Volume 17 Issue 1, January 2021

#BlackInPhysics Week aimed to build community among physicists by celebrating, supporting and increasing the visibility of Black physicists. The week accomplished all of this, and more.

The uncertainty associated with epidemic forecasts is often simulated with ensembles of epidemic trajectories based on combinations of parameters. We show that the standard approach for summarizing such ensembles systematically suppresses critical epidemiological information.

The interplay of topological properties and non-Hermitian symmetry breaking has been implemented for a range of classical-wave systems. Recent advances, challenges and opportunities are reviewed across the different physical platforms.

Although quantum mechanics is essential to understand microscopic systems, it has little effect on heavier objects. Experiments have now put strict constraints on theories that use gravity to explain the absence of large-scale quantum effects.

When a semiconductor is embedded inside a microcavity, infrared photons have been shown to bind electrons and holes together as excitons. This result opens the door for quantum material engineering based on light–matter interactions.

Populations of organisms can be regarded as clouds of genetic variants evolving passively in response to mutation and natural selection. Counterdiabatic driving — a tool borrowed from quantum control — now offers the possibility of actively controlling both the rate and route followed by an evolving population.

Magnons are collective excitations that dictate many of a magnet’s low-temperature properties. By means of Raman scattering, the magnon spectra of CrI₃ are measured in the monolayer limit.

Transport measurements show that spontaneous symmetry breaking plays a crucial role in the correlated insulating and metallic states in twisted double bilayer graphene.

Electrons and holes in doped quantum wells cannot form bound states from usual Coulomb interaction. However, when the system is embedded in a cavity, the exchange of photons provides an effective attraction, leading to the creation of bound excitons.

A three-dimensional topological magnetic superlattice structure exhibits the quantum anomalous Hall effect when the Fermi energy is tuned into the correct energy window.

By incorporating a ferromagnetic layer in their superconductor–semiconductor nanowire hybrid device, Vaitiekėnas et al. show that zero-bias peaks—potential Majorana bound states—can be induced without an external magnetic field.

Composite fermions can be tuned to very low effective density in a clean two-dimensional electron gas, which allows the formation of a Bloch ferromagnet.

X-ray scattering experiments show that the quantum fluctuations associated with charge order take a form that is incompatible with the idea of competition between charge order and superconductivity.

The strange metal phase in unconventional superconductors is probed by Hall measurements. This reveals that quantum criticality drives the Hall effect, which also correlates with the superconductivity. This indicates that all three may be linked.

Numerical calculations that include the quantum fluctuations of protons explain the optical properties of hydrogen at high pressure.

Bacteria live in heterogeneous environments, so it is important to investigate their behaviour in porous media. Here the authors show that flow disorder enhances the effect of chemical gradients in micropockets in a porous medium, which then aid the transport of bacteria.

The radiation emission rate from gravity-related wave function collapse is calculated and the results of a dedicated experiment at the Gran Sasso laboratory are reported, ruling out the natural parameter-free version of the Diósi–Penrose model.

The presence of axion-like dark matter candidates is expected to induce an oscillating magnetic field, enhanced by a ferromagnet. Limits on the electromagnetic coupling strength of axion-like particles are reported over a mass range spanning three decades.

Analogous to the radiation-pressure coupling known in optomechanics, photon-pressure interaction between superconducting circuits can reach the strong coupling regime, which allows flexible control of the electromagnetic resonator’s quantum state.

High-quality WSe₂–MoSe₂ heterostructures support strong coupling between the two layers, which is associated with tight hybridization and effective charge separation. In these structures, the bands of the interlayer excitons can be pressure-engineered.

A new form of superradiance is predicted that ‘in contrast to the standard effect’ arises even for vanishing numbers of particles per wavelength. This finding may enable coherent emission in plasma accelerators.

The phrase ‘arrow of time’ refers to the asymmetry in the flow of events. A machine learning algorithm trained to infer its direction identifies entropy production as the relevant underlying physical principle in the decision-making process.

Softness, a machine-learned structural quantity, has been recently identified as a parameter that characterizes glassy dynamics. Here, the authors observe devitrification in 3D soft colloidal glasses and find that softness may indicate regions predisposed to crystallization.

The authors investigate the role of spherical confinement and curvature-induced topological defects on the crystallization of charged colloids. They conclude that crystallization in spherical confinement is due to a combination of thermodynamics and kinetic pathways.

The authors investigate out-of-equilibrium crystallization of a binary mixture of sphere-like nanoparticles in small droplets. They observe the spontaneous formation of an icosahedral structure with stable MgCu₂ phases, which are promising for photonic applications.

The unpredictability of evolution makes it difficult to deal with drug resistance because over the course of a treatment there may be mutations that we cannot predict. The authors propose to use quantum methods to control the speed and distribution of potential evolutionary outcomes.

Simulations are as much a part of science as hypothesis and experiment. But can their outcomes be considered observations? Wendy S. Parker investigates.