Interference in complex canonical variables is not quantum

Abstract

We formally represent the quantum interference of a single qubit embodied by a photon in the Mach-Zehnder interferometer using the classical Hamiltonian framework but with complex canonical variables. Although all operations on a single qubit can be formally expressed using the complex classical Hamiltonian dynamics, we show that the resulting system is still not a proper qubit. The reason is that it is not capable of getting entangled to another bona fide qubit and hence it does not have the information-processing capacity of a fully-fledged quantum system. This simple example powerfully illustrates the failure of all hybrid quantum-classical models in accounting for the full range of behaviour of even a single quantum bit.