  For superconducting quantum processors, microwave signals are delivered to
each qubit from room-temperature electronics to the cryogenic environment
through coaxial cables. Limited by the heat load of cabling and the massive
cost of electronics, such an architecture is not viable for millions of qubits
required for fault-tolerant quantum computing. Monolithic integration of the
control electronics and the qubits provides a promising solution, which,
however, requires a coherent cryogenic microwave pulse generator that is
compatible with superconducting quantum circuits. Here, we report such a signal
source driven by digital-like signals, generating pulsed microwave emission
with well-controlled phase, intensity, and frequency directly at millikelvin
temperatures. We showcase high-fidelity readout of superconducting qubits with
the microwave pulse generator. The device demonstrated here has a small
footprint, negligible heat load, great flexibility to operate, and is fully
compatible with today's superconducting quantum circuits, thus providing an
enabling technology for large-scale superconducting quantum computers.
