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# Volume 15 Issue 11, November 2019

## Odd viscosity

A chiral fluid comprising spinning colloidal magnets exhibits macroscopic dynamics reminiscent of the free surface flows of Newtonian fluids, together with unique features suggestive of Hall—or odd—viscosity.

Image: Vishal Soni, Ephraim Bililign, Sofia Magkiriadou, University of Chicago. Cover Design: David Shand

## Editorial

• ### Big Bang theory

Half of the 2019 Nobel Prize in Physics was awarded to James Peebles “for theoretical discoveries in physical cosmology” and the other half is shared by Michel Mayor and Didier Queloz “for the discovery of an exoplanet orbiting a solar-type star”.

Editorial

• ### A supreme achievement

The demonstration of a quantum computational advantage is a milestone worth celebrating.

Editorial
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## Thesis

• ### Leibniz’s principle of indistinguishability

• Mark Buchanan
Thesis
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## Research Highlights

• ### Triplet no more

• David Abergel
Research Highlight
• ### Skyrmion school

• Elizaveta Dubrovina
Research Highlight
• ### Fall forward

• Federico Levi
Research Highlight
• ### Reconfigurable routing

• Nina Meinzer
Research Highlight
• ### Of all shapes

• Stefanie Reichert
Research Highlight
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## News & Views

• ### Engaged in gauge theory

Two independent cold-atom experiments have demonstrated the building blocks for the quantum simulation of dynamical gauge fields — an advance that holds promise for our understanding of computationally intractable problems in high-energy physics.

• Cheng Chin
News & Views
• ### A needle in a moiré stack

Spatially resolved measurements of twisted bilayer graphene reveal more details of the strongly correlated electrons.

News & Views
• ### The magic in molecules

The demonstration of high-resolution spectroscopy of Sr2 molecules trapped in an optical lattice at the ‘magic’ wavelength opens the way to precision control of molecular excitations.

• Nicola Poli
News & Views
• ### Model oddity

A model fluid comprising rotating magnetic particles behaves according to the equations of hydrodynamics, but for a few key differences due to broken mirror symmetry. The resulting active chiral fluid is characterized by parity-odd Hall viscosity.

• Alexander Abanov
News & Views
• ### Nearly perfect quark–gluon fluid

A statistical analysis of data from ultra-relativistic heavy-ion collisions has uncovered the specific viscosities of the quark–gluon plasma — suggesting that the hottest matter in the current Universe behaves like a near-perfect fluid.

• Kari J. Eskola
News & Views
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## Letters

• ### Bayesian estimation of the specific shear and bulk viscosity of quark–gluon plasma

As the quark–gluon plasma is a short-lived state of matter, its properties cannot be measured directly. A Bayesian parameter estimation method now provides a reliable estimation of the temperature-dependent specific shear and bulk viscosities.

• Jonah E. Bernhard
• J. Scott Moreland
• Steffen A. Bass
Letter
• ### Molecular lattice clock with long vibrational coherence

The realization of a molecular lattice clock based on vibrations in diatomic molecules is reported with coherence times lasting over tens of milliseconds, which is enabled by the use of a state-insensitive magic lattice trap.

• S. S. Kondov
• C.-H. Lee
• T. Zelevinsky
Letter
• ### Non-exponential decay of a giant artificial atom

By coupling a superconducting qubit to surface acoustic waves the ‘giant atom’ regime is realized, where an atom is coupled to a field with wavelength orders of magnitude smaller than the atomic size. This leads to non-Markovian qubit dynamics.

• Per Delsing
Letter
• ### Vanishing carrier-envelope-phase-sensitive response in optical-field photoemission from plasmonic nanoantennas

The carrier-envelope-phase-sensitive component of field-driven photoemission at the tip of a nanostructure shows a dip with a sudden phase shift. This is a consequence of its nonlinear dependence on the tunnel ionization and is not limited to solids.

• P. D. Keathley
• W. P. Putnam
• F. X. Kärtner
Letter
• ### Giant gate-controlled proximity magnetoresistance in semiconductor-based ferromagnetic–non-magnetic bilayers

The authors demonstrate magnetoresistance of 80% from a two-dimensional electron gas proximity coupled to a ferromagnetic layer. This extends spintronics functionality to semiconductor devices.

• Kosuke Takiguchi
• Le Duc Anh
• Masaaki Tanaka
Letter
• ### Identification of spin, valley and moiré quasi-angular momentum of interlayer excitons

Stacked 2D materials can host excitons with distinct valley selection rules due to the spatial variation of the moiré pattern. The authors demonstrate this via optical spectroscopy, opening a route for control of optoelectronic devices.

• Chenhao Jin
• Emma C. Regan
• Feng Wang
Letter
• ### Attosecond screening dynamics mediated by electron localization in transition metals

Experiments with attosecond time resolution reveal many-body electron dynamics in transition metals before thermalization sets in. Ultrafast electronic localization on d orbitals is found to dominate the collective dynamic response of the system.

• M. Volkov
• S. A. Sato
• U. Keller
Letter
• ### Photonic Weyl points due to broken time-reversal symmetry in magnetized semiconductor

Photonic Weyl points—topologically chiral singularity points in three-dimensional momentum space—have been realized in a homogeneous non-reciprocal material without a crystal lattice structure.

• Dongyang Wang
• Biao Yang
• Shuang Zhang
Letter
• ### Generation and acceleration of electron bunches from a plasma photocathode

Electron bunches are generated and accelerated to relativistic velocities by tunnel ionization of neutral gas species in a plasma. This represents a step towards ultra-bright, high-emittance beams in plasma wakefield accelerators. [This summary has been amended from ‘laser-plasma’ to ‘plasma wakefield’ accelerators.]

• A. Deng
• O. S. Karger
• B. Hidding
Letter
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## Articles

• ### Realization of density-dependent Peierls phases to engineer quantized gauge fields coupled to ultracold matter

Non-trivial Peierls phases that depend on the site occupations for ultracold fermions in an optical lattice have been engineered in a Floquet approach, providing a fundamental ingredient for a density-dependent gauge field acting on ultracold matter.

• Frederik Görg
• Kilian Sandholzer
• Tilman Esslinger
Article
• ### Floquet approach to ℤ2 lattice gauge theories with ultracold atoms in optical lattices

An effective Hamiltonian exhibiting \$\${\Bbb Z}_2\$\$ ${\mathbb{Z}}_{2}$ symmetry has been engineered by implementing a Floquet-based method on ultracold bosons in an optical lattice, providing a first step towards quantum simulation of \$\${\Bbb Z}_2\$\$ ${\mathbb{Z}}_{2}$ lattice gauge theories with ultracold matter.

• Christian Schweizer
• Fabian Grusdt
• Monika Aidelsburger
Article
• ### Electronic correlations in twisted bilayer graphene near the magic angle

Scanning tunnelling microscopy shows that electrons in twisted bilayer graphene are strongly correlated for a wide range of density. In particular, a correlated regime appears near charge neutrality and theory suggests nematic ordering.

• Youngjoon Choi
• Jeannette Kemmer
Article
• ### Half-integer level shift of vortex bound states in an iron-based superconductor

The authors use STM to show that there are two different classes of zero-bias peak in vortex cores of Fe(Te,Se). One class is topological, one not. These are distinguished by a shift in the energy levels of the excited states.

• Lingyuan Kong
• Shiyu Zhu
• Hong Ding
Article
• ### The odd free surface flows of a colloidal chiral fluid

A chiral fluid comprising spinning colloidal magnets exhibits macroscopic dynamics reminiscent of the free surface flows of Newtonian fluids, together with unique features suggestive of Hall—or odd—viscosity.

• Vishal Soni
• Ephraim S. Bililign
• William T. M. Irvine
Article
• ### Tissue fluidity promotes epithelial wound healing

An observation that cells at the edge of a healing wound readily undergo intercalation leads to the finding that tissue fluidity is crucial for effective wound closure.

• Robert J. Tetley
• Yanlan Mao
Article
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## Amendments & Corrections

• ### Author Correction: Electronic correlations in twisted bilayer graphene near the magic angle

• Youngjoon Choi
• Jeannette Kemmer
Author Correction
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## Measure for Measure

• ### The sound of black holes fusing

• Joe Wolfe
• Oleg P. Sushkov
Measure for Measure
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