Experimental creation of superposition of unknown photonic quantum states

TL;DR

This paper demonstrates a heralded quantum machine that probabilistically creates superpositions of unknown photonic qubits with high fidelity, advancing quantum superposition understanding and applications.

Contribution

It presents the first experimental realization of a probabilistic protocol for superposing arbitrary unknown photonic qubits with nonzero overlaps, achieving high fidelity.

Findings

Average fidelity of 0.99 in superposition creation

Successful superposition of 11 different qubit pairs

Deepens understanding of quantum superposition and its applications

Abstract

As one of the most intriguing intrinsic properties of quantum world, quantum superposition provokes great interests in its own generation. Oszmaniec [Phys. Rev. Lett. 116, 110403 (2016)] have proven that though a universal quantum machine that creates superposition of arbitrary two unknown states is physically impossible, a probabilistic protocol exists in the case of two input states have nonzero overlaps with the referential state. Here we report a heralded quantum machine realizing superposition of arbitrary two unknown photonic qubits as long as they have nonzero overlaps with the horizontal polarization state $|H\rangle$. A total of 11 different qubit pairs are chosen to test this protocol by comparing the reconstructed output state with theoretical expected superposition of input states. We obtain the average fidelity as high as 0.99, which shows the excellent reliability of our realization. This realization not only deepens our understanding of quantum superposition but also has significant applications in quantum information and quantum computation, e.g., generating non-classical states in the context of quantum optics and realizing information compression by coherent superposition of results of independent runs of subroutines in a quantum computation.