Researchers at the University of Virginia developed a tiny optical frequency comb, or microcomb, that uses two-mode squeezing to create unconditional entanglement between continuous optical fields. The miniature, chip-based device lays the groundwork for mass production of deterministic quantum frequency combs that could see use in quantum computing, quantum metrology, and quantum sensing. The microcomb is designed for quantum information protocols based on continuous-variable entangled states that generate qumodes (entangled states) for entire optical fields rather than single photons. Unlike qubit-based methods, there is no requirement for single photons for special optical modulation. “Unlike qubit approaches, continuous-variable approaches enable the number of entangled qumodes in a quantum state to be scaled up through frequency, time, or spatial multiplexing without the need of quantum memory or the repeat-until-success strategies,” said Zijiao Yang, who presented the research at the Frontiers in Optics + Laser Science Conference (FiO LS) all-virtual meeting. The quantum microcomb is generated in a 3-mm-diameter silica wedge microresonator with a 22-GHz free spectral range on a silicon chip, and with a single-mode tapered fiber that is used as the coupling waveguide. The microcomb uses two-mode squeezing to create unconditional entanglement between continuous optical fields. The researchers tested the device by measuring 20 qumode pairs created by the new microcomb. They found that the qumodes exhibited a maximum raw squeezing of 1.6 dB and maximum anti-squeezing of 6.5 dB. The raw squeezing is primarily limited by the 83% cavity escape efficiency, 1.7-dB optical loss, and approximately 89% photodiode quantum efficiency. The team reports a total efficiency after the tapered fiber of 60%. The squeezing measurements provide convincing evidence for quantum correlations among the qumodes, but the squeezing level needs to be further increased for quantum information-processing applications. According to the team, the raw squeezing could be improved by reducing system losses, improving photodiode quantum efficiency, and achieving higher resonator-waveguide escape efficiency.