Femtosecond laser direct writing of photonic quantum logic gate chips for generating path-encoded entangled states

Abstract

Femtosecond laser direct writing (FLDW) technique has been widely applied for fabrications of various photonic quantum chips in glass, such as chips for quantum computation, quantum walk, quantum simulation and quantum metrology. Photonic quantum logic gates are the building blocks for the realization of linear optical universal quantum computation. Up to now, several photonic quantum logic gates have been fabricated by FLDW, such as polarization encoded Hadamard (H) gate, Controlled NOT (CNOT) gate, herald CNOT gate and path encoded herald Controlled phase (CZ) gate and CNOT gate. By combining several single-qubit and two-qubit gates together, the constructed quantum circuits can realize some special functions, such as generating entangled states and perform quantum computation algorithms. Based on the successful fabrication of path encoded CNOT gate by FLDW, we further realized the fabrication of photonic quantum chips by cascading one H gate and one CNOT gate at the control qubit to generate path encoded Bell states, whose fidelity of truth table can reach 97.60.3%. Further, we cascaded one H gate and two parallel CNOT gates at the same control qubit to generate path encoded GHZ states, but which need three photons. Both Bell states and GHZ states are important entangled photon resources, which are widely used in quantum communication and quantum computation, and both combinations of logic gates above can be applied in many quantum circuits, so this work is of great importance and lays the technical foundation for the FLDW of more complex and powerful photonic quantum computation chips.