9 research outputs found
A Stronger Theorem Against Macro-realism
Macro-realism is the position that certain "macroscopic" observables must
always possess definite values: e.g. the table is in some definite position,
even if we don't know what that is precisely. The traditional understanding is
that by assuming macro-realism one can derive the Leggett-Garg inequalities,
which constrain the possible statistics from certain experiments. Since quantum
experiments can violate the Leggett-Garg inequalities, this is taken to rule
out the possibility of macro-realism in a quantum universe. However, recent
analyses have exposed loopholes in the Leggett-Garg argument, which allow many
types of macro-realism to be compatible with quantum theory and hence violation
of the Leggett-Garg inequalities. This paper takes a different approach to
ruling out macro-realism and the result is a no-go theorem for macro-realism in
quantum theory that is stronger than the Leggett-Garg argument. This approach
uses the framework of ontological models: an elegant way to reason about
foundational issues in quantum theory which has successfully produced many
other recent results, such as the PBR theorem.Comment: Accepted journal version. 10 + 7 pages, 1 figur
No -epistemic model can fully explain the indistinguishability of quantum states
According to a recent no-go theorem (M. Pusey, J. Barrett and T. Rudolph,
Nature Physics 8, 475 (2012)), models in which quantum states correspond to
probability distributions over the values of some underlying physical variables
must have the following feature: the distributions corresponding to distinct
quantum states do not overlap. This is significant because if the distributions
do not overlap, then the quantum state itself is encoded by the physical
variables. In such a model, it cannot coherently be maintained that the quantum
state merely encodes information about underlying physical variables. The
theorem, however, considers only models in which the physical variables
corresponding to independently prepared systems are independent. This work
considers models that are defined for a single quantum system of dimension ,
such that the independence condition does not arise. We prove a result in a
similar spirit to the original no-go theorem, in the form of an upper bound on
the extent to which the probability distributions can overlap, consistently
with reproducing quantum predictions. In particular, models in which the
quantum overlap between pure states is equal to the classical overlap between
the corresponding probability distributions cannot reproduce the quantum
predictions in any dimension . The result is noise tolerant, and an
experiment is motivated to distinguish the class of models ruled out from
quantum theory.Comment: 5+5 page
Does a Computer have an Arrow of Time?
In [Sch05a], it is argued that Boltzmann's intuition, that the psychological
arrow of time is necessarily aligned with the thermodynamic arrow, is correct.
Schulman gives an explicit physical mechanism for this connection, based on the
brain being representable as a computer, together with certain thermodynamic
properties of computational processes. [Haw94] presents similar, if briefer,
arguments. The purpose of this paper is to critically examine the support for
the link between thermodynamics and an arrow of time for computers. The
principal arguments put forward by Schulman and Hawking will be shown to fail.
It will be shown that any computational process that can take place in an
entropy increasing universe, can equally take place in an entropy decreasing
universe. This conclusion does not automatically imply a psychological arrow
can run counter to the thermodynamic arrow. Some alternative possible explana-
tions for the alignment of the two arrows will be briefly discussed.Comment: 31 pages, no figures, publication versio
Running CMS software on GRID Testbeds
Starting in the middle of November 2002, the CMS experiment undertook an evaluation of the European DataGrid Project (EDG) middleware using its event simulation programs. A joint CMS-EDG task force performed a "stress test" by submitting a large number of jobs to many distributed sites. The EDG testbed was complemented with additional CMS-dedicated resources. A total of ~ 10000 jobs consisting of two different computational types were submitted from four different locations in Europe over a period of about one month. Nine sites were active, providing integrated resources of more than 500 CPUs and about 5 TB of disk space (with the additional use of two Mass Storage Systems). Descriptions of the adopted procedures, the problems encountered and the corresponding solutions are reported. Results and evaluations of the test, both from the CMS and the EDG perspectives, are described
Coordination properties of dithiobutylamine (DTBA), a newly introduced protein disulfide reducing agent
The acid-base properties and metal-binding abilities of (2S)-2-amino-1,4-dimercaptobutane, otherwise termed dithiobutylamine (DTBA), which is a newly introduced reagent useful for reducing protein and peptide disulfides, were studied in solution using potentiometry, (1)H NMR spectroscopy, spectropolarimetry, and UV-vis spectroscopy. The list of metal ions studied here includes Zn(II), Cd(II), Ni(II), Co(II), and Cu(I). We found that DTBA forms specific and very stable polynuclear and mononuclear complexes with all of these metal ions using both of its sulfur donors. DTBA forms complexes more stable than those of the commonly used disulfide reducing agent DTT, giving it more interference capacity in studies of metal binding in thiol-containing biomolecules. The ability of DTBA to strongly bind metal ions is reflected in its limited properties as a thiol protectant in their presence, which is manifested through slower disulfide reduction kinetics. We found that this effect correlated with the stabilities of the complexes. Additionally, the reducing properties of DTBA toward MMTS-modified papain (MMTS = S-methylmethanethiosulfonate) were also significantly affected by the investigated metal ions. In this case, however, electrostatic interactions and stereospecific effects, rather than metal-binding abilities, were found to be responsible for the reduced protective properties of DTBA. Despite its limitations, a high affinity toward metal ions makes DTBA an attractive agent in competition studies with metalloproteins