Waveguides were fabricated with a 1.85-μm proton exchange depth followed by annealing for 8 hours at 328°C and reverse proton exchange for 10 hours at the same temperature. Inputs of the periodically poled waveguides were designed with a channel width of 2.5 μm to get nearly single-mode operation at 775 nm and inject efficiently the pump beam into the fundamental mode of the waveguides. Channel width at the beginning of the poling region was increased to 8 μm with a 7-mm adiabatic taper to work in a noncritical condition for quasi-phase matching (39). After the poling region, the channel widths were decreased to 6 μm with a second adiabatic taper of 1.5 mm in length to get single-mode operation at 1550 nm. S-bends were designed with a sinusoidal function and a minimum bend radius of 40 mm. Separation between waveguide centers at the input and the output of the device was set to 127 μm to match the standard pitch of fiber V-groove arrays. To prevent back reflections into the waveguides and cavity effects inside the chip, the output facet was polished at 8°. Total length of the chip was 62 mm. Light was coupled into the HDs using bulk optical elements to avoid the coupling losses with a second fiber array. In future implementations, the deposition of an AR coating on the output facet would enable polishing the chip at a 0° angle and reduction of the Fresnel losses.

The poling pattern was generated by standard electric field poling with a period Λ = 16.12 μm and a 50:50 duty cycle. After poling and waveguide fabrication, aluminum electrodes were realized on a 200-nm-thick SiO2 buffer layer to prevent optical absorption from the metal. Aluminum thickness was 250 nm, while electrodes were patterned using electron beam lithography and wet etching.

Directional couplers were designed with separation between waveguide centers of 11.3 μm for DC1, DC4, and DC5, and 10.6 μm for DC2 and DC3. The lengths of the directional couplers were 6.1 mm for DC1 and 3.5 mm for DC2, DC3, DC4, and DC5. For DC2 and DC3, we chose a smaller center-to-center separation to achieve an SR of 1 while minimizing the length of the couplers. Electrodes (12 mm long) act as phase shifters on the LO arms. At a zero applied voltage, the SRs of the reconfigurable couplers were 0.72 for DC1, 0.85 for DC4, and 0.75 for DC5. Application of a voltage to the electrodes induced a mismatch between the propagation constants of the coupled waveguides, which reduced the SR of the DCs. For this reason, both positive and negative voltages reduced the SRs below their zero-voltage values. Application of a square modulation with positive and negative amplitudes resulted in typical traces such as the one reported in Fig. 2B.

The isolation of the pump beams from the homodyne detectors was achieved by DC2 and DC3 and a dichroic mirror after the chip for a total of 40-dB isolation. Furthermore, since the output facet of the chip was angled polished, pump and signal beams were refracted in slightly different directions, and only 1550-nm light gets coupled into the HDs. During the experiments, the shot-noise level was measured while blocking the pump beam with a chopper.

To prevent photorefractive damage, the chip was bonded with an ultraviolet curing glue to a custom-made aluminum oven and heated at 125°C. Photorefractive effect must be avoided because it can locally change phase-matching wavelength of the waveguide and reduce the interaction length of the down conversion process, thus reducing the maximum attainable level of squeezing in our device. Two printed circuit boards with SubMiniature version A connectors were mounted on the sides of the oven and wire bonded to the electrodes to control the voltage applied to phase shifters and directional couplers.

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