Universal Linear Optics

TL;DR

This paper demonstrates a reprogrammable six-mode linear optical circuit capable of implementing any linear optical protocol, including quantum gates and boson sampling, with high fidelity, advancing quantum technology applications.

Contribution

The authors built and tested a reprogrammable six-mode optical circuit that can implement all linear optical protocols up to its size, a longstanding goal in the field.

Findings

Implemented 100 Haar random unitaries with 0.999 fidelity

Successfully programmed quantum gates and boson sampling protocols

System switches protocols within seconds

Abstract

Linear optics underpins tests of fundamental quantum mechanics and computer science, as well as quantum technologies. Here we experimentally demonstrate the longstanding goal of a single reprogrammable optical circuit that is sufficient to implement all possible linear optical protocols up to the size of that circuit. Our six-mode universal system consists of a cascade of 15 Mach-Zehnder interferometers with 30 thermo-optic phase shifters integrated into a single photonic chip that is electrically and optically interfaced for arbitrary setting of all phase shifters, input of up to six photons and their measurement with a 12 single-photon detector system. We programmed this system to implement heralded quantum logic and entangling gates, boson sampling with verification tests, and six-dimensional complex Hadamards. We implemented 100 Haar random unitaries with average fidelity 0.999 $\pm$ 0.001. Our system is capable of switching between these and any other linear optical protocol in seconds. These results point the way to applications across fundamental science and quantum technologies.