Indistinguishable telecom band photons from a single Er ion in the solid state

Area of Science:

• Quantum communication
• Solid-state physics
• Quantum information science

Background:

• Atomic defects are crucial for quantum repeater networks.
• Rare earth ions, especially Er3+, are promising for telecom band quantum communication due to their optical transitions.
• Optical spectral diffusion has hindered indistinguishable single-photon generation, limiting repeater node development.

Purpose of the Study:

• To significantly reduce optical spectral diffusion in rare earth ions for quantum repeater applications.
• To enable indistinguishable single-photon generation for long-distance quantum communication.

Main Methods:

• Implantation of Er3+ ions into CaWO4 crystals, chosen for non-polar site symmetry, low nuclear spin decoherence, and absence of background rare earth ions.
• Coupling shallow implanted ions to nanophotonic cavities with a high Purcell factor.
• Characterization of optical linewidths, long-term spectral diffusion, and spin relaxation times.

Main Results:

• Achieved significantly reduced optical spectral diffusion with single-scan optical linewidths of 150 kHz and long-term diffusion of 63 kHz.
• Observed Hong-Ou-Mandel interference with 80(4)% visibility between successively emitted photons after a 36 km delay.
• Measured long spin relaxation times (T1,s = 3.7 s, T2,s > 200 μs), with T2,s limited by impurities rather than nuclear spins.

Conclusions:

• The developed Er3+-doped CaWO4 system demonstrates substantially reduced optical spectral diffusion, overcoming a key limitation for quantum repeaters.
• The observed indistinguishable photon generation and long spin coherence times represent a significant advancement towards practical telecom band quantum repeater networks.