Boosting the dimensionality of frequency entanglement using a reconfigurable microring resonator

Integrated quantum frequency combs (QFCs) based on microring resonators supplies as an essential resource for expanding the Hilbert-space dimensionality for high-dimensional quantum computing and information processing. In this work, we propose and demonstrate a reconfigurable ring resonator with tunable quality factors to efficiently increase the dimensionality of frequency entanglement, simultaneously, ensuring a high on-chip pair generation rate (PGR) and coincidence-to-accidental ratio (CAR). Our method exploits the asymmetric Mach-Zehnder interferometer instead of the traditional straight waveguide as the coupler of resonators which offer a tunable external coupling coefficient to modulate the quality factor to enlarge the QFCs’ bandwidth and thus increase the dimensionality of frequency entanglement. We measured the QFCs’ joint spectral intensity of 28 frequency pairs under various quality factors ranging from 16.6 × 10 4 to 3.4 × 10 4 . Meanwhile, the measured Schmidt number increased from 11.01 to 24.77, denoting a huge expansion of the Hilbert-space dimensionality from 121 to a record number of 613 dimensions, which agrees well with our theoretical calculations. In addition, the PGR and CAR—another two key parameters for high-quality QFCs—were all measured under different quality factors to verify that our method can significantly increase the Schmidt number and CAR while maintaining a high PGR. In fact, bright QFCs with a total PGR of 4.3 MHz under a 0.48 mW pump power and a mean CAR of 1578 were simultaneously obtained at the highest Schmidt number. This method is widely applicable to other material-based ring resonators and can act as a general solution for high-dimensional QFCs.