911 research outputs found
Variational transition state theory based surface chemistry for the C2H6/H2/O2/Pt system
A reaction class-based framework for the development of heterogeneous mechanisms is applied to study the (partial) oxidation of ethane over platinum. The rate parameters for the surface chemistry were derived using a systematic application of variational transition state theory (VTST) for adsorption, desorption, and EleyâRideal reactions coupled with two-dimensional (2D) collision theory for reactions occurring on the surface. The approach removes the need for the experimental determination of surface sticking coefficients and the associated major uncertainties. The barrier heights were determined using the unity bond indexâquadratic exponential potential (UBIâQEP) method. The combined gas- and surface-phase chemistry was evaluated against independent data sets obtained from three experimental configurations. The associated 18 cases cover a wide range of residence times, stoichiometries (0.1 < Ï < 10.4), and inlet pressures (1â12 bar). The work highlights the generality of the VTST approach that is shown to outperform the customary sticking coefficient-based methods for key aspects. A sensitivity analysis highlights the importance of the O2 and CO adsorption pathways
It's a gas: oxidative dehydrogenation of propane over boron nitride catalysts
Boron nitride and related boron-containing materials have recently been suggested as very promising catalysts in the oxidative dehydrogenation of propane. The high selectivity toward propylene at comparably high conversion significantly exceeds the performance of established vanadium-based catalysts. In the current work we show that the high selectivity toward propylene and ethylene is fully consistent with a gas-phase conversion mechanism and that it can be modeled reasonably well by the recent detailed microkinetic reaction mechanism of Hashemi and co-workers [ Proc. Combust. Inst. 2019, 37, 461]. Our analysis, using six heterogeneous catalytic reaction pathways, each representing a hypothetical limit case, shows that the boron nitride catalyst is responsible for initiating the gas-phase chemistry. The increased conversion of propane in cases with water cofeed, as well as the trends in the selectivities of minor species upon dilution of the catalytic bed and upon varying the C3H8/O2 inlet ratio, as observed by Venegas and Hermans [ Org. Process Res. Dev. 2018, 22, 1644], are here explained as gas-phase phenomena. Hence, the oxidative dehydrogenation of propane over boron nitride catalysts is an example of a coupled gas and catalytic chemistry system. The current work also highlights the importance of modeling of the complete heated zone, including the rear heat shields and reactor padding if present
A Weakly Supervised Deep Learning Approach for Detecting Malaria and Sickle Cells in Blood Films
Machine vision analysis of blood films imaged under a brightfield microscope could provide scalable malaria diagnosis solutions in resource constrained endemic urban settings. The major bottleneck in successfully analyzing blood films with deep learning vision techniques is a lack of object-level annotations of disease markers such as parasites or abnormal red blood cells. To overcome this challenge, this work proposes a novel deep learning supervised approach that leverages weak labels readily available from routine clinical microscopy to diagnose malaria in thick blood film microscopy. This approach is based on aggregating the convolutional features of multiple objects present in one hundred high resolution image fields. We show that this method not only achieves expert-level malaria diagnostic accuracy without any hard object-level labels but can also identify individual malaria parasites in digitized thick blood films, which is useful in assessing disease severity and response to treatment. We demonstrate another application scenario where our approach is able to detect sickle cells in thin blood films. We discuss the wider applicability of the approach in automated analysis of thick blood films for the diagnosis of other blood disorders
Realisation of a programmable two-qubit quantum processor
The universal quantum computer is a device capable of simulating any physical
system and represents a major goal for the field of quantum information
science. Algorithms performed on such a device are predicted to offer
significant gains for some important computational tasks. In the context of
quantum information, "universal" refers to the ability to perform arbitrary
unitary transformations in the system's computational space. The combination of
arbitrary single-quantum-bit (qubit) gates with an entangling two-qubit gate is
a gate set capable of achieving universal control of any number of qubits,
provided that these gates can be performed repeatedly and between arbitrary
pairs of qubits. Although gate sets have been demonstrated in several
technologies, they have as yet been tailored toward specific tasks, forming a
small subset of all unitary operators. Here we demonstrate a programmable
quantum processor that realises arbitrary unitary transformations on two
qubits, which are stored in trapped atomic ions. Using quantum state and
process tomography, we characterise the fidelity of our implementation for 160
randomly chosen operations. This universal control is equivalent to simulating
any pairwise interaction between spin-1/2 systems. A programmable multi-qubit
register could form a core component of a large-scale quantum processor, and
the methods used here are suitable for such a device.Comment: 7 pages, 4 figure
Demon-like Algorithmic Quantum Cooling and its Realization with Quantum Optics
The simulation of low-temperature properties of many-body systems remains one
of the major challenges in theoretical and experimental quantum information
science. We present, and demonstrate experimentally, a universal cooling method
which is applicable to any physical system that can be simulated by a quantum
computer. This method allows us to distill and eliminate hot components of
quantum states, i.e., a quantum Maxwell's demon. The experimental
implementation is realized with a quantum-optical network, and the results are
in full agreement with theoretical predictions (with fidelity higher than
0.978). These results open a new path for simulating low-temperature properties
of physical and chemical systems that are intractable with classical methods.Comment: 7 pages, 5 figures, plus supplementarity material
A Rydberg Quantum Simulator
Following Feynman and as elaborated on by Lloyd, a universal quantum
simulator (QS) is a controlled quantum device which reproduces the dynamics of
any other many particle quantum system with short range interactions. This
dynamics can refer to both coherent Hamiltonian and dissipative open system
evolution. We investigate how laser excited Rydberg atoms in large spacing
optical or magnetic lattices can provide an efficient implementation of a
universal QS for spin models involving (high order) n-body interactions. This
includes the simulation of Hamiltonians of exotic spin models involving
n-particle constraints such as the Kitaev toric code, color code, and lattice
gauge theories with spin liquid phases. In addition, it provides the
ingredients for dissipative preparation of entangled states based on
engineering n-particle reservoir couplings. The key basic building blocks of
our architecture are efficient and high-fidelity n-qubit entangling gates via
auxiliary Rydberg atoms, including a possible dissipative time step via optical
pumping. This allows to mimic the time evolution of the system by a sequence of
fast, parallel and high-fidelity n-particle coherent and dissipative Rydberg
gates.Comment: 8 pages, 4 figure
The sudden change phenomenon of quantum discord
Even if the parameters determining a system's state are varied smoothly, the
behavior of quantum correlations alike to quantum discord, and of its classical
counterparts, can be very peculiar, with the appearance of non-analyticities in
its rate of change. Here we review this sudden change phenomenon (SCP)
discussing some important points related to it: Its uncovering,
interpretations, and experimental verifications, its use in the context of the
emergence of the pointer basis in a quantum measurement process, its appearance
and universality under Markovian and non-Markovian dynamics, its theoretical
and experimental investigation in some other physical scenarios, and the
related phenomenon of double sudden change of trace distance discord. Several
open questions are identified, and we envisage that in answering them we will
gain significant further insight about the relation between the SCP and the
symmetry-geometric aspects of the quantum state space.Comment: Lectures on General Quantum Correlations and their Applications, F.
F. Fanchini, D. O. Soares Pinto, and G. Adesso (Eds.), Springer (2017), pp
309-33
Quantum dynamics in strong fluctuating fields
A large number of multifaceted quantum transport processes in molecular
systems and physical nanosystems can be treated in terms of quantum relaxation
processes which couple to one or several fluctuating environments. A thermal
equilibrium environment can conveniently be modelled by a thermal bath of
harmonic oscillators. An archetype situation provides a two-state dissipative
quantum dynamics, commonly known under the label of a spin-boson dynamics. An
interesting and nontrivial physical situation emerges, however, when the
quantum dynamics evolves far away from thermal equilibrium. This occurs, for
example, when a charge transferring medium possesses nonequilibrium degrees of
freedom, or when a strong time-dependent control field is applied externally.
Accordingly, certain parameters of underlying quantum subsystem acquire
stochastic character. Herein, we review the general theoretical framework which
is based on the method of projector operators, yielding the quantum master
equations for systems that are exposed to strong external fields. This allows
one to investigate on a common basis the influence of nonequilibrium
fluctuations and periodic electrical fields on quantum transport processes.
Most importantly, such strong fluctuating fields induce a whole variety of
nonlinear and nonequilibrium phenomena. A characteristic feature of such
dynamics is the absence of thermal (quantum) detailed balance.Comment: review article, Advances in Physics (2005), in pres
TESS hunt for young and maturing exoplanets (THYME). VI. an 11 Myr giant planet transiting a very-low-mass star in lower centaurus crux
Mature super-Earths and sub-Neptunes are predicted to be â Jovian radius when younger than 10 Myr. Thus, we expect to find 5â15 Râ planets around young stars even if their older counterparts harbor none. We report the discovery and validation of TOI 1227b, a 0.85 ± 0.05 RJ (9.5 Râ) planet transiting a very-low-mass star (0.170 ± 0.015 Mâ) every 27.4 days. TOI 1227's kinematics and strong lithium absorption confirm that it is a member of a previously discovered subgroup in the Lower Centaurus Crux OB association, which we designate the Musca group. We derive an age of 11 ± 2 Myr for Musca, based on lithium, rotation, and the colorâmagnitude diagram of Musca members. The TESS data and ground-based follow-up show a deep (2.5%) transit. We use multiwavelength transit observations and radial velocities from the IGRINS spectrograph to validate the signal as planetary in nature, and we obtain an upper limit on the planet mass of â0.5 MJ. Because such large planets are exceptionally rare around mature low-mass stars, we suggest that TOI 1227b is still contracting and will eventually turn into one of the more common <5 Râ planets
- âŠ