We describe a CMOS photonic integrated circuit for fully on-chip generation of frequency-bin and polarization-
entangled photon pairs. The Sagnac-inspired design uses an on-chip polarization splitter-rotator to bidirectionally
pump a microring resonator and generate entangled photon pairs through spontaneous four-wave mixing in
frequency bins spaced 38.4 GHz apart with < 6 GHz linewidth. By recombining the counterpropagating outputs
into orthogonal polarization modes with a second polarization splitter-rotator, the source outputs polarization
Bell states with high fidelity (95% on average for ≥ 10 bins away from the pump) across the C- and L-bands
(> 9 THz)—a bandwidth currently limited only by the passband of our wavelength-selective switch. Our source
has applications in flex-grid entanglement distribution, where adjacent frequency bins may be combined to
improve the flux and coincidences received by an end-user. Additionally, the source can support a high density
of information per photon pair as a hyperentangled resource in the polarization and frequency-bin degrees of
freedom.
Frequency-bin encoding is massively parallelizable and robust for optical fiber transmission. When coupled with an additional degree of freedom (DoF), the expansion of the Hilbert space allows for deterministic controlled operations between two DoFs within a single photon. Such capabilities, when combined with photonic hyperentanglement, are of great value for quantum communication protocols, including dense coding and single-copy entanglement distillation. In this talk, we present an all-fiber-coupled, ultrabroadband polarization–frequency hyperentangled source and conduct comprehensive quantum state tomography across multiple dense wavelength division multiplexing channels spanning the optical C+L-band (1530–1625 nm). In addition, we design and implement a high-fidelity controlled-NOT (cnot) operation between polarization and frequency DoFs by exploiting electro-optic phase modulation within a fiber Sagnac loop. Collectively, our hyperentangled source and two-qubit gate should unlock new opportunities for harnessing polarization–frequency resources in established telecommunication fiber networks for future quantum applications.
Quantum networking holds tremendous promise in transforming computation and communication. Entangled-photon sources are critical for quantum repeaters and networking, while photonic integrated circuits are vital for miniaturization and scalability. In this talk, we focus on generating and manipulating frequency-bin entangled states within integrated platforms. We encode quantum information as a coherent superposition of multiple optical frequencies; this approach is favorable due to its amenability to high-dimensional entanglement and compatibility with fiber transmission. We successfully generate and measure the density matrix of biphoton frequency combs from integrated silicon nitride microrings, fully reconstructing the state in an 8 × 8 two-qudit Hilbert space, the highest so far for frequency bins. Moreover, we employ Vernier electro-optic phase modulation methods to perform time-resolved measurements of biphoton correlation functions. Currently, we are exploring bidirectional pumping of microrings to generate indistinguishable entangled pairs in both directions, aiming to demonstrate key networking operations such as entanglement swapping and Greenberger–Horne–Zeilinger state generation in the frequency domain.
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