  Quantum correlated, highly non-degenerate photons can be used to synthesize
disparate quantum nodes and link quantum processing over incompatible
wavelengths, thereby constructing heterogeneous quantum systems for otherwise
unattainable superior performance. Existing techniques for correlated photons
have been concentrated in the visible and near-IR domains, with the photon
pairs residing within one octave. Here, we demonstrate direct generation and
detection of high-purity photon pairs at room temperature that are four octaves
apart, one at 780 nm to match the rubidium D2 line, and the other at 3950 nm
that falls in a transparent, low-scattering optical window for free space
applications. The pairs are created via spontaneous parametric downconversion
in a lithium niobate waveguide with specially designed geometry and periodic
poling. The 780 nm photons are measured with a silicon avalanche photodiode,
and the 3950 nm photons are measured with an upconversion photon detector using
a similar waveguide, which attains 34% internal conversion efficiency. Quantum
correlation measurement yields a high coincidence-to-accidental ratio of 54,
which indicates the strong correlation with the extremely non-degenerate photon
pairs. Our system bridges existing quantum technology to the challenging mid-IR
regime, where unprecedented applications are expected in metrology, sensing,
communications, medical diagnostics, and so on.
