Experimental implementation of optimal linear-optical controlled-unitary gates

TL;DR

This paper demonstrates a method to reduce the number of two-qubit gates in linear optical quantum computing by using tunable controlled-phase gates, improving success probability and resource efficiency.

Contribution

The authors introduce a technique to replace pairs of controlled-NOT gates with a single tunable controlled-phase gate, optimizing controlled-unitary operations in linear optics.

Findings

Reduced the number of two-qubit gates needed for controlled-unitary transformations.

Increased success probability of two-qubit gates by about an order of magnitude.

Experimental demonstration with a controlled single-qubit unitary gate.

Abstract

We show that it is possible to reduce the number of two-qubit gates needed for the construction of an arbitrary controlled-unitary transformation by up to two times using a tunable controlled-phase gate. On the platform of linear optics, where two-qubit gates can only be achieved probabilistically, our method significantly reduces the amount of components and increases success probability of a two-qubit gate. The experimental implementation of our technique presented in this paper for a controlled single-qubit unitary gate demonstrates that only one tunable controlled-phase gate is needed instead of two standard controlled-NOT gates. Thus, not only do we increase success probability by about one order of magnitude (with the same resources), but also avoid the need for conducting quantum non-demolition measurement otherwise required to join two probabilistic gates. Subsequently, we generalize our method to a higher order, showing that n-times controlled gates can be optimized by replacing blocks of controlled-NOT gates with tunable controlled-phase gates.