842 research outputs found
Dissipative mean-field theory of IBM utility experiment
In spite of remarkable recent advances, quantum computers have not yet found
any useful applications. A promising direction for such utility is offered by
the simulation of the dynamics of many-body quantum systems, which cannot be
efficiently computed classically. Recently, IBM used a superconducting quantum
computer to simulate a kicked quantum Ising model for large numbers of qubits
and time steps. By employing powerful error mitigation techniques, they were
able to obtain an excellent agreement with the exact solution of the model.
This result is very surprising, considering that the total error accumulated by
the circuit is prohibitively large. In this letter, we address this paradox by
introducing a dissipative mean-field approximation based on Kraus operators.
Our effective theory reproduces the many-body unitary dynamics and matches
quantitatively local and non-local observables. These findings demonstrate that
the observed dynamics is equivalent to a single qubit undergoing rotations and
dephasing. Our emergent description can explain the success of the quantum
computer in solving this specific problem.Comment: 4 pages, 2 figure
Acid catalyzed synthesis of dimethyl isosorbide via dimethyl carbonate chemistry
Dimethyl isosorbide (DMI) is a bio-based solvent that can be used as green alternative for conventional dipolar
media (dimethyl sulfoxide, dimethylformamide, and dimethylacetamide). The main synthetic procedures to DMI
reported in the literature are based on the methylation of isosorbide employing different alkylating agents
including toxic halogen compounds such as alkyl halides. A more sustainable alternative would be to employ
dimethyl carbonate (DMC), a well-known green reagent and solvent, considered one of the most promising
methylating agents for its good biodegradability and low toxicity. Indeed, in recent years, DMC-promoted
methylation of isosorbide has been extensively exploited although mostly in the presence of a base or an
amphoteric catalyst. In this work, we report for the first time a comprehensive investigation on the synthesis of
DMI via DMC chemistry promoted by heterogeneous acid catalyst (Amberlyst-36 and Purolite CT275DR). Re-
action conditions were optimized and then applied for the methylation of isosorbide and its epimers, isoidide and
isomannide. Considerations on the related reaction mechanism were reported highlighting the difference in the
preferred reaction pathways among this new synthetic approach and the previously reported base-catalyzed
procedures
Dissipative Preparation of Spin Squeezed Atomic Ensembles in a Steady State
We present and analyze a new approach for the generation of atomic spin
squeezed states. Our method involves the collective coupling of an atomic
ensemble to a decaying mode of an open optical cavity. We demonstrate the
existence of a collective atomic dark-state, decoupled from the radiation
field. By explicitly constructing this state we find that it can feature spin
squeezing bounded only by the Heisenberg limit. We show that such dark states
can be deterministically prepared via dissipative means, thus turning
dissipation into a resource for entanglement. The scaling of the phase
sensitivity taking realistic imperfections into account is discussed.Comment: 5 pages, 4 figure
The interaction of aluminum with catecholamine-based neurotransmitters: Can the formation of these species be considered a potential risk factor for neurodegenerative diseases?
The potential neurotoxic role of Al(iii) and its proposed link with the insurgence of Alzheimer's Disease (AD) have attracted increasing interest towards the determination of the nature of bioligands that are propitious to interact with aluminum. Among them, catecholamine-based neurotransmitters have been proposed to be sensitive to the presence of this non-essential metal ion in the brain. In the present work, we characterize several aluminum-catecholamine complexes in various stoichiometries, determining their structure and thermodynamics of formation. For this purpose, we apply a recently validated computational protocol with results that show a remarkably good agreement with the available experimental data. In particular, we employ Density Functional Theory (DFT) in conjunction with continuum solvation models to calculate complexation energies of aluminum for a set of four important catecholamines: l-DOPA, dopamine, noradrenaline and adrenaline. In addition, by means of the Quantum Theory of Atoms in Molecules (QTAIM) and Energy Decomposition Analysis (EDA) we assessed the nature of the Al-ligand interactions, finding mainly ionic bonds with an important degree of covalent character. Our results point at the possibility of the formation of aluminum-catecholamine complexes with favorable formation energies, even when proton/aluminum competition is taken into account. Indeed, we found that these catecholamines are better aluminum binders than catechol at physiological pH, because of the electron withdrawing effect of the positively-charged amine that decreases their deprotonation penalty with respect to catechol. However, overall, our results show that, in an open biological environment, the formation of Al-catecholamine complexes is not thermodynamically competitive when compared with the formation of other aluminum species in solution such as Al-hydroxide, or when considering other endogenous/exogenous Al(iii) ligands such as citrate, deferiprone and EDTA. In summary, we rule out the possibility, suggested by some authors, that the formation of Al-catecholamine complexes in solution might be behind some of the toxic roles attributed to aluminum in the brain. An up-to-date view of the catecholamine biosynthesis pathway with sites of aluminum interference (according to the current literature) is presented. Alternative mechanisms that might explain the deleterious effects of this metal on the catecholamine route are thoroughly discussed, and new hypotheses that should be investigated in future are proposed
RHIP, a Radio-controlled High-Voltage Insulated Picoammeter and its usage in studying ion backflow in MPGD-based photon detectors
A picoammeter system has been developed and engineering. It consists in a
current-voltage converter, based on an operational amplifier with very low
input current, a high precision ADC, a radio controlled data acquisition unit
and the computer-based control, visualization and storage. The precision is of
the order of a tenth of picoampers and it can measure currents between
electrodes at potentials up to 8 kV. The system is battery powered and a number
of strategies have been implemented to limit the power consumption. The system
is designed for multichannel applications, up to 256 parallel channels. The
overall implementation is cost-effective to make the availability of
multichannel setups easily affordable. The design, implementation and
performance of the picoammeter system are described in detail as well as a an
application: the measurement of ion backflow in MPGD-based photon detectors.Comment: 5th International Conference on Micro-Pattern Gas Detectors
(MPGD2017), presentation by Silvia Dalla Torr
Study of MicroPattern Gaseous detectors with novel nanodiamond based photocathodes for single photon detection in EIC RICH
Identification of high momentum hadrons at the future EIC is crucial, gaseous
RICH detectors are therefore viable option. Compact collider setups impose to
construct RICHes with small radiator length, hence significantly limiting the
number of detected photons. More photons can be detected in the far UV region,
using a windowless RICH approach. QE of CsI degrades under strong irradiation
and air contamination. Nanodiamond based photocathodes (PCs) are being
developed as an alternative to CsI. Recent development of layers of
hydrogenated nanodiamond powders as an alternative photosensitive material and
their performance, when coupled to the THick Gaseous Electron Multipliers
(THGEM)-based detectors, are the objects of an ongoing R\&D. We report about
the initial phase of our studies.Comment: 3 pages, 5 figures, RICH2018 conference proceedin
Introduction to the Dicke model : from equilibrium to nonequilibrium, and vice versa
P.K. acknowledges support from EPSRC (EP/M010910/1) and the Austrian Academy of Sciences (ÖAW). P.K. and J.K. acknowledge support from EPSRC program “Hybrid Polaritonics” (EP/M025330/1).The Dicke model describes the coupling between a quantized cavity field and a large ensemble of two-level atoms. When the number of atoms tends to infinity, this model can undergo a transition to a superradiant phase, belonging to the mean-field Ising universality class. The superradiant transition was first predicted for atoms in thermal equilibrium, but its experimental realizations required driven-dissipative systems. In this Progress Report, we offer an introduction to some theoretical concepts relevant to the Dicke model, reviewing the critical properties of the superradiant phase transition, and the distinction between equilibrium and nonequilibrium conditions. In addition, we explain the fundamental difference between the superradiant phase transition and the more common lasing transition. Our report mostly focuses on the steady states of single-mode optical cavities, but we also mention some aspects of real-time dynamics, as well as applications to multimode cavities, superconducting circuits, and trapped ions.PostprintPeer reviewe
Rise and fall of hidden string order of lattice bosons
We investigate the ground state properties of a newly discovered phase of one
dimensional lattice bosons with extended interactions (see E. G. Dalla Torre et
al., Phys. Rev. Lett. \textbf{97}, 260401 (2006)). The new phase, termed the
Haldane Insulator (HI) in analogy with the gapped phase of spin-1 chains, is
characterized by a non local order parameter, which can only be written as an
infinite string in terms of the bosonic densities. We show that the string
order can nevertheless be probed with physical fields that couple locally, via
the effect those fields have on the quantum phase transitions separating the
exotic phase from the conventional Mott and density wave phases. Using a field
theoretical analysis we show that a perturbation which breaks lattice inversion
symmetry gaps the critical point separating the Mott and Haldane phases and
eliminates the sharp distinction between them. This is remarkable given that
neither of these phases involves broken inversion symmetry. We also investigate
the evolution of the phase diagram with the tunable coupling between parallel
chains in an optical lattice setup. We find that inter-chain tunneling destroys
the direct phase transition between the Mott and Haldane insulators by
establishing an intermediate superfluid phase. On the other hand coupling the
chains only by weak repulsive interactions does not modify the structure of the
phase diagram. The theoretical predictions are confirmed with numerical
calculations using the Density Matrix Renormalization Group (DMRG).Comment: 15 pages, 8 figures. Minor corrections, published versio
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