938 research outputs found
Mean field approach to breakdown phenomena in disordered systems
Treballs Finals de Grau de FÃsica, Facultat de FÃsica, Universitat de Barcelona, Curs: 2018, Tutor: Antoni PlanesBreakdown phenomena can be understood as a phase transition that occur in athermal systems constituted of many interacting elements governed by the interplay between an external driving force and disorder. Representative examples are magnets driven by a magnetic field out of equilibrium and ruptures, in a stress driven fiber bundle. In this work we analyze critical behaviors of both cases within the mean field approach. The magnetic case has been compared with the Weiss model for a ferromagnet with no disorder at finite temperature. Finally, in the case of the fiber bundle model we show the existence of certain conditions for criticality to occu
Development of a Reference Wafer for On-Wafer Testing of Extreme Impedance Devices
This paper describes the design, fabrication, and testing of an on-wafer substrate that has been developed specifically for measuring extreme impedance devices using an on-wafer probe station. Such devices include carbon nano-tubes (CNTs) and structures based on graphene which possess impedances in the κ Ω range and are generally realised on the nano-scale rather than the micro-scale that is used for conventional on-wafer measurement. These impedances are far removed from the conventional 50- reference impedance of the test equipment. The on-wafer substrate includes methods for transforming from the micro-scale towards the nano-scale and reference standards to enable calibrations for extreme impedance devices. The paper includes typical results obtained from the designed wafer
A contextuality witness inspired by optimal state discrimination
Many protocols and tasks in quantum information science rely inherently on
the fundamental notion of contextuality to provide advantages over their
classical counterparts, and contextuality represents one of the main
differences between quantum and classical physics. In this work we present a
witness for preparation contextuality inspired by optimal two-state
discrimination. The main idea is based on finding the accessible averaged
success and error probabilities in both classical and quantum models. We can
then construct a noncontextuality inequality and associated witness which we
find to be robust against depolarising noise and loss in the form of
inconclusive events.Comment: 5 pages main text, 3 figures, 3 pages supplemental materia
Storage and Retrieval of a Microwave Field in a Spin Ensemble
We report the storage and retrieval of a small microwave field from a
superconducting resonator into collective excitations of a spin ensemble. The
spins are nitrogen-vacancy centers in a diamond crystal. The storage time of
the order of 30 ns is limited by inhomogeneous broadening of the spin ensemble.Comment: 4 pages + supplementary material. Submitted to PR
Contextual advantages and certification for maximum confidence discrimination
One of the most fundamental results in quantum information theory is that no
measurement can perfectly discriminate between non-orthogonal quantum states.
In this work, we investigate quantum advantages for discrimination tasks over
noncontextual theories by considering a maximum confidence measurement that
unifies different strategies of quantum state discrimination, including
minimum-error and unambiguous discrimination. We first show that maximum
confidence discrimination, as well as unambiguous discrimination, contains
contextual advantages. We then consider a semi-device independent scenario of
certifying maximum confidence measurement. The scenario naturally contains
undetected events, making it a natural setting to explore maximum confidence
measurements. We show that the certified maximum confidence in quantum theory
also contains contextual advantages. Our results establish how the advantages
of quantum theory over a classical model may appear in a realistic scenario of
a discrimination task
The Ground State Energy of Heavy Atoms According to Brown and Ravenhall: Absence of Relativistic Effects in Leading Order
It is shown that the ground state energy of heavy atoms is, to leading order,
given by the non-relativistic Thomas-Fermi energy. The proof is based on the
relativistic Hamiltonian of Brown and Ravenhall which is derived from quantum
electrodynamics yielding energy levels correctly up to order Ry
Essential spectra of difference operators on \sZ^n-periodic graphs
Let (\cX, \rho) be a discrete metric space. We suppose that the group
\sZ^n acts freely on and that the number of orbits of with respect to
this action is finite. Then we call a \sZ^n-periodic discrete metric
space. We examine the Fredholm property and essential spectra of band-dominated
operators on where is a \sZ^n-periodic discrete metric space.
Our approach is based on the theory of band-dominated operators on \sZ^n and
their limit operators.
In case is the set of vertices of a combinatorial graph, the graph
structure defines a Schr\"{o}dinger operator on in a natural way. We
illustrate our approach by determining the essential spectra of Schr\"{o}dinger
operators with slowly oscillating potential both on zig-zag and on hexagonal
graphs, the latter being related to nano-structures
Influence of pressure and radio frequency power on deposition rate and structural properties of hydrogenated amorphous silicon thin films prepared by plasma deposition
The influence of radio frequency (rf) power and pressure on deposition rate and structural properties of hydrogenated amorphous silicon (a-Si:H) thin films, prepared by rf glow discharge decomposition of silane, have been studied by phase modulated ellipsometry and Fourier transform infrared spectroscopy. It has been found two pressure regions separated by a threshold value around 20 Pa where the deposition rate increases suddenly. This behavior is more marked as rf power rises and reflects the transition between two rf discharges regimes. The best quality films have been obtained at low pressure and at low rf power but with deposition rates below 0.2 nm/s. In the high pressure region, the enhancement of deposition rate as rf power increases first gives rise to a reduction of film density and an increase of content of hydrogen bonded in polyhydride form because of plasma polymerization reactions. Further rise of rf power leads to a decrease of polyhydride bonding and the material density remains unchanged, thus allowing the growth of a-Si:H films at deposition rates above 1 nm/s without any important detriment of material quality. This overcoming of deposition rate limitation has been ascribed to the beneficial effects of ion bombardment on the a-Si:H growing surface by enhancing the surface mobility of adsorbed reactive species and by eliminating hydrogen bonded in polyhydride configurations
Competition between electric field and magnetic field noise in the decoherence of a single spin in diamond
We analyze the impact of electric field and magnetic field fluctuations in
the decoherence of the electronic spin associated with a single
nitrogen-vacancy (NV) defect in diamond by engineering spin eigenstates
protected either against magnetic noise or against electric noise. The
competition between these noise sources is analyzed quantitatively by changing
their relative strength through modifications of the environment. This study
provides significant insights into the decoherence of the NV electronic spin,
which is valuable for quantum metrology and sensing applications.Comment: 8 pages, 4 figures, including supplementary information
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