  The ability of phase-change materials to reversibly and rapidly switch
between two stable phases has driven their use in a number of applications such
as data storage and optical modulators. Incorporating such materials into
metasurfaces enables new approaches to the control of optical fields. In this
article we present the design of novel switchable metasurfaces that enable the
control of the nonclassical two-photon quantum interference. These structures
require no static power consumption, operate at room temperature, and have high
switching speed. For the first adaptive metasurface presented in this article,
tunable nonclassical two-photon interference from -97.7% (anti-coalescence) to
75.48% (coalescence) is predicted. For the second adaptive geometry, the
quantum interference switches from -59.42% (anti-coalescence) to 86.09%
(coalescence) upon a thermally driven crystallographic phase transition. The
development of compact and rapidly controllable quantum devices is opening up
promising paths to brand-new quantum applications as well as the possibility of
improving free space quantum logic gates, linear-optics bell experiments, and
quantum phase estimation systems.
