A quantum Fredkin gate

TL;DR

This paper reports the first implementation of a quantum Fredkin gate using photonic qubits, demonstrating high-fidelity three-photon GHZ states and a novel control technique for black-box unitaries, advancing scalable quantum computing.

Contribution

The paper introduces a method to add control to black-box unitaries, enabling the first quantum Fredkin gate and paving the way for larger controlled quantum circuits.

Findings

First quantum Fredkin gate realized with photonic qubits

Generated highest-fidelity three-photon GHZ states to date

Introduced a technique to control black-box unitaries

Abstract

Key to realising quantum computers is minimising the resources required to build logic gates into useful processing circuits. While the salient features of a quantum computer have been shown in proof-of-principle experiments, difficulties in scaling quantum systems have made more complex operations intractable. This is exemplified in the classical Fredkin (controlled-SWAP) gate for which, despite theoretical proposals, no quantum analogue has been realised. By adding control to the SWAP unitary, we use photonic qubit logic to demonstrate the first quantum Fredkin gate, which promises many applications in quantum information and measurement. We implement example algorithms and generate the highest-fidelity three-photon GHZ states to-date. The technique we use allows one to add a control operation to a black-box unitary, something impossible in the standard circuit model. Our experiment represents the first use of this technique to control a two-qubit operation and paves the way for larger controlled circuits to be realised efficiently.