On-chip telecom-band single photon delay lines approaching a microsecond

We use spectral hole burning-induced slow light in nanophotonic waveguides in erbium-doped thin-film lithium niobate to create long delays for single telecom-band photons on chip. We show delays of nearly 300 ns with internal efficiencies approaching 3.5% in a 3 cm long waveguide. The same delay would require a path length of tens of meters in a waveguide without hole burning, incurring >30 dB of loss under the most optimistic assumptions about propagation losses. Our method allows tunable and reconfigurable delays set by the width of the prepared spectral hole, which is controlled by the optical power used to prepare the hole. We investigate the achievable regime for this method in terms of the primary metrics: delay-bandwidth product, internal efficiency, and single photon noise. In particular, the use of heavily erbium-doped samples allows a large delay-bandwidth product without excessive optical loss, but makes burning persistent spectral holes challenging, which means a spontaneous emission background that prevents operation at the single photon level. We investigate the temperature and magnetic field dependence of the spectral hole burning to identify requirements for eliminating this background noise.