  A quantum memory that can store quantum states faithfully and retrieve them
on demand has wide applications in quantum information science. An efficient
quantum memory in the microwave regime working alongside quantum processors
based on superconducting quantum circuits may serve as an important
architecture for quantum computers. Here we realize on-demand storage and
retrieval of weak coherent microwave photon pulses at the single-photon level.
We implement a superconducting multi-resonator quantum memory which is composed
of a set of frequency-tunable coplanar transmission line (CPW) resonators. By
dynamically tuning the resonant frequencies of the resonators, we achieve
tunable memory bandwidth from 10 MHz to 55 MHz, with an overall storage
efficiency up to 12 % with well preserved phase coherence. We further
demonstrate on-demand storage and retrieval of a time-bin flying qubit. This
result opens up a prospect to integrate our chip-based quantum memory with the
state-of-the-art superconducting quantum circuit technology for quantum
information processing.
