Volume 14 Issue 7, July 2018

Many-body quantum systems fail to reach thermalization only under specific circumstances. An analysis now reveals a new, different kind of non-equilibrating dynamics based on the many-body analogue of quantum scars in single-particle quantum chaos.

Streams of motile cells appear in both healthy development and the evolution of tumours. A study of cells under lateral confinement now suggests their activity plays a key role in triggering these flows.

Cells in embryonic tissues generate coordinated forces to close small wounds rapidly without scarring. New research shows that large cell-to-cell variations in these forces are a key system feature that surprisingly speeds up wound healing.

Bedforms in deserts include both small ripples and sand dunes that can reach tens to hundreds of metres in length — with seemingly little in between. It now looks as though intermediate-sized megaripples do appear if the conditions are just right.

Understanding the behaviour of almost any biological object is a fundamentally multiscale problem — a challenge that biophysicists have been increasingly embracing, building on two centuries of biophysical studies at a variety of length scales.

Robust and responsive, the surface of a cell is as important as its interior when it comes to mechanically regulating form and function. New techniques are shedding light on this role, and a common language to describe its properties is now needed.

It may look like little more than slime, but the glycocalyx coating our cells plays a key role in cell signalling. And changes to its physical structure have been linked to cancer, triggering emergent behaviours that form the focus of this Review.

The behaviour of cells and tissues can be understood in terms of emergent mesoscale states that are determined by a set of physical properties. This Review surveys experimental evidence for these states and the physics underpinning them.

Evidence that ants communicate mechanically to move objects several times their size has prompted a theory that places the group near a transition between uncoordinated and coordinated motion. These findings and their implications are reviewed here.

Light fields of energy comparable to the Coloumb field that binds valence electrons in atoms generate states where nearly free electrons oscillate in the laser field. These are now shown to exist in rare gases, acting as gain for laser filamentation.

When an electron with specific orbit — either clockwise or anticlockwise — in a rare gas atom is selectively ionized, the remaining ion will possess a stationary ring current, which can be probed in a time-delayed second ionization step.

Sending quantum states as shaped microwave photonic wavepackets realizes on-demand, high-fidelity quantum state transfer and entanglement between two superconducting cavity quantum memories.

A detailed and systematic neutron scattering study of rare-earth pyrochlore magnet Pr₂Hf₂O₇ provides evidence for a quantum spin ice state, and emergent lattice quantum electrodynamics consistent with theoretical predictions.

A neutron scattering study of an Ising-like quasi-one-dimensional antiferromagnet, BaCo₂V₂O₈, reveals a topological quantum phase transition when it is subjected to a transverse magnetic field.

Experiments on the Shakti geometry of artificial spin ice show that its low-energy excitations are topologically protected, and that an emergent classical topological order influences the ergodicity and equilibration of this nanomagnetic system.

Antiparallel streams of nematically oriented cells arise in both embryonic development and cancer. In vitro experiments and a hydrodynamic active gel theory suggest that these cells are subject to a transition that is driven by their activity.

Attosecond XUV spectroscopy is reported, focussing on non-Born–Oppenheimer dynamics in molecular gases of light elements. It is shown that the phase of the detected photoelectrons carries information from both vibrational and electronic degrees of freedom.

Surface plasmon polaritons in an array of metallic nanoparticles evolve quickly into the band minimum by interacting with a molecule bath, forming a Bose–Einstein condensate at room temperature within picoseconds.

Ergodicity can be strongly broken by integrable or many-body localized systems. A new form of weak ergodicity breaking is shown to arise from the presence of special eigenstates in the many-body spectrum akin to quantum scars in chaotic systems.

The motor proteins and contractile forces involved in wound closure are both shown to be heterogeneously distributed around a wound. Theory suggests that this heterogeneity speeds up wound closure, as long as the proteins are mechanically regulated.

Wind-mediated ripples form on a centimetre scale in sand, and in dunes on a scale spanning tens of metres, but patterns on intermediate scales are rare. A theory now fills the gap by predicting megaripples, which resemble structures seen on Mars.

Hans-Georg Menzel walks us through the complex set of units characterizing radioactivity and ionizing radiation.