Realization of a Knill-Laflamme-Milburn C-NOT gate -a photonic quantum circuit combining effective optical nonlinearities

TL;DR

This paper reports the experimental realization of a heralded photonic CNOT gate based on the KLM scheme, demonstrating a key step towards scalable all-optical quantum computing using linear optics and measurement.

Contribution

The authors experimentally implement a KLM CNOT gate using a stable interferometric architecture, advancing the practical development of optical quantum circuits.

Findings

Successful demonstration of a heralded CNOT gate with four-photon interferometers

Validation of the KLM scheme for scalable optical quantum computation

Potential applications in entanglement generation and quantum communication

Abstract

Quantum information science addresses how uniquely quantum mechanical phenomena such as superposition and entanglement can enhance communication, information processing and precision measurement. Photons are appealing for their low noise, light-speed transmission and ease of manipulation using conventional optical components. However, the lack of highly efficient optical Kerr nonlinearities at single photon level was a major obstacle. In a breakthrough, Knill, Laflamme and Milburn (KLM) showed that such an efficient nonlinearity can be achieved using only linear optical elements, auxiliary photons, and measurement. They proposed a heralded controlled-NOT (CNOT) gate for scalable quantum computation using a photonic quantum circuit to combine two such nonlinear elements. Here we experimentally demonstrate a KLM CNOT gate. We developed a stable architecture to realize the required four-photon network of nested multiple interferometers based on a displaced-Sagnac interferometer and several partially polarizing beamsplitters. This result confirms the first step in the KLM `recipe' for all-optical quantum computation, and should be useful for on-demand entanglement generation and purification. Optical quantum circuits combining giant optical nonlinearities may find wide applications across telecommunications and sensing.