Frontiers of quantum physics

Abstract:

A fully local quantum account of the interactions experienced between charges requires us to use all four modes of the electromagnetic vector potential in the Lorenz gauge. However, it is frequently stated that only the two transverse modes of the vector potential are “real” in that they contain photons that can actually be detected. The photons present in the other two modes, the scalar and the longitudinal, are considered unobservable and are referred to as “virtual particles” or “ghosts”. Here we argue that this view, which is rooted in standard quantum electrodynamics, is a consequence of assuming that charges are always dressed in such modes and that naked charges do not have an independent existence. In particular, we present a thought experiment where, assuming that naked charges can be independently manipulated, one can then measure the entanglement generated between a charge and the scalar modes. This entanglement is a direct function of the number of photons present in the scalar field. Our conclusion, therefore, is that the scalar quantum variables, under this assumption, would be as “real” as the transverse ones, where reality is defined by their ability to affect the charge. A striking consequence of this is that there is a critical value of charge beyond which we cannot detect its spatial superposition by local means.

Abstract:

Achieving efficient optical coupling between the emission from perovskite quantum dots (PQDs) and photonic integrated elements requires ultranarrow linewidths and highly directional emission. These are challenging goals at room temperature due to the broad and isotropic nature of perovskite emission. Here, we demonstrate ultranarrow‐linewidth emission from CsPbBr3 PQDs at room temperature, in both spontaneous and stimulated regimes, by coupling to state‐of‐the‐art open‐access curved dielectric cavities under continuous wave excitation. The emission is confined to a single transverse electromagnetic mode of the cavity, achieving a remarkably narrow linewidth of 0.2 nm, ≈100× narrower than free‐space emission in both the emission regime. Single‐mode lasing from a small number of PQDs is observed, yielding a quality factor of ≈2590, among the highest reported for single‐mode lasing. The open‐access design enables precise tuning of cavity length and selective coupling of emitters in their native state, overcoming the limitations associated with closed and fixed‐length vertical‐cavity surface emitting laser geometries. The geometry's low divergence and tunability provide an efficient route for integrating perovskite emitters with on‐chip photonic circuits, advancing their use in quantum and optoelectronic technologies.

Abstract:

Take quantum theory seriously and a surprising, beautiful new vision of reality opens up to us, says physicist Vlatko Vedral

Abstract:

Advancements in utilizing low refractive index dielectric particles have implications for sensing, lasing, and strong‐coupling at nano and microscopic scales. These cavities offer benefits like ease of fabrication and biocompatibility, making them promising for a wide range of technologies by utilizing their narrow linewidth modes. However, optical modes sustained in these dispersive systems can show distinct behaviors depending on the detection configuration. This study shows the influence of numerical aperture (NA) of the objective lens on the detection of Mie modes in a dielectric microsphere under far‐field excitation and collection. It is demonstrated experimentally and numerically that Mie modes from microspheres outcouple at different angles, with variations in mode amplitudes contingent on the NA of the objective lens, thus leading to distinct linewidths while probing with different NA objectives. Furthermore, it is shown that metallic substrates can facilitate efficient detection of Mie modes by redirecting scattered modes towards low angles. This enables mode detection with low NA lenses and further preventing the inclusion of incident scattered light from higher angles which otherwise perturb the modes. The results underline the importance of careful detection strategies to fully harness dielectric particles as optical platforms for applications in particle detection and characterization.