We report photonic quantum circuits created using an ultrafast laser processing technique that is rapid, requires no lithographic mask and can be used to create three-dimensional networks of waveguide devices. We have characterized directional couplers—the key functional elements of photonic quantum circuits—and found that they perform as well as lithographically produced waveguide devices. We further demonstrate high-performance interferometers and an important multi-photon quantum interference phenomenon for the first time in integrated optics. This direct-write approach will enable the rapid development of sophisticated quantum optical circuits and their scaling into three-dimensions.

The doped nonlinear optical crystal, ytterbium: yttrium aluminum borate, ${\mathrm{Yb}}^{3+}:{\mathrm{YAl}}_3({\mathrm{BO}}_3{)}_4,$ has been grown with good optical quality in a high-temperature solution from ${\mathrm{K}}_2{\mathrm{Mo}}_3{\mathrm{O}}_{10}$ and ${\mathrm{B}}_2{\mathrm{O}}_3$ fluxes. Room- and low-temperature absorption and fluorescence spectra have been measured, and spectral parameters have been calculated. The ground-state energy-level splitting is 581 cm^-1. The emission lifetime, the absorption cross section, and the peak emission cross section are 0.68 ms, $3.4\times {10}^{-20} {\mathrm{cm}}^2,$ and $0.8\times {10}^{-20} {\mathrm{cm}}^2,$ respectively. In comparison with established laser crystals, the results for the cross sections, lifetime, and energy splitting suggest that this crystal has potential as an efficient self-frequency-doubled laser operating in the green.

We report a continuous-wave self-Raman laser based on diode-pumped Nd:GdVO₄ giving first-Stokes output at 1173 nm and intracavity frequency-doubled output at 586.5 nm. A maximum cw output power at 1173 nm of 2 W was obtained for diode pump powers of 22 W and a maximum cw power at 586 nm of 678 mW with 16.3 W pump power. Infrared and yellow powers were limited by thermal lensing in the gain medium and parasitic oscillations of weak Nd³⁺ transitions. Quasi-cw operation at 50% duty-cycle reduced the thermal load in the laser/Raman crystal, allowing cavity stability to be obtained near maximum available pump power (25.7 W) for which the maximum quasi-cw yellow output power was 1.88 W.