  In recent years, remarkable advances in photonic computing have highlighted
the need for photonic memory, particularly high-speed and coherent
random-access memory. Addressing the ongoing challenge of implementing photonic
memories is required to fully harness the potential of photonic computing. A
photonic-phononic memory based on stimulated Brillouin scattering is a possible
solution as it coherently transfers optical information into sound waves at
high-speed access times. Such an optoacoustic memory has shown great potential
as it fulfils key requirements for high-performance optical random-access
memory due to its coherence, on-chip compatibility, frequency selectivity, and
high bandwidth. However, the storage time has so far been limited to a few
nanoseconds due to the nanosecond decay of the acoustic wave. In this work, we
experimentally enhance the intrinsic storage time of an optoacoustic memory by
more than one order of magnitude and coherently retrieve optical information
after a storage time of 120 ns. This is achieved by employing the optoacoustic
memory in a highly nonlinear fiber at 4.2 K, increasing the intrinsic phonon
lifetime by a factor of six. We demonstrate the capability of our scheme by
measuring the initial and readout optical data pulse with a direct and double
homodyne detection scheme. Finally, we analyze the dynamics of the optoacoustic
memory at different cryogenic temperatures in the range of 4.2 K to 20 K and
compare the findings to continuous wave measurements. The extended storage time
is not only beneficial for photonic computing, but also for Brillouin
applications that require long phonon lifetimes, such as optoacoustic filters,
true-time delay networks, and synthesizers in microwave photonics.
