Femtosecond laser direct writing of an integrated path-encoded CNOT quantum gate

Abstract

Femtosecond lasers enable three-dimensional direct writing of waveguides inside bulk transparent materials and have been applied to the fabrication of integrated photonic quantum logic gates. Up to now, the controlled-NOT (CNOT) gate, the key two-qubit quantum gate, has been realized only for polarization-encoded photonic qubits, which consists of three partially polarizing directional couplers (DCs) or two polarizing DCs. In this work, we demonstrate the femtosecond laser direct writing of a path-encoded CNOT gate formed by five DCs all with precisely controlled splitting ratios and stable symmetric phases inside glass for the first time. The quantum CNOT operation is performed using single photons with an average fidelity higher than 0.98. This provides a novel venue for the fabrication of large-scale 3D quantum computation circuits based on femtosecond laser writing.