Reviews of Modern Physics

Gravitational waves offer a potential window into the first moments of cosmic history. If the Universe underwent and abruptly exited an early matter-dominated phase, as predicted by many particle-physics models involving heavy particles, primordial black holes, or oscillating scalar fields forming $Q$-balls, the resulting gravitational-wave signal could be amplified via the recently discovered “poltergeist mechanism” discussed in this review. The authors explain the physical origin of this amplification, analyze the mechanism across several cosmological scenarios, and discuss prospects for detecting the resulting signals with current and future gravitational-wave observatories.

Astronomers observe thousands of planets beyond our Solar System, many with extreme interior conditions. Deep inside these worlds, gravity produces pressures millions to billions of times stronger than Earth’s atmosphere. At these pressures, matter behaves in surprising ways: hydrogen can become metallic, water and ammonia may form exotic superionic states, and hydrocarbons can turn into diamond. This Colloquium describes how studying these unusual materials helps scientists explain how planets form, evolve, and generate magnetic fields and magma oceans. Using powerful lasers and pulsed-power facilities, researchers recreate such pressures in laboratories, revealing new physics and improving understanding of the hidden interiors of distant planets.

Syukuro “Suki” Manabe won part of the 2021 Nobel Prize in Physics for his role in predicting global warming using early climate models. This Colloquium traces that decades-long effort, showing how Manabe combined bedrock physical laws with a keen intuition for simplifying assumptions to construct a hierarchy of climate models, ranging from a simple 1D model of the globally averaged atmosphere to a 3D model of the fully coupled atmosphere-ocean system with realistic continental geometry. Each rung of this hierarchy yielded new insight and specific predictions, not just for global warming but also for the spatial patterns of warming—predictions borne out by subsequent observations.

The observation by Berry that adiabatic transport around a closed loop leads to a phase in addition to that determined by the energy spectrum lies at the heart of topology in solids. Such geometrical effects have measurable physical consequences, notably quantum Hall physics in the absence of an external magnetic field—all of which are encapsulated by the quantum geometric tensor. The imaginary part of this tensor is the Berry curvature, and the real part encodes the distance between nearby wave functions and hence is referred to as the quantum metric. In recent years, the quantum geometric tensor has proven central to novel many-body effects in quantum matter, including superconductivity in flat-band systems, third-order nonlinear Hall effects, and second-harmonic nonlinear Hall effects, to name a few. This review catalogs these advances with an eye toward showing how new many-body effects arise from quantum geometry.

Social systems, when considered on an appropriately coarse-grained scale, display phenomena reminiscent of the behavior of physical many-body systems and can be studied using the methods of statistical mechanics. A particularly interesting subfield is opinion dynamics, which aims to understand the emergence of collective social phenomena, such as consensus, polarization, and fragmentation. This review systematizes the terminology and methods of opinion dynamics, surveys empirical findings alongside theoretical models, and summarizes the current state and future directions of this field.

It is sometimes said that if one uses quantum cryptography methods to distribute keys, their secrecy is guaranteed by the laws of physics. This is only partly true: while quantum physics provides a strong boost to security, the proper mathematical analysis of the full detailed protocol is still nontrivial. This review provides details of this analysis for one of the most important quantum protocols for key distribution and its variants, in which weak (few photon) coherent pulses are transmitted and threshold detectors are used for measurements.

The polaron, a single impurity embedded in a quantum many-body environment, conceptually bridges few- and many-body physics. Its properties provide both a test bed for many-body theories and physical insight into the phase structure of more complicated many-body systems. This review discusses two pristine experimental platforms in which polarons have recently been realized: ultracold atomic gases and atomically thin transition-metal dichalcogenides. The authors discuss the theory of Bose and Fermi polarons, compare theory to experiment, and provide a perspective on how polarons may serve as precise sensors in complex environments.

Recent advances in molecular cooling have enabled the realization of strongly dipolar molecular Bose-Einstein condensates. Such systems provide a unique platform for investigating new states of matter, from quantum droplets to supersolids. This Colloquium surveys the collisional shielding techniques that make stable molecular condensates possible, the theoretical challenges that arise in the strongly dipolar regime, and the exotic quantum phases now within experimental reach.