Tunable cavity coupling of a single <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>Sn</mml:mi> </mml:math> - <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msup> <mml:mi>V</mml:mi>

Efficient coupling between quantum emitters and optical cavities is essential for scalable quantum photonic technologies. Group-IV vacancy centers in diamond, particularly the negatively charged tin-vacancy center, have emerged as promising candidates due to their spectral stability, high Debye-Waller factor, and large orbital splitting in ground states. Here, we demonstrate controlled coupling of a single negatively charged tin-vacancy center hosted in a nanodiamond to a fully tunable Fabry-Pérot microcavity. At cryogenic temperatures, we access the weak-coupling regime and observe a transition from the bad-emitter regime to the bad-cavity regime as the optical transition of the color center narrows. At 4 K, a Purcell factor exceeding 1.7 is measured, confirming cavity-enhanced emission. The obtained results demonstrate the potential of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>Sn</a:mi> <a:mstyle displaystyle="false" scriptlevel="0"> <a:mtext>−</a:mtext> </a:mstyle> <a:msup> <a:mi>V</a:mi> <a:mo>−</a:mo> </a:msup> </a:math> centers in nanodiamonds as a coherent single-photon source for quantum networks.