Long baseline diffraction-limited optical aperture synthesis technology by interferometry plays an important role in scientific study and practical application. In contrast to amplitude (phase) interferometry, intensity interferometry - which exploits the second-order coherence of thermal light - is robust against atmospheric turbulence and optical defects. However, a thermal light source typically has a broadband spectrum, a low average photon number per mode, and a wide divergence angle, forestalling extended applications. Here, we propose and demonstrate active intensity interferometry for optical synthetic aperture imaging over the kilometer range. Our scheme employs multiple phase-independent laser emitters to generate thermal illumination and utilizes a flexible computational algorithm for image reconstruction. Through outdoor experiments, we have successfully imaged millimeter-scale targets located at 1.36 km away, achieving a resolution enhancement by about 14 times over the diffraction limit of a single telescope. The application of long-baseline active intensity interferometry holds promise for advancing high-resolution optical imaging and sensing.

Postulating the identification of ψ^∗(x,t)ψ(x,t) with a physical probability density is unsatisfactory conceptually and overly limited practically. For electrons, there is a simple, calculable relativistic correction proportional to ∇ψ^∗⋅∇ψ. In particular, zeroes of the wave function do not indicate vanishing probability density of presence. We derive a correction of this kind from a Lagrangian, in a form suitable for wide generalization and use in effective field theories. Thus we define a large new class of candidate models for (quasi-)particles and fields, featuring modified kinetic terms. We solve for the stationary states and energy spectrum in some representative problems, finding striking effects including the emergence of negative effective mass at high energy and of localization by energy.

We propose simple quantitative criteria, based on counting statistics in resonant harmonic detectors, that probe the quantum mechanical character of radiation fields. They provide, in particular, practical means to test the null hypothesis that a given field is "maximally classical,"i.e., accurately described by a coherent state. We suggest circumstances in which that hypothesis plausibly fails, notably including gravitational radiation involving nonlinear or stochastic sourcing.

Many widely different problems have a common mathematical structure wherein limited knowledge leads to ambiguity that can be captured conveniently using a concept of invisibility that requires the introduction of negative values for quantities that are inherently positive. Here I analyze three examples taken from perception theory, rigid body mechanics, and quantum measurement.

Distinguishing whether a system supports alternate low-energy (locally stable) states—stable (true vacuum) versus metastable (false vacuum)—by direct observation can be difficult when the lifetime of the state is very long but otherwise unknown. Here we demonstrate, in a tractable model system, that there are physical phenomena on much shorter timescales that can diagnose the difference. Specifically, we study the time evolution of the magnetization following a quench in the tilted quantum Ising model, and show that its magnitude spectrum is an effective diagnostic. Small transition bubbles are more common than large ones, and we see characteristic differences in the size dependence of bubble lifetimes even well below the critical size for false vacuum decay. We expect this sort of behavior to be generic in systems of this kind. We show such signatures persist in a continuum field theory. This also opens the possibility of similar signatures of the potential metastable false vacuum of our universe well before the beginning of a decay process to the true vacuum.

Parity-violating superconductors can support a low-dimension local interaction that becomes, upon condensation, a purely spatial Chern-Simons term. Solutions to the resulting generalized London equations can be obtained from solutions of the ordinary London equations with a complex penetration depth, and suggest several remarkable physical phenomena. The problem of flux exclusion by a sphere brings in an anapole moment, the problem of current-carrying wires brings in an azimuthal magnetic field, and the problem of vortices brings in currents along the vortices. We demonstrate that interactions of this kind, together with a conceptually related dimensionally reduced Chern-Simons interaction, can arise from physically plausible microscopic interactions.

The quantum-mechanical description of assemblies of particles whose motion is confined to two (or one) spatial dimensions offers many possibilities that are distinct from bosons and fermions. We call such particles anyons. The simplest anyons are parameterized by an angular phase parameter theta. theta = 0,pi correspond to bosons and fermions, respectively; at intermediate values, we say that we have fractional statistics. In two dimensions, theta describes the phase acquired by the wave function as two anyons wind around one another counterclockwise. It generates a shift in the allowed values for the relative angular momentum. Composites of localized electric charge and magnetic flux associated with an abelian U(1) gauge group realize this behavior. More complex charge-flux constructions can involve nonabelian and product groups acting on a spectrum of allowed charges and fluxes, giving rise to nonabelian and mutual statistics. Interchanges of nonabelian anyons implement unitary transformations of the wave function within an emergent space of internal states. Anyons of all kinds are described by quantum field theories that include Chern-Simons terms. The crossings of one-dimensional anyons on a ring are unidirectional, such that a fractional phase theta acquired upon interchange gives rise to fractional shifts in the relative momenta between the anyons.The quasiparticle excitations of fractional quantum Hall states have long been predicted to include anyons. Recently, the anyon behavior predicted for quasiparticles in the v = 1/3 fractional quantum Hall state has been observed in both scattering and interferometric experiments. Excitations within designed systems, notably including superconducting circuits, can exhibit anyon behavior. Such systems are being developed for possible use in quantum information processing.

Handedness, or chirality, has been a continuing source of inspiration across a wide range of scientific problems. After a quick review of some important, instructive historical examples, I present three contemporary case studies involving sophisticated applications of chirality at the frontier of present-day science in the measurement of the muon magnetic moment, in topological physics, and in exploring the “chirality” of time. Finally, I briefly discuss chirality as a source of fertile questions.

The spontaneous breaking of time-translation symmetry has led to the discovery of a new phase of matter: the discrete time crystal. Discrete time crystals exhibit rigid subharmonic oscillations that result from a combination of many-body interactions, collective synchronization, and ergodicity breaking. This Colloquium reviews recent theoretical and experimental advances in the study of quantum and classical discrete time crystals. The breaking of ergodicity is focused upon as the key to discrete time crystals and the delaying of ergodicity as the source of numerous phenomena that share many of the properties of discrete time crystals, including the ac Josephson effect, coupled map lattices, and Faraday waves. Theoretically, there is a diverse array of strategies to stabilize time crystalline order in both closed and open systems, ranging from localization and prethermalization to dissipation and error correction. Experimentally, many-body quantum simulators provide a natural platform for investigating signatures of time-crystalline order; recent work utilizing trapped ions, solid-state spin systems, and superconducting qubits are reviewed. Finally, this Colloquium concludes by describing outstanding challenges in the field and a vision for new directions on both the experimental and theoretical fronts.

We consider the design of metamaterials whose behavior embodies the equations of axion electrodynamics. We derive an effective medium description of an assembly of magneto-optical layers with out-of-plane magnetization analytically and show how to achieve effective axion response with tunable parameters. We display some key predictions and validate them numerically.