  The phenomenon of upconversion, in which a system sequentially absorbs two or
more photons and emits a photon of a higher frequency, has been used in
numerous applications. These include high-resolution non-destructive
bioimaging, deep-penetrating photodynamic therapy, and photovoltaic devices.
Due to the multi-photon mechanism of upconversion, its quantum yield cannot
exceed 50%. We propose a new mechanism of upconversion, which is based on
single-photon absorption; in this process, unlike in multiple-photon
upconversion, the quantum yield can be higher than 50%. We show that in a
system of two atoms interacting with a reservoir, a low-frequency excitation of
one atom can be upconverted into a high-frequency excitation of another atom.
The energy required for such an upconversion is drawn from the reservoir, which
destroys coherence. Decoherence leads to the transition of the system from the
pure state with a small energy dispersion to the mixed state with greater
dispersion of energy, while the system entropy increases. The phenomenon of
single-photon upconversion can be used to increase the efficacy of devices
utilizing upconversion.
