Spin-Photon Entanglement of a Single <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi>Er</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow>

Entanglement between photons and a quantum memory is a key component of quantum repeaters, which allow long-distance quantum entanglement distribution in the presence of fiber losses. Spin-photon entanglement has been implemented with a number of different atomic and solid-state qubits with long spin coherence times, but none directly emit photons into the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mn>1.5</a:mn><a:mtext>−</a:mtext><a:mi mathvariant="normal">μ</a:mi><a:mi mathvariant="normal">m</a:mi></a:mrow></a:math> telecom band where losses in optical fibers are minimized. Here, we demonstrate spin-photon entanglement using a single rare earth ion in the solid-state <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:msup><e:mrow><e:mi>Er</e:mi></e:mrow><e:mrow><e:mn>3</e:mn><e:mo>+</e:mo></e:mrow></e:msup></e:mrow></e:math> coupled to a silicon nanophotonic cavity, which directly emits photons at 1532.6 nm. We infer an entanglement fidelity of 73(3)% after propagating through 15.6 km of optical fiber. This work opens the door to large-scale quantum networks based <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mrow><g:msup><g:mrow><g:mi>Er</g:mi></g:mrow><g:mrow><g:mn>3</g:mn><g:mo>+</g:mo></g:mrow></g:msup></g:mrow></g:math> ions, leveraging scalable silicon device fabrication and spectral multiplexing.