  Nonclassical light sources are highly sought after as they are an integral
part of quantum communication and quantum computation devices. Typical sources
rely on bulk crystals that are not compact and have limited bandwidth due to
phase-matching conditions. In this work, we demonstrate the generation of
photon pairs from a free-standing lithium niobate microcube at the
telecommunication wavelength through the spontaneous parametric down-conversion
process. The maximum photon pair generation rate obtained from a single
microcube with the size of ~4 microns is ~80 Hz, resulting in an efficiency of
~1.2 GHz/Wm per unit volume, which is an order of magnitude higher than the
efficiency of photon-pair generation in bulky nonlinear crystals. The
microcubes are synthesized through a solvothermal method, offering the
possibility for scalable devices via bottom-up assembly. Our work constitutes
an important step forward in the realization of compact nonclassical light
sources with broadband tunability for various applications in quantum
communication, quantum computing, and quantum metrology.
