  Complete characterization of the state of a quantum system made up of
subsystems requires determination of relative phase, because of interference
effects between the subsystems. For a system of qubits used as a quantum
computer this is especially vital, because the entanglement, which is the basis
for the quantum advantage in computing, depends intricately on phase. We
present here a first step towards that determination, in which we use a
two-qubit quantum system as a quantum neural network, which is trained to
compute and output its own relative phase.
