Quantum phase measurement for two-qubit states in an open waveguide

We present a new method for quantum state tomography within a single-excitation subspace of two-qubit states in an open waveguide. The system under investigation consists of three qubits in an open waveguide, separated by a distance comparable to the wavelength of the electromagnetic field. We show that the modulation of the frequency of the central ancillary qubit allows us to obtain unambiguous information about the initial phase difference $\varphi_1-\varphi_3$ of the edge qubits via the measurement of the evolution of their probability amplitudes.