponential decay of a process that has exactly two
eigenvalues close to one and the rest close to zero. This framework also allows
us to construct a gauge in which the average gate-set error is a depolarizing
channel parameterized by the RB decay rates, as well as a gauge which maximizes
the fidelity with respect to the ideal gate-set
Exchange-Only Dynamical Decoupling in the 3-Qubit Decoherence Free Subsystem
The Uhrig dynamical decoupling sequence achieves high-order decoupling of a
single system qubit from its dephasing bath through the use of bang-bang Pauli
pulses at appropriately timed intervals. High-order decoupling of single and
multiple qubit systems from baths causing both dephasing and relaxation can
also be achieved through the nested application of Uhrig sequences, again using
single-qubit Pauli pulses. For the 3-qubit decoherence free subsystem (DFS) and
related subsystem encodings, Pauli pulses are not naturally available
operations; instead, exchange interactions provide all required encoded
operations. Here we demonstrate that exchange interactions alone can achieve
high-order decoupling against general noise in the 3-qubit DFS. We present
decoupling sequences for a 3-qubit DFS coupled to classical and quantum baths
and evaluate the performance of the sequences through numerical simulations
On Measuring Condensate Fraction in Superconductors
An analysis of off-diagonal long-range order in superconductors shows that
the spin-spin correlation function is significantly influenced by the order if
the order parameter is anisotropic on a microscopic scale. Thus, magnetic
neutron scattering can provide a direct measurement of the condensate fraction
of a superconductor. It is also argued that recent measurements in high
temperature superconductors come very close to achieving this goal.Comment: 4 pages, 1 eps figure, RevTex. A new possibility in the underdoped
regime is added. Other corrections are mino
Leptogenesis from Soft Supersymmetry Breaking (Soft Leptogenesis)
Soft leptogenesis is a scenario in which the cosmic baryon asymmetry is
produced from a lepton asymmetry generated in the decays of heavy sneutrinos
(the partners of the singlet neutrinos of the seesaw) and where the relevant
sources of CP violation are the complex phases of soft supersymmetry-breaking
terms. We explain the motivations for soft leptogenesis, and review its basic
ingredients: the different CP-violating contributions, the crucial role played
by thermal corrections, and the enhancement of the efficiency from lepton
flavour effects. We also discuss the high temperature regime GeV in
which the cosmic baryon asymmetry originates from an initial asymmetry of an
anomalous -charge, and soft leptogenesis reembodies in -genesis.Comment: References updated. Some minor corrections to match the published
versio
Quantum Impurities and the Neutron Resonance Peak in : Ni versus Zn
The influence of magnetic (S=1) and nonmagnetic (S=0) impurities on the spin
dynamics of an optimally doped high temperature superconductor is compared in
two samples with almost identical superconducting transition temperatures:
YBa(CuNi)O (T=80 K) and
YBa(CuZn)O (T=78 K). In the Ni-substituted
system, the magnetic resonance peak (which is observed at E40 meV in
the pure system) shifts to lower energy with a preserved E/T ratio
while the shift is much smaller upon Zn substitution. By contrast Zn, but not
Ni, restores significant spin fluctuations around 40 meV in the normal state.
These observations are discussed in the light of models proposed for the
magnetic resonance peak.Comment: 3 figures, submitted to PR
A "kilonova" associated with short-duration gamma-ray burst 130603B
Short-duration gamma-ray bursts (SGRBs) are intense flashes of cosmic
gamma-rays, lasting less than ~2 s, whose origin is one of the great unsolved
questions of astrophysics today. While the favoured hypothesis for their
production, a relativistic jet created by the merger of two compact stellar
objects (specifically, two neutron stars, NS-NS, or a neutron star and a black
hole, NS-BH), is supported by indirect evidence such as their host galaxy
properties, unambiguous confirmation of the model is still lacking. Mergers of
this kind are also expected to create significant quantities of neutron-rich
radioactive species, whose decay should result in a faint transient in the days
following the burst, a so-called "kilonova". Indeed, it is speculated that this
mechanism may be the predominant source of stable r-process elements in the
Universe. Recent calculations suggest much of the kilonova energy should appear
in the near-infrared (nIR) due to the high optical opacity created by these
heavy r-process elements. Here we report strong evidence for such an event
accompanying SGRB 130603B. If this simplest interpretation of the data is
correct, it provides (i) support for the compact object merger hypothesis of
SGRBs, (ii) confirmation that such mergers are likely sites of significant
r-process production and (iii) quite possibly an alternative, un-beamed
electromagnetic signature of the most promising sources for direct detection of
gravitational waves.Comment: preprint of paper appearing in Nature (3 Aug 2013
A New Normal: How COVID-19 and Digital Contact Tracing Highlight a Need for New Fourth Amendment Norms
Contact tracing helps epidemiologists identify individuals who have been exposed to a virus. Manual contact tracing has been used for decades to interrupt the transmission of disease and reduce the number of infections within a population. It is a pillar of disease control. But the manual process has certain limitations—it is time-intensive, expensive, and subject to human error. Digital contact tracing overcomes these limitations. Using GPS and Bluetooth technologies, digital contact tracing applications automate and expedite the tracing and notification processes, with life-saving implications. In 2020, countries that implemented contact tracing technology in response to COVID-19 contained outbreaks, minimized incidence of the virus, and kept death tolls comparatively low.
Notwithstanding the urgent public health need COVID-19 created, privacy-minded Americans were and continue to be resistant to digital contact tracing. Instead of widespread adoption of the technology, there is widespread concern that data collected via contact tracing apps will be co-opted, de-anonymized, and used by law enforcement for non-public health purposes.
Is this concern warranted? Can the government demand a record of your location data from Apple and Google without implicating your Fourth Amendment rights? Can it secure this data without a warrant or probable cause? The answer to all these questions is, most likely, yes. Although the Fourth Amendment limits the government’s search and seizure powers, Americans who opt to use contact tracing apps—for the sake of their health and the public health at large—position themselves outside the bounds of Fourth Amendment protections. In other words, Americans can choose health or privacy, but not both.
Surely, that should not be our norm. We need a new normal. This Comment, therefore, discusses how jurisprudence fails to protect the rights of U.S. citizens using contact tracing applications. It details the current Fourth Amendment tests and doctrines, including the Katz test (which centers around reasonable expectations of privacy) and the third-party doctrine (which says a person has no legitimate expectation of privacy in information supplied to third parties). Given the public health benefits of an effective contact tracing system, this Comment considers why changes to the Fourth Amendment framework—ones that accommodate the competing privacy and welfare needs of the twenty-first century—are warranted. Ultimately, this Comment proposes that the Supreme Court eliminate the Katz test and overturn the third-party doctrine to extend Fourth Amendment protections to information like location data captured by life-saving technologies
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