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    Single-photon sources (SPSs) are essential components for quantum information technology as single photons are needed as fast-flying qubits for various applications such as quantum cryptography, quantum computing,and quantum communication. One of the most promising methods for realizing high-performance SPSs is through the use of strong coupling (SC) between a single quantum emitter (QE) and an optical cavity. For example, the use of a semiconductor quantum dot coupled with an on-chip photonic cavity with an ultrahigh quality (Q) factor and an extremely small mode volume has resulted in impressive on-demand SPSs with high degrees of indistinguishability and high collection efficiencies. However, the configurations of these SPSs are not ideal for long-distance quantum communication as they are intrinsically incompatible with optical fibers and require additional couplers to couple the generated single photons into the optical fibers for long-distance transmission. This results in a bulky and complex setup, leading to additional insertion loss and a high error rate of qubits in quantum communication.

In this work, we propose a SC system based on a single QE directly coupled with an in-fiber microcavity for long-distance quantum communication. The in-fiber microcavity is adiabatically connected with single-mode fibers, which avoids using bulky and complex couplers while enabling negligible insertion loss. The in-fiber microcavity is designed to have a high Q factor and an ultrasmall mode volume by utilizing twenty-five air holes with precisely tailored sizes in each reflector of the in-fiber microcavity. A square through hole is introduced into the in-fiber microcavity center to boost the Purcell factor and the coupling efficiency with the single QE. With this in-fiber microcavity design, an anti-crossing behavior with a vacuum Rabi splitting is obtained to enter the strong-coupling regime, which enables a high quantum efficiency for a SPS. Our work presents a promising platform for realizing high-performance SPSs for long-distance quantum communication.