  A nonclassical light source is essential for implementing a wide range of
quantum information processing protocols, including quantum computing,
networking, communication, and metrology. In the microwave regime, propagating
photonic qubits that transfer quantum information between multiple
superconducting quantum chips serve as building blocks of large-scale quantum
computers. In this context, spectral control of propagating single photons is
crucial for interfacing different quantum nodes with varied frequencies and
bandwidth. Here we demonstrate a microwave quantum light source based on
superconducting quantum circuits that can generate propagating single photons,
time-bin encoded photonic qubits and qudits. In particular, the frequency of
the emitted photons can be tuned in situ as large as 200 MHz. Even though the
internal quantum efficiency of the light source is sensitive to the working
frequency, we show that the fidelity of the propagating photonic qubit can be
well preserved with the time-bin encoding scheme. Our work thus demonstrates a
versatile approach to realizing a practical quantum light source for future
distributed quantum computing.
