Robust and Stable Delay Interferometers with Application to d -Dimensional Time-Frequency Quantum Key Distribution

We experimentally investigate a cascade of temperature-compensated unequal-path interferometers that can be used to measure frequency states in a high-dimensional quantum distribution system. In particular, we demonstrate that commercially available interferometers have sufficient environmental isolation so that they maintain an interference visibility greater than 98.5% at a wavelength of 1550 nm over extended periods with only moderate passive control of the interferometer temperature (<±0.50 °C). Specifically, we characterize two interferometers that have matched delays: one with a free spectral range of 2.5 GHz and the other with 1.25 GHz. We find that the relative path of these interferometers drifts less than 3 nm over a period of 1 h during which the temperature fluctuates by <±0.10 °C. When we purposely heat the interferometers over a temperature range of 20-50 °C, we measure a path-length shift of 26±9 nm/°C for the 2.5-GHz interferometer. For the 1.25-GHz interferometer, the path-length shift is nonlinear and is locally equal to zero at a temperature of 37.1 °C and is 50±17 nm/°C at 22 °C. With these devices, we realize a proof-of-concept quantum key distribution experiment and achieve quantum bit error rates of 1.94% and 3.69% in time and frequency basis, respectively, at a quantum channel loss of 14 dB.

Duke Scholars

Author Jungsang Kim Electrical and Computer Engineering