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A method involving intensity correlation measurements is described, which allows for the complete removal of Doppler broadening in the emission of electromagnetic radiation from faraway sources that are inaccessible to conventional Doppler-free measurements. The technique, relying on a correction to ${g}^{(2)}$ of order ${N}^{\ensuremath{-}1}$, probes the separation between neighboring spectral lines and is also applicable to the elimination of broadening due to collisions ($N$ is the number of emitting particles and ${g}^{(2)}$ is the second-order field correlation function). Possible applications include a determination of cosmological parameters from redshifts of gravitationally lensed quasars.

Temporal Bell inequalities, or Leggett-Garg Inequalities (LGI), are studied for continuous-variable systems placed in a squeezed state. The importance of those systems lies in their broad applicability which allows the description of many different physical settings in various branches of physics, ranging from cosmology to condensed matter physics and from optics to quantum information theory. LGI violations are explored and systematically mapped in squeezing parameter space. Configurations for which LGI violation occurs are found but it is shown that no violation can be obtained if all squeezing angles vanish, contrary to what happens for the spatial Bell inequalities. We also assess the effect of decoherence on the detectability of such violations. Our study opens up the possibility of new experimental designs for the observation of LGI violation., Comment: typo in equation (5) corrected, otherwise matches published version in Physical Review A

Low-mass (sub-eV) spin-0 dark matter particles, which form a coherently oscillating classical field $\phi = \phi_0 \cos(m_\phi t)$, can induce oscillating variations in the fundamental constants through their interactions with the Standard Model sector. We calculate the effects of such possible interactions, which may include the linear interaction of $\phi$ with the Higgs boson, on atomic and molecular transitions. Using recent atomic clock spectroscopy measurements, we derive new limits on the linear interaction of $\phi$ with the Higgs boson, as well as its quadratic interactions with the photon and light quarks. For the linear interaction of $\phi$ with the Higgs boson, our derived limits improve on existing constraints by up to $2-3$ orders of magnitude., Comment: 6 pages, 5 figures