  The ability to control the direction of scattered light in integrated devices
is crucial to provide the flexibility and scalability for a wide range of
on-chip applications, such as integrated photonics, quantum information
processing and nonlinear optics. In the optical and microwave frequency ranges
tunable directionality can be achieved by applying external magnetic fields,
that modify optical selection rules, by using nonlinear effects, or
interactions with vibrations. However, these approaches are less suitable to
control propagation of microwave photons inside integrated superconducting
quantum devices, that is highly desirable. Here, we demonstrate tunable
directional scattering with just two transmon qubits coupled to a transmission
line based on periodically modulated transition frequency. By changing the
symmetry of the modulation, governed by the relative phase between the local
modulation tones, we achieve directional forward or backward photon scattering.
