  Frequency combs, specialized laser sources emitting multiple equidistant
frequency lines, have revolutionized science and technology with unprecedented
precision and versatility. Recently, integrated frequency combs are emerging as
scalable solutions for on-chip photonics. Here, we demonstrate a fully
integrated superconducting microcomb that is easy to manufacture, simple to
operate, and consumes ultra-low power. Our turnkey apparatus comprises a basic
nonlinear superconducting device, a Josephson junction, directly coupled to a
superconducting microstrip resonator. We showcase coherent comb generation
through self-started mode-locking. Therefore, comb emission is initiated solely
by activating a DC bias source, with power consumption as low as tens of
picowatts. The resulting comb spectrum resides in the microwave domain and
spans multiple octaves. The linewidths of all comb lines can be narrowed down
to 1 Hz through a unique coherent injection-locking technique. Our work
represents a critical step towards fully integrated microwave photonics and
offers the potential for integrated quantum processors.
