Light Sci. Appl. 4, e354 (2015)

Fulvio Flamini and co-workers from Italy have now developed a reconfigurable integrated optical circuit by using an aluminoborosilicate glass substrate, and investigated the performance of the circuit in the quantum regime. The waveguide circuits for a tunable Mach–Zehnder interferometer were directly inscribed in the aluminoborosilicate glass by tightly focused laser pulses at a wavelength of 1,030 nm and a duration of 300 fs. The fabricated waveguides yielded single-mode operation at 1,550 nm and propagation losses of ∼0.6 dB cm−1. The coupling losses to standard single-mode telecom fibres were 0.2 dB per facet. The phase in the interferometer was thermally controlled by tuning the voltage on a gold electrode on the glass substrate. Pairs of horizontally polarized identical photons were generated at a wavelength of 1,550 nm via spontaneous parametric down-conversion in a bismuth borate crystal and sent to the circuits. Tunable Hong–Ou–Mandel interference and super-resolved fringes based on N00N states were successfully demonstrated with two-photon input states.