We report an experiment that demonstrates full function of a quantum router using entangled photons, where the paths of a single-photon pulse are controlled in a coherent fashion by polarization of another single photon. Through a projective measurement, we prepare the polarization of the control photon in arbitrary superposition states, leading to coherent routing of the target photon in quantum superposition of different paths. We demonstrate quantum nature of this router through optical measurements based on quantum state tomography and show an average fidelity of 93.24% for the quantum routing operation.
Technology Review – It is an interesting step forward but the new router has significant limitations. The most significant of these is that it can handle only one quantum bit or qubit at a time. And because the process of parametric down conversion cannot handle more qubits, it cannot be scaled to more qubits.
That’s a significant drawback. It means that this is a proof-of-principle device but not one that will ever form the basis of a future quantum internet.
In a sense, it’s a little like the first quantum computers which relied on nuclear magnetic resonance to manipulate the spins of the molecules in a tub of acetone. These performed trivial calculations using a handful of qubits but couldn’t be scaled up to do anything interesting.
That’s not to say that we’ll never have scalable quantum routers. Various groups are working on different approaches that have the potential to scale. Progress is steady but slow.