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 ($<\pm 0.50°\mathrm{C}$). 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 $<\pm 0.10°\mathrm{C}$. When we purposely heat the interferometers over a temperature range of $20–50°\mathrm{C}$, we measure a path-length shift of $26\pm 9\mathrm{nm}/°\mathrm{C}$ 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 $37.1°\mathrm{C}$ and is $50\pm 17\mathrm{nm}/°\mathrm{C}$ at $22°\mathrm{C}$. 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.

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• Quantum communication
• Quantum cryptography
• Quantum information with atoms & light
• Interferometry