Surpassing 99% detection efficiency by cascading two superconducting nanowires on one waveguide with self-calibration

Integrated quantum photonics (IQP) allows for on-chip generation, manipulation and detection of quantum states of light, fostering advancements in quantum communication, quantum computing, and quantum information technologies. Single-photon detector is a key device in IQP that allows for efficient readout of quantum information through the detection of single-photon statistics and measurement of photonic quantum states. The efficacy of quantum information retrieval hinges on the ability to simultaneously detect every single photon with high efficiency, a relationship that grows exponentially with the number of photons (n). Even a slight decrease in single photon detection efficiency can lead to a significant reduction in probability as n grows larger. Here, we introduce a superconductor-semiconductor heterogeneous integration technology that allows for the integration of transversal superconducting nanowires single-photon detectors that eliminate corner loss on various optical waveguides with exceptional efficiency and versatility. Two cascaded nanowires have been integrated on one silicon waveguide, which not only boosts the detection efficiency to 99.73% at a wavelength of 1550 nm but also provides an on-chip calibration setup, allowing such high efficiency to be measured despite the large loss from fiber-to-waveguide coupling and uncertainties from absolute power calibrations. These advancements represent a substantial improvement compared to previous records, approaching the theoretical limit achievable on silicon waveguide, and demonstrate the versatility of heterogeneous integration technology. This breakthrough in ultra-high detection efficiency establishes a new baseline for assessing quantum measurement capabilities on scalable IQP platforms.