143 research outputs found
Reaching Agreement in Quantum Hybrid Networks
We consider a basic quantum hybrid network model consisting of a number of nodes each holding a qubit, for which the aim is to drive the network to a consensus in the sense that all qubits reach a common state. Projective measurements are applied serving as control means, and the measurement results are exchanged among the nodes via classical communication channels. In this way the quantum-opeartion/classical-communication nature of hybrid quantum networks is captured, although coherent states and joint operations are not taken into consideration in order to facilitate a clear and explicit analysis. We show how to carry out centralized optimal path planning for this network with all-to-all classical communications, in which case the problem becomes a stochastic optimal control problem with a continuous action space. To overcome the computation and communication obstacles facing the centralized solutions, we also develop a distributed Pairwise Qubit Projection (PQP) algorithm, where pairs of nodes meet at a given time and respectively perform measurements at their geometric average. We show that the qubit states are driven to a consensus almost surely along the proposed PQP algorithm, and that the expected qubit density operators converge to the average of the network’s initial values.The work of B.L. is partially supported by National Natural Science Foundation of China (NSFC) under Grant
11301518 and the National Center for Mathematics and Interdisciplinary Sciences, Chinese Academy of Sciences
SuperScaler: Supporting Flexible DNN Parallelization via a Unified Abstraction
With the growing model size, deep neural networks (DNN) are increasingly
trained over massive GPU accelerators, which demands a proper parallelization
plan that transforms a DNN model into fine-grained tasks and then schedules
them to GPUs for execution. Due to the large search space, the contemporary
parallelization plan generators often rely on empirical rules that couple
transformation and scheduling, and fall short in exploring more flexible
schedules that yield better memory usage and compute efficiency. This tension
can be exacerbated by the emerging models with increasing complexity in their
structure and model size. SuperScaler is a system that facilitates the design
and generation of highly flexible parallelization plans. It formulates the plan
design and generation into three sequential phases explicitly: model
transformation, space-time scheduling, and data dependency preserving. Such a
principled approach decouples multiple seemingly intertwined factors and
enables the composition of highly flexible parallelization plans. As a result,
SuperScaler can not only generate empirical parallelization plans, but also
construct new plans that achieve up to 3.5X speedup compared to
state-of-the-art solutions like DeepSpeed, Megatron and Alpa, for emerging DNN
models like Swin-Transformer and AlphaFold2, as well as well-optimized models
like GPT-3
Intrinsic Electronic Structure and Nodeless Superconducting Gap of Observed by Spatially-Resolved Laser-Based Angle Resolved Photoemission Spectroscopy
The spatially-resolved laser-based high resolution ARPES measurements have
been performed on the optimally-doped
(Y123) superconductor. For the first time, we found the region from the cleaved
surface that reveals clear bulk electronic properties. The intrinsic Fermi
surface and band structures of Y123 are observed. The Fermi surface-dependent
and momentum-dependent superconducting gap is determined which is nodeless and
consistent with the d+is gap form
Electronic Origin of High-Tc Maximization and Persistence in Trilayer Cuprate Superconductors
In high temperature cuprate superconductors, it was found that the
superconducting transition temperature Tc depends on the number of CuO2 planes
(n) in the structural unit and the maximum Tc is realized in the trilayer
system (n=3). It was also found that the trilayer superconductors exhibit an
unusual phase diagram that Tc keeps nearly constant in the overdoped region
which is in strong contrast to the Tc decrease usually found in other cuprate
superconductors. The electronic origin of the Tc maximization in the trilayer
superconductors and its high Tc persistence in the overdoped region remains
unclear. By taking high resolution laser-based angle resolved photoemission
(ARPES) measurements, here we report our revelation of the microscopic origin
of the unusual superconducting properties in the trilayer superconductors. For
the first time we have observed the trilayer splitting in Bi2Sr2Ca2Cu3O10+d
(Bi2223) superconductor. The observed Fermi surface, band structures,
superconducting gap and the selective Bogoliubov band hybridizations can be
well described by a three-layer interaction model. Quantitative information of
the microscopic processes involving intra- and interlayer hoppings and pairings
are extracted. The electronic origin of the maximum Tc in Bi2223 and the
persistence of the high Tc in the overdoped region is revealed. These results
provide key insights in understanding high Tc superconductivity and pave a way
to further enhance Tc in the cuprate superconductors
Ubiquitous Coexisting Electron-Mode Couplings in High Temperature Cuprate Superconductors
In conventional superconductors, the electron-phonon coupling plays a
dominant role in pairing the electrons and generating superconductivity. In
high temperature cuprate superconductors, the existence of the electron
coupling with phonons and other boson modes and its role in producing high
temperature superconductivity remain unclear. The evidence of the
electron-boson coupling mainly comes from the angle-resolved photoemission
(ARPES) observations of the ~70meV nodal dispersion kink and the ~40meV
antinodal kink. However, the reported results are sporadic and the nature of
the involved bosons are still under debate. Here we report new findings of
ubiquitous two coexisting electron-mode couplings in cuprate superconductors.
By taking ultra-high resolution laser-based ARPES measurements, combined with
the improved second derivative analysis method, we discovered that the
electrons are coupled simultaneously with two sharp phonon modes with energies
of ~70meV and ~40meV in different superconductors with different doping levels,
over the entire momentum space and at different temperatures above and below
the superconducting transition temperature. The observed electron-phonon
couplings are unusual because the associated energy scales do not exhibit an
obvious change across the superconducting transition. We further find that the
well-known "peak-dip-hump" structure, which has long been considered as a
hallmark of superconductivity, is also omnipresent and consists of finer
structures that originates from electron coupling with two sharp phonon modes.
These comprehensive results provide a unified picture to reconcile all the
reported observations and pinpoint the origin of the electron-mode couplings in
cuprate superconductors. They provide key information to understand the role of
the electron-phonon coupling in generating high temperature superconductivity
Orbital-Dependent Electron Correlation in Double-Layer Nickelate La3Ni2O7
The latest discovery of high temperature superconductivity near 80K in
La3Ni2O7 under high pressure has attracted much attention. Many proposals are
put forth to understand the origin of superconductivity. The determination of
electronic structures is a prerequisite to establish theories to understand
superconductivity in nickelates but is still lacking. Here we report our direct
measurement of the electronic structures of La3Ni2O7 by high-resolution
angle-resolved photoemmission spectroscopy. The Fermi surface and band
structures of La3Ni2O7 are observed and compared with the band structure
calculations. A flat band is formed from the Ni-3dz2 orbitals around the zone
corner which is 50meV below the Fermi level. Strong electron correlations are
revealed which are orbital- and momentum-dependent. Our observations will
provide key information to understand the origin of high temperature
superconductivity in La3Ni2O7.Comment: 18 pages, 4 figure
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