Integrated quantum photonics is one of the leading approaches for the realization of a universal quantum computing machine because it merges together the excellent coherence property of photons with the benefit of using a compact and stable micron-size waveguide platform. One of the key challenges for the realization of a fully integrated optical quantum machine is the integration of high efficiency, photon-number-resolving detectors with optical waveguides. In this Optics Express article Calkins et al. demonstrate high efficiency, photon-number-resolving detectors based on superconducting transition edge sensors (TES). This detector is capable of detecting the small energy of a single photon in the infrared region by measuring the amount of heat deposited with its absorption. The authors fabricated the detectors on an UV-written silica-on-silicon optical waveguide working at telecom wavelengths around 1550 nm. Using an innovative design that takes into account the thermal and optical properties of the detector-waveguide system they reached a detector quantum efficiency of 43%. This result improves by a factor of six the value previously reported by the authors in a similar geometry. With a clever configuration the authors integrated three TES detectors and several Bragg grating structures on a single waveguide. With this geometry they were able to characterize the performance of each individual detector and the losses associated with coupling and propagation in the waveguide. The fact that the efficiencies of the three detectors are within 0.5% of each other is a sign that shows the reproducibility of their design, which is crucial for applications over larger optical networks. A single pass efficiency of 79% was measured for the three detectors in series which increased to 88% in double pass configuration with light reflected back by a Bragg grating located at the end of the device. These results are extremely exciting since these types of detectors have a great potential in terms of multiplexing and scalability into large integrated optical network. This work paves the way towards the realization of integrated quantum photonic devices capable of generating, manipulating and detecting nonclassical light all on a single substrate.

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