Correlated photon pairs generated from a warm atomic ensemble

We present measurements of the cross-correlation function of photon pairs at 780 nm and 1367 nm, generated in a hot rubidium vapor cell. The temporal character of the biphoton is determined by the dispersive properties of the medium where the pair generation takes place. We show that short correlation times occur for optically thick samples, which can be understood in terms of off-resonant pair generation. By modifying the linear response of the sample, we produce near-resonant photon pairs, which could in principle be used for entanglement distribution

The q\textit{q}-metric naked singularity: A viable explanation for the nature of the central object in the Milky Way

In this work, we investigate whether the compact object at the center of the Milky Way is a naked singularity described by the $\textit{q}$-metric spacetime. Our fitting of the astrometric and spectroscopic data for the S2 star implies that similarly to the Schwarzschild black hole, the $\textit{q}$-metric naked singularity offers a satisfactory fit to the observed measurements. Additionally, it is shown that the shadow produced by the naked singularity is consistent with the shadow observed by the Event Horizon Telescope collaboration for Sgr-A*. It is worth mentioning that the spatial distribution of the S-stars favors the notion that the compact object at the center of our Galaxy can be described by an almost static spacetime. Based on these findings, the $\textit{q}$-metric naked singularity turns up as a compelling candidate for further investigation.Comment: Accepted for publication in Classical and Quantum Gravit

GRB 180620A: Evidence for Late-time Energy Injection

The early optical emission of gamma-ray bursts (GRBs) gives an opportunity to understand the central engine and first stages of these events. About 30% of GRBs present flares whose origin is still a subject of discussion. We present optical photometry of GRB 180620A with the COATLI telescope and RATIR instrument. COATLI started to observe from the end of prompt emission at T + 39.3 s and RATIR from T + 121.4 s. We supplement the optical data with the X-ray light curve from Swift/XRT. We observe an optical flare from T + 110 s to T + 550 s, with a temporal index decay α O,decay = 1.32 ± 0.01, and Δt/t = 1.63, which we interpret as the signature of a reverse shock component. After the initial normal decay the light curves show a long plateau from T + 500 s to T + 7800 s in both X-rays and the optical before decaying again after an achromatic jet break at T + 7800 s. Fluctuations are seen during the plateau phase in the optical. Adding to the complexity of GRB afterglows, the plateau phase (typically associated with the coasting phase of the jet) is seen in this object after the "normal" decay phase (associated with the deceleration phase of the jet), and the jet break phase occurs directly after the plateau. We suggest that this sequence of events can be explained by a rapid deceleration of the jet with t d ≲ 40 s due to the high density of the environment (≈100 cm-3) followed by reactivation of the central engine, which causes the flare and powers the plateau phase