Indistinguishable telecom band photons from a single Er ion in the solid state
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Abstract
Atomic defects in the solid state are a key component of quantum repeater networks for long-distance quantum communication<sup>1</sup>. Recently, there has been significant interest in rare earth ions<sup>2-4</sup>, in particular Er<sup>3+</sup> for its telecom band optical transition<sup>5-7</sup> that allows long-distance transmission in optical fibres. However, the development of repeater nodes based on rare earth ions has been hampered by optical spectral diffusion, precluding indistinguishable single-photon generation. Here, we implant Er<sup>3+</sup> into CaWO<sub>4</sub>, a material that combines a non-polar site symmetry, low decoherence from nuclear spins<sup>8</sup> and is free of background rare earth ions, to realize significantly reduced optical spectral diffusion. For shallow implanted ions coupled to nanophotonic cavities with large Purcell factor, we observe single-scan optical linewidths of 150 kHz and long-term spectral diffusion of 63 kHz, both close to the Purcell-enhanced radiative linewidth of 21 kHz. This enables the observation of Hong-Ou-Mandel interference<sup>9</sup> between successively emitted photons with a visibility of V = 80(4)%, measured after a 36 km delay line. We also observe spin relaxation times T<sub>1,s</sub> = 3.7 s and T<sub>2,s</sub> > 200 μs, with the latter limited by paramagnetic impurities in the crystal instead of nuclear spins. This represents a notable step towards the construction of telecom band quantum repeater networks with single Er<sup>3+</sup> ions.