Defining the solar radius is not as easy as it might seem. José Vaquero tells us a story of eclipses, meridian lines, astronomical archives and space observations.
Quantum-mechanical interaction potentials support localized bound states below the dissociation threshold or delocalized scattering states above it. Although bound states are energetically isolated, scattering states couple a quantum system to environmental modes, making dissipation and loss intrinsic to open systems. Bound states in the continuum are an exception, as destructive interference keeps them localized within the scattering continuum. Theory predicts that such states can arise through interference between two Feshbach resonances coupled to a common continuum. Here we form such an interference-stabilized state in ultracold collisions of 6Li atoms by coherently coupling two tunable Feshbach resonances using Floquet engineering. At a critical parameter point, elastic and inelastic coupling to the continuum vanish, producing a molecular state above the dissociation threshold. Consequently, resonant scattering features disappear and the colliding atoms become effectively transparent to one another. Full coupled-channel calculations and a minimal non-Hermitian model quantitatively reproduce our observations and identify the state as a Friedrich–Wintgen bound state in the continuum. These results show how quantum interference can control openness in quantum matter and enable the engineering of non-Hermitian Hamiltonians. Resonances in ultracold collisions couple to scattering states, causing interactions and loss. Now it is shown that the interference of Feshbach resonances creates a bound state in the continuum that decouples from elastic and inelastic scattering.
The conservation of phase-space volume in Hamiltonian systems rules out a host of desirable processes across the physical sciences, forbidding the enhancement of radiance through linear and passive optical elements and purification of incoherent states through unitary transformations in isolated settings. Apparent violations are typically sought using gain media and active feedback. Here we introduce an entropic framework that harnesses a fully conservative mechanism, whereby nonlinearity reshapes phase space to relax a multimode system into a pure ground state, even when excited with strongly incoherent inputs. The resulting near-zero-temperature state exhibits minimal statistical fluctuations without the need for active phase alignment. We experimentally observe this behaviour in nonlinear photonic mesh lattices, where efficient spatial coherence enhancement manifests despite the irreversible maximization of entropy. The generality of the underlying mechanism amongst conservative weakly nonlinear multimode systems suggests broad applicability across disciplines, from high-brightness laser sources to controlled condensation in many-body quantum systems such as Bose–Einstein condensates. Phase-space conservation bars coherence enhancement in passive linear optics, a limit often bypassed by gain or feedback. Here conservative nonlinearity reshapes phase space, thermodynamically driving incoherent light towards a coherent equilibrium.
Computer simulation has become an essential tool for investigating the complex behaviour of cold, non-crystalline forms of water. In the past few years, improved computational capabilities, the use of advanced sampling algorithms and the rapid adoption of machine learning methods have driven major progress in the computational exploration of the properties of supercooled and glassy water. Here we review recent progress in this area, focusing on pure bulk water, and covering simulations of supercooled water using classical potentials, simulations of supercooled water using machine learning potentials and simulations of glassy water. The article selectively reviews work that, either methodologically or in its findings, has helped to define the current understanding of cold, disordered states of water that are metastable with respect to crystalline phases. This Review discusses recent advances in the computational exploration of the properties of supercooled and glassy water.
