215 research outputs found
China’s new foreign policy strategy and Russia’s concerns
The recent trends in Beijing’s foreign policy have become a broadly discussed topic throughout the world. China’s economic success over the last ten years has led Beijing to take a more assertive approach to China’s relationship with the outside world. This shift has manifested itself in a more hard-line approach to China’s relationship with her partners, less inclination toward compromise, and a tendency to respond to the external pressure with more pressure, to the external bumps with harder bumps. The new assertiveness of China can be understood. After all, it is merely the natural urge of a new, large, and successful regime to actively pursue its interests. At the same time, it is true that the successful economic development of the last ten years has led to the growth of nationalism among the elite. If the nationalist tendency prevails in the Chinese foreign policy, China’s neighbors, including Russia, will have to do some serious rethinking of their approach to the growing giant
Unconventional superfluidity and quantum geometry of topological bosons
We investigate superfluidity of bosons in gapped topological bands and
discover a new phase that has no counterparts in the previous literature. This
phase is characterized by a highly unconventional modulation of the order
parameter, breaking the crystallographic symmetry, and for which the
condensation momentum is neither zero nor any other high-symmetry vector of the
Brillouin zone. This unconventional structure impacts the spectrum of
Bogoliubov excitations and, consequently, the speed of sound in the system.
Even in the case of perfectly flat bands, the speed of sound and Bogoliubov
excitations remain nonvanishing, provided that the underlying topology and
quantum geometry are nontrivial. Furthermore, we derive detailed expressions
for the superfluid weight using the Popov hydrodynamic formalism for
superfluidity and provide estimates for the Berezinskii-Kosterlitz-Thouless
temperature, which is significantly enhanced by the nontriviality of the
underlying quantum metric. These results are applicable to generic topological
bosonic bands, with or without dispersion. To illustrate our findings, we
employ the Haldane model with a tunable bandwidth, including the narrow
lowest-band case. Within this model, we also observe a re-entrant superfluid
behavior: As the Haldane's magnetic flux is varied, the
Berezinskii-Kosterlitz-Thouless transition temperature initially decreases to
almost zero, only to resurface with renewed vigor.Comment: 23 pages, 10 figure
Measuring entanglement entropy through the interference of quantum many-body twins
Entanglement is one of the most intriguing features of quantum mechanics. It
describes non-local correlations between quantum objects, and is at the heart
of quantum information sciences. Entanglement is rapidly gaining prominence in
diverse fields ranging from condensed matter to quantum gravity. Despite this
generality, measuring entanglement remains challenging. This is especially true
in systems of interacting delocalized particles, for which a direct
experimental measurement of spatial entanglement has been elusive. Here, we
measure entanglement in such a system of itinerant particles using quantum
interference of many-body twins. Leveraging our single-site resolved control of
ultra-cold bosonic atoms in optical lattices, we prepare and interfere two
identical copies of a many-body state. This enables us to directly measure
quantum purity, Renyi entanglement entropy, and mutual information. These
experiments pave the way for using entanglement to characterize quantum phases
and dynamics of strongly-correlated many-body systems.Comment: 14 pages, 12 figures (6 in the main text, 6 in supplementary
material
Regimes of charged particle dynamics in current sheets: the machine learning approach
Current sheets are spatially localized almost-1D structures with intense
plasma currents. They play a key role in storing the magnetic field energy and
they separate different plasma populations in planetary magnetospheres, the
solar wind, and the solar corona. Current sheets are primary regions for the
magnetic field line reconnection responsible for plasma heating and charged
particle acceleration. One of the most interesting and widely observed type of
1D current sheets is the rotational discontinuity, that can be force-free or
include plasma compression. Theoretical models of such 1D current sheets are
based on the assumption of adiabatic motion of ions, i.e. ion adiabatic
invariants are conserved. We focus on three current sheet configurations,
widely observed in the Earth magnetopause and magnetotail and in the near-Earth
solar wind. Magnetic field in such current sheets is supported by currents
carried by transient ions, which exist only when there is a sufficient number
of invariants. In this paper, we apply a novel machine learning approach, AI
Poincar'e, to determine parametrical domains where adiabatic invariants are
conserved. For all three current sheet configurations, these domains are quite
narrow and do not cover the entire parametrical range of observed current
sheets. We discuss possible interpretation of obtained results indicating that
1D current sheets are dynamical rather than static plasma equilibria
Quantum Error Correction for Metrology
We propose and analyze a new approach based on quantum error correction (QEC) to improve quantum metrology in the presence of noise. We identify the conditions under which QEC allows one to improve the signal-to-noise ratio in quantum-limited measurements, and we demonstrate that it enables, in certain situations, Heisenberg-limited sensitivity. We discuss specific applications to nanoscale sensing using nitrogen-vacancy centers in diamond in which QEC can significantly improve the measurement sensitivity and bandwidth under realistic experimental conditions.Chemistry and Chemical BiologyPhysic
Probing many-body dynamics on a 51-atom quantum simulator
Controllable, coherent many-body systems can provide insights into the
fundamental properties of quantum matter, enable the realization of new quantum
phases and could ultimately lead to computational systems that outperform
existing computers based on classical approaches. Here we demonstrate a method
for creating controlled many-body quantum matter that combines
deterministically prepared, reconfigurable arrays of individually trapped cold
atoms with strong, coherent interactions enabled by excitation to Rydberg
states. We realize a programmable Ising-type quantum spin model with tunable
interactions and system sizes of up to 51 qubits. Within this model, we observe
phase transitions into spatially ordered states that break various discrete
symmetries, verify the high-fidelity preparation of these states and
investigate the dynamics across the phase transition in large arrays of atoms.
In particular, we observe robust manybody dynamics corresponding to persistent
oscillations of the order after a rapid quantum quench that results from a
sudden transition across the phase boundary. Our method provides a way of
exploring many-body phenomena on a programmable quantum simulator and could
enable realizations of new quantum algorithms.Comment: 17 pages, 13 figure
Ultra-Slow Light and Enhanced Nonlinear Optical Effects in a Coherently Driven Hot Atomic Gas
We report the observation of small group velocities of order 90 meters per
second, and large group delays of greater than 0.26 ms, in an optically dense
hot rubidium gas (~360 K). Media of this kind yield strong nonlinear
interactions between very weak optical fields, and very sharp spectral
features. The result is in agreement with previous studies on nonlinear
spectroscopy of dense coherent media
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