Quantum frequency conversion for networking via the telecom E-band

High-efficiency quantum frequency conversion (QFC) for group-IV color centers, such as silicon vacancies (SnV), is required for long-distance quantum networking protocols. SiV QFC has previously been demonstrated at external eGiciencies of 12% [1] and 30% [2], using periodically poled lithium niobate (PPLN) waveguides of lengths 3.5 and 5 mm, and with eGiciencies limited by the available pump power (120 mW and 320 mW, respectively). Here, the eGiciency scales with the square of the waveguide length. These results are also obtained with pump wavelength of 1623 nm (L-band), networking the SiV wavelength of 737 nm into the O-band (1350 nm). Here, we improve these results by replacing the pump (L-band in [1],[2]) with a pump with wavelength in the C-band (1561 nm), where (i) high-power erbium-doped fiber amplifiers (EDFAs) are available (ii) SMF-28e fibers have lower losses in the E-band than O-band. Using an EDFA and a 1.5-cm-long PPLN waveguide We measure a maximum external photon conversion eGiciency of 43±1.8% (corresponding internal eGiciency of 96±1.8%), which we reach at an external pump power of 1.51 W. The external eGiciency is limited by waveguide-fiber coupling. As such, we have demonstrated an external eGiciency for SiV-telecom quantum frequency conversion exceeding prior results from MIT, Lincoln Labs, and Harvard. To further improve these results in the future, the waveguide will be replaced with a longer one to reduce the power required to reach unit internal eGiciency, which further reduces noise. We may also optimize fiber coupling or switch to a free-space-coupled device, which we estimate to increase the external conversion eGiciency to approx. 70-80%.