Increasing efficiency of a linear-optical quantum gate using an electronic feed forward

TL;DR

This paper demonstrates that using a fast electronic feed forward in a fiber-optic linear optical quantum gate doubles its success probability from 25% to 50%, without compromising fidelity, by applying conditional unitary operations.

Contribution

The work introduces a real-time electronic feed forward technique to enhance the success rate of a linear optical quantum gate, achieving the theoretical maximum success probability.

Findings

Success probability increased from 25% to 50%.

Gate fidelity remains unchanged with feed forward.

Quantum process tomography confirms unchanged process fidelity.

Abstract

We have successfully used a fast electronic feed forward to increase the success probability of a linear optical implementation of a programmable phase gate from 25% to its theoretical limit of 50%. The feed forward applies a conditional unitary operation which changes the incorrect output states of the data qubit to the correct ones. The gate itself rotates an arbitrary quantum state of the data qubit around the z-axis of the Bloch sphere with the angle of rotation being fully determined by the state of the program qubit. The gate implementation is based on fiber optics components. Qubits are encoded into spatial modes of single photons. The signal from the feed-forward detector is led directly to a phase modulator using only a passive voltage divider. We have verified the increase of the success probability and characterized the gate operation by means of quantum process tomography. We have demonstrated that the use of the feed forward does not affect either the process fidelity or the output-state fidelities.