  Highly nonclassical quantum states of light, characterized by Wigner
functions with negative values, have been created so far only in a heralded
fashion. In this case, the desired output emerges rarely and randomly from a
quantum-state generator. An important example is the heralded production of
high-purity single-photon states, typically based on some nonlinear optical
interaction. In contrast, on-demand single-photon sources were also reported,
exploiting the quantized level structure of matter systems. These sources,
however, lead to highly impure output states, composed mostly of vacuum. While
such impure states may still exhibit certain single-photon-like features such
as anti-bunching, they are not enough nonclassical for advanced quantum
information processing. On the other hand, the intrinsic randomness of pure,
heralded states can be circumvented by first storing and then releasing them on
demand. Here we propose such a controlled release, and we experimentally
demonstrate it for heralded single photons. We employ two optical cavities,
where the photons are both created and stored inside one cavity, and finally
released through a dynamical tuning of the other cavity. We demonstrate storage
times of up to 300 ns, while keeping the single-photon purity around 50% after
storage. This is the first demonstration of a negative Wigner function at the
output of an on-demand photon source or a quantum memory. In principle, our
storage system is compatible with all kinds of nonclassical states, including
those known to be essential for many advanced quantum information protocols.
