  Orbital angular momentum (OAM) entangled photon pairs with narrow bandwidths
play a crucial role in the interaction of light and quantum states of matter.
In this article, we demonstrate an approach for generating OAM entangled photon
pairs with a narrow bandwidth by using a single driving beam in a $^{85}$Rb
atomic vapor cell. This single driving beam is able to simultaneously couple
two atomic transitions and directly generate OAM entangled biphotons by
leveraging the OAM conservation law through the spontaneous four-wave mixing
(SFWM) process. The photon pairs exhibit a maximum cross-correlation function
value of 27.7 and a linewidth of 4 MHz. The OAM entanglement is confirmed
through quantum state tomography, revealing a fidelity of 95.7\% and a
concurrence of 0.926 when compared to the maximally entangled state. Our scheme
is notably simpler than previously proposed schemes and represents the first
demonstration of generating subnatural-linewidth entangled photon pairs in hot
atomic systems.
