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We measured the covariance matrix of the fields generated in an integrated third-order optical parametric oscillator operating above threshold. We observed up to (2.3 ± 0.3) dB of squeezing in amplitude difference and inferred (4.9 ± 0.7) dB of on-chip squeezing, while an excess of noise for the sum of conjugated quadratures hinders the entanglement. The degradation of amplitude correlations and state purity for increasing the pump power is consistent with the observed growth of the phase noise of the fields, showing the necessity of strategies for phase noise control aiming at entanglement generation in these systems. 

Discrete frequency modes, or bins, present a blend of opportunities and challenges for photonic quantum information processing. Frequency-bin-encoded photons are readily generated by integrated quantum light sources, naturally high-dimensional, stable in optical fiber, and massively parallelizable in a single spatial mode. Yet quantum operations on frequency-bin states require coherent and controllable multifrequency interference, making them significantly more challenging to manipulate than more traditional spatial degrees of freedom. In this mini-review, we describe recent developments that have transformed these challenges and propelled frequency bins forward. Focusing on sources, manipulation schemes, and detection approaches, we introduce the basics of frequency-bin encoding, summarize the state of the art, and speculate on the field’s next phases. Given the combined progress in integrated photonics, high-fidelity quantum gates, and proof-of-principle demonstrations, frequency-bin quantum information is poised to emerge from the lab and leave its mark on practical quantum information processing—particularly in networking where frequency bins offer unique tools for multiplexing, interconnects, and high-dimensional communications. 

We demonstrate two-photon interference of three frequency modes via three-pump Bragg-scattering four-wave mixing in analogy to a three-level system. Correlated photon input pairs display distinct behavior from that of coherent-state input fields. 

We demonstrate a novel approach to actively and continuously tune the coupling condition of microresonators. Our approach allows for wavelength-dependent coupling and dispersion modification after fabrication. 

We propose multipartite Bragg-scattering to perform all-to-all transformation among N frequency modes, realizing a bosonic N-level system. We demonstrate the N = 3 case illustrating a pathway towards scalability for frequency-domain optical quantum information systems. 

We theoretically and experimentally investigate the noise properties of four-wave mixing-based optical-parametric oscillators (OPOs) in silicon nitride microresonators. Such OPOs can operate at ultralow-noise levels and serve as a dual-point source for optical- frequency division. 

Using silicon-nitride microresonators with integrated Moiré-Bragg gratings to suppress parasitic nonlinear processes, we demonstrate on-chip frequency conversion to a single idler tone with a record-high 71% efficiency using Bragg scattering four-wave-mixing.