  Biphoton frequency combs (BFCs) are promising quantum sources for large-scale
and high-dimensional quantum information and networking systems. In this
context, the spectral purity of individual frequency bins will be critical for
realizing quantum networking protocols like teleportation and entanglement
swapping. Measurement of the temporal auto-correlation function of the
unheralded signal or idler photons comprising the BFC is a key tool for
characterizing their spectral purity and in turn verifying the utility of the
biphoton state for networking protocols. Yet the experimentally obtainable
precision for measuring BFC correlation functions is often severely limited by
detector jitter. The fine temporal features in the correlation function$-$not
only of practical value in quantum information, but also of fundamental
interest in the study of quantum optics$-$are lost as a result and have
remained unexplored. We propose a scheme to circumvent this challenge through
electro-optic phase modulation, experimentally demonstrating time-resolved
Hanbury Brown-Twiss characterization of BFCs generated from an integrated 40.5
GHz Si$_3$N$_4$ microring, up to a 3$\times$3-dimensional two-qutrit Hilbert
space. Through slight detuning of the electro-optic drive frequency from the
comb's free spectral range, our approach leverages Vernier principles to
magnify temporal features which would otherwise be averaged out by detector
jitter. We demonstrate our approach under both continuous-wave and pulsed
pumping regimes, finding excellent agreement with theory. Importantly, our
method reveals not only the collective statistics of the contributing frequency
bins but also their temporal shapes$-$features lost in standard fully
integrated auto-correlation measurements.
