648 Hilbert-space dimensionality in a biphoton frequency comb*

Qudit entanglement is a valuable resource for quantum information processing because increasing dimensionality provides a pathway to higher capacity and increased error resilience in quantum communications, cluster-state quantum computation, and quantum phase measurements. Time-frequency entanglement enables qudit encoding equivalent to multiple qubits per particle that is bounded only by the spectral correlation bandwidth and readout timing jitter. Our interest is in the discrete-variable time-frequency entanglement afforded by filtering the signal and idler outputs from a continuous-wave-pumped spontaneous parametric downconverter (SPDC) to create a biphoton frequency comb (BFC). Using a fiber Fabry- Pérot cavity with 45.32 GHz free-spectral range and 1.56 GHz full-width-at-half-maximum (FWHM) linewidth to filter the outputs from a type-II quasi-phase-matched SPDC source, we generate a BFC whose time-binned Hilbert space dimensionality is at least 324. When combined with its post-selected polarization entanglement, this BFC's dimensionality doubles to at least 648, implying it has a 6.28 bits/photon classical-information capacity.