We discuss the statistics of correlations between two resonant detectors. We show that this allows simple null tests of the coherent state hypothesis, free of vacuum (quantum) noise. Complementary aspects of the radiation field, e.g. squeezing in number or phase, can be revealed through appropriate detection strategies.

Many physicists, including some of the best and brightest, describe what they do as working on quantum gravity. Many different activities, with different motivations, approaches, and achievements, go by that name. In conceptualizing this situation, I have found it clarifying and enlightening to distinguish several different levels within quantum gravity, and to compare them with corresponding levels within quantum electrodynamics. I will share those thoughts here.

We estimate the degree of squeezing possible in gravitational waves due to nonlinear gravitational effects in the weakly perturbative regime. Using the predicted amplitude ratios for higher harmonic generation in the ringdown phase of a black hole merger event, we estimate the relevant degree of squeezing produced by a Schwarzschild singularity to be of the order of 1%.

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.

We demonstrate that statistical fluctuations in resonant radiation detectors operating in homodyne and heterodyne modes offer additional, complementary information to that obtained from their direct operation as click detectors. We use this to refine tests of the coherent-state hypothesis, which is of special interest for gravitational wave fields.

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.

Gravitational radiation from known astrophysical sources is conventionally treated classically. This treatment corresponds, implicitly, to the hypothesis that a particular class of quantum-mechanical states — the so-called coherent states — adequately describe the gravitational radiation field. We propose practicable, quantitative tests of that hypothesis using resonant bar detectors monitored in coincidence with LIGO-style interferometers. Our tests readily distinguish fields that contain significant thermal components or squeezing. We identify concrete circumstances in which the classical (i.e. coherent state) hypothesis is likely to fail. Such failures are of fundamental interest, in that addressing them requires us to treat the gravitational field quantum-mechanically, and they open a new window into the dynamics of gravitational wave sources.

Spatial structuring of materials at subwavelength scales underlies the concept of metamaterials possessing exotic properties beyond those of the constituent media. Temporal modulation of material parameters enables further functionalities. Here, we show that high-frequency oscillations of spatially uniform magnetization generate an effective dynamic axion field embedding the amplitude and phase of magnetization oscillations. This allows one to map ultrafast magnetization dynamics using a probe signal with much lower frequency.

We compare two quantum Hamiltonian algorithms that address the maximum independent set problem: one based on the emergent non-Abelian gauge matrix in adiabatic evolution of an energetically isolated manifold of states; the other based on designed application of single-qubit operations. We demonstrate that they are mathematically equivalent in the sense that one is the other’s interaction picture. Despite their mathematical equivalence, our numerical simulations show significant differences between them in performance, which is explained analytically. Intriguingly, this equivalence unveils that the PXP model, recently prominent in quantum dynamics research, can be viewed as quantum diffusion over the median graph of all independent sets governed by the non-Abelian gauge matrix.

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.