Octave-spanning spectral phase control for single-cycle bi-photons

The quantum correlation of octave-spanning time-energy entangled bi-photons can be as short as a single optical cycle. Many experiments designed to explore and exploit this correlation require a uniform spectral phase (transform-limited) with very low loss. So far, transform-limited single-cycle bi-photons have not been demonstrated, primarily due to the lack of precise, broadband control of their spectral phase. Here, we demonstrate the correction of the spectral-phase of near-octave spanning bi-photons to over an octave in frequency-2600 nm). Using a prism-pair with an effectively negative separation for shaping the bi-photons' spectral phase, we obtain a tuned, very low-loss compensation of both the second and fourth dispersion orders. An essential requisite for precise tuning over such a broad bandwidth is a measure of the spectral phase that provides feedback for the tuning even when the overall dispersion is far from compensated. This is achieved by a non-classical bi-photon interference, which enables direct verification of the corrected bi-photon spectral phase.