Researchers from the School of Physics at Nanjing University, led by Professors Hui-Tian Wang and Xi-Lin Wang, have proposed a solution to achieve the first postselection-free cavity-enhanced narrow-band entangled photon sources. Cavity-enhanced spontaneous parametric down-conversion (SPDC) provides a significant way to produce ∼10 MHz narrow-band photon pairs, which matches the bandwidth of photon for quantum memory. However, the output photon pairs directly from the cavity are not entangled, and postselection is required to create the entanglement so far, so the direct output of cavity-enhanced narrow-band entangled photon pairs is still an open challenge. This challenge has been overcome recently. The entanglement is achieved in degree of freedom of orbital angular momentum (OAM) by an OAM-conservation SPDC process in an actively and precisely controlled cavity supporting degenerate high-order OAM modes. The directly generated OAM entangled two photons have a linewidth of 13.8 MHz and a fidelity of 0.969(3). Moreover, the researchers have also produced narrow-band OAM-polarization hyperentangled photon pairs, which is realized by interfering the two photons on a polarizing beam splitter (PBS) and postselecting the events of one and only one photon on each PBS port. Their results provide an efficient and promising approach to create narrow-band entangled photon sources for memory-based long-distance quantum communication and network. Novel cavity can also find applications in cavity-based light-matter interaction. Their findings were published in the journal Physical Review Letters [Phys. Rev. Lett. 134, 053801(2025)] entitled “Postselection-Free Cavity-enhanced Narrow-Band Orbital Angular Momentum Entangled photon source”.

Publication link: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.134.053801