Room-temperature biphoton source with a spectral brightness near the ultimate limit

Biphotons, generated from a hot atomic vapor via the process of spontaneous four-wave mixing (SFWM), have following merits: Stable and tunable frequencies as well linewidth. Such merits are very useful in application long-distance quantum communication. However, hot-atom SFWM biphoton sources previously had far lower values generation rate per linewidth, i.e., spectral brightness, compared with biphotons by parametric down-conversion process. Here we report source linewidth 960 kHz $3.7\ifmmode\times\else\texttimes\fi{}{10}^{5}$ pairs/s. The high rate, together narrow results brightness $3.8\ifmmode\times\else\texttimes\fi{}{10}^{5}$ pairs/s/MHz, which is 17 times previous best result vapors also better than all known kinds media. all-co-propagating scheme large optical depth (OD) key improvement, enabling achieved to be about one quarter ultimate limit. Furthermore, this signal-to-background ratio (SBR) 2.7, violated Cauchy-Schwartz inequality for classical light 3.6-fold. Although an increasing usually leads decreasing SBR, our systematic study indicates that both present SBR can enhanced further OD. This work demonstrates significant advancement provides knowledge technology using photons.