1,034 research outputs found
Weak values are quantum: you can bet on it
The outcome of a weak quantum measurement conditioned to a subsequent
postselection (a weak value protocol) can assume peculiar values. These results
cannot be explained in terms of conditional probabilistic outcomes of
projective measurements. However, a classical model has been recently put
forward that can reproduce peculiar expectation values, reminiscent of weak
values. This led the authors of that work to claim that weak values have an
entirely classical explanation. Here we discuss what is quantum about weak
values with the help of a simple model based on basic quantum mechanics. We
first demonstrate how a classical theory can indeed give rise to non-trivial
conditional values, and explain what features of weak values are genuinely
quantum. We finally use our model to outline some main issues under current
research.Comment: 6 pages, 1 figur
The Interplay of Spin and Charge Channels in Zero Dimensional Systems
We present a full fledged quantum mechanical treatment of the interplay
between the charge and the spin zero-mode interactions in quantum dots. Quantum
fluctuations of the spin-mode suppress the Coulomb blockade and give rise to
non-monotonic behavior near this point. They also greatly enhance the dynamic
spin susceptibility. Transverse fluctuations become important as one approaches
the Stoner instability. The non-perturbative effects of zero-mode interaction
are described in terms of charge (U(1)) and spin (SU(2)) gauge bosons.Comment: 4.5 pages, 2 figure
The Interplay of Charge and Spin in Quantum Dots: The Ising Case
The physics of quantum dots is succinctly depicted by the {\it Universal
Hamiltonian}, where only zero mode interactions are included. In the case where
the latter involve charging and isotropic spin-exchange terms, this would lead
to a non-Abelian action. Here we address an Ising spin-exchange interaction,
which leads to an Abelian action. The analysis of this simplified yet
non-trivial model shed some light on a more general case of charge and spin
entanglement. We present a calculation of the tunneling density of states and
of the dynamic magnetic susceptibility. Our results are amenable to
experimental study and may allow for an experimental determination of the
exchange interaction strength.Comment: 11 pages, 7 figure
U(1) and SU(2) quantum dissipative systems: The Caldeira-Leggett vs. the Amegaokar-Eckern-Sch\"on approaches
There are two paradigmatic frameworks for treating quantum systems coupled to
a dissipative environment: the Caldeira-Leggett and the
Ambegaokar-Eckern-Sch\"on approaches. Here we recall the differences between
them, and explain the consequences when each is applied to a zero dimensional
spin (possessing an SU(2) symmetry) in a dissipative environment (a dissipative
quantum dot near or beyond the Stoner instability point).Comment: Contribution for Leonid Keldysh 85 Festschrif
Proteasome Lid Bridges Mitochondrial Stress with Cdc53/Cullin1 NEDDylation Status
Cycles of Cdc53/Cullin1 rubylation (a.k.a NEDDylation) protect ubiquitin-E3 SCF (Skp1-Cullin1-F-box protein) complexes from self-destruction and play an important role in mediating the ubiquitination of key protein substrates involved in cell cycle progression, development, and survival. Cul1 rubylation is balanced by the COP9 signalosome (CSN), a multi-subunit derubylase that shows 1:1 paralogy to the 26 S proteasome lid. The turnover of SCF substrates and their relevance to various diseases is well studied, yet, the extent by which environmental perturbations influence Cul1 rubylation/derubylation cycles per se is still unclear. In this study, we show that the level of cellular oxidation serves as a molecular switch, determining Cullin1 rubylation/derubylation ratio. We describe a mutant of the proteasome lid subunit, Rpn11 that exhibits accumulated levels of Cullin1-Rub1 conjugates, a characteristic phenotype of csn mutants. By dissecting between distinct phenotypes of rpn11 mutants, proteasome and mitochondria dysfunction, we were able to recognize the high reactive oxygen species (ROS) production during the transition of cells into mitochondrial respiration, as a checkpoint of Cullin1 rubylation in a reversible manner. Thus, the study adds the rubylation cascade to the list of cellular pathways regulated by redox homeostasis
Metric characterization of cluster dynamics on the Sierpinski gasket
We develop and implement an algorithm for the quantitative characterization
of cluster dynamics occurring on cellular automata defined on an arbitrary
structure. As a prototype for such systems we focus on the Ising model on a
finite Sierpsinski Gasket, which is known to possess a complex thermodynamic
behavior. Our algorithm requires the projection of evolving configurations into
an appropriate partition space, where an information-based metrics (Rohlin
distance) can be naturally defined and worked out in order to detect the
changing and the stable components of clusters. The analysis highlights the
existence of different temperature regimes according to the size and the rate
of change of clusters. Such regimes are, in turn, related to the correlation
length and the emerging "critical" fluctuations, in agreement with previous
thermodynamic analysis, hence providing a non-trivial geometric description of
the peculiar critical-like behavior exhibited by the system. Moreover, at high
temperatures, we highlight the existence of different time scales controlling
the evolution towards chaos.Comment: 20 pages, 8 figure
Spin and Charge Correlations in Quantum Dots: An Exact Solution
The inclusion of charging and spin-exchange interactions within the Universal
Hamiltonian description of quantum dots is challenging as it leads to a
non-Abelian action. Here we present an {\it exact} analytical solution of the
probem, in particular, in the vicinity of the Stoner instabilty point. We
calculate several observables, including the tunneling density of states (TDOS)
and the spin susceptibility. Near the instability point the TDOS exhibits a
non-monotonous behavior as function of the tunneling energy, even at
temperatures higher than the exchange energy. Our approach is generalizable to
a broad set of observables, including the a.c. susceptibility and the
absorption spectrum for anisotropic spin interaction. Our results could be
tested in nearly ferromagnetic materials.Comment: JETPL class, 6 pages, 2 figure
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