High-dimensional photonic entanglement holds substantial promise for advancing quantum communication, computation, and metrology. For example, large-alphabet quantum communication protocols are known to benefit from enhanced noise resilience and information capacity via multibit time-bin encoding. Yet, characterizing high-dimensional entangled states is challenging, as full-state tomography becomes prohibitively costly and often requires unrealizable measurements. Here, we demonstrate a scan-free method to characterize high-dimensional entanglement in the time-frequency domain. Our reconstruction achieves a record 5.70 ± 0.07 ebits and a fidelity of 65.4 ± 0.4% with the maximally entangled state of local dimension 1021, certifying the presence of 668-dimensional entanglement. We further prove the attainability of a secure key rate of 15.6 kilobits per second in a composable finite-size, entanglement-based protocol and show that in continuous operation, the setup can quickly approach asymptotic key rates. Using commercial telecom components and state-of-the-art low-jitter single-photon detectors, our scalable architecture offers a practical path toward high-rate, noise-resilient quantum communication test beds.
Chang et al. (Wed,) studied this question.