Progress Report
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Development of Large-scale Fault-tolerant Universal Optical Quantum Computers[6] Research and development on photon number counters with high detection efficiency and high counting rete made of Titanium superconductors
Progress until FY2025
1. Outline of the project
In this research topic, we have developed a photon number resolving detector (PNRD) based on superconducting transition-edge sensors (TES), which is a key component for implementing non-Gaussian operations required for fault-tolerant universal optical quantum computing. In particular, we focused on realizing high system detection efficiency, high energy resolution, and high-speed operation simultaneously at telecommunication wavelengths. By exploiting titanium-based superconducting TES devices combined with optical absorption cavities composed of dielectric multilayer films, we successfully established a detector platform that satisfies the stringent requirements for photon-number-resolving measurements in large-scale quantum optical systems. In addition, a national-standard-traceable evaluation system was established to quantitatively guarantee the performance of quantum optical sensors, enabling internationally comparable and reliable performance assessment.
2. Outcomes achieved
(a) Design and fabrication of high-efficiency TES devices
We designed and fabricated TES devices with optical absorption cavities optimized for the telecommunication wavelength band (around 1550 nm). The cavity structure consists of dielectric multilayer films (Ta2O5/SiO2) deposited using ion-beam sputtering, with precisely controlled stoichiometry and thickness. The complex refractive indices of the films were experimentally determined by multi-angle spectroscopic ellipsometry and incorporated into the cavity design.
The optimized structure includes a total of 16 dielectric layers on the backside of the TES together with an Au reflective mirror, as well as three anti-reflection layers on the front side. Numerical simulations predicted a photon absorption probability of approximately 99.7 % at 1550 nm. Microfabricated TES devices based on this design were successfully realized.
(b) Evaluation of high-efficiency and high-speed TES performance
The photon-number-resolving performance of the fabricated TES devices was experimentally evaluated using pulsed coherent light at a wavelength of 1.5 µm. Clear photon-number discrimination was observed in the reconstructed photon-number distributions.
Thanks to the optimized titanium/gold TES design and readout configuration, we achieved a rise time of 27 ns and a decay time constant of 32 ns, representing one of the fastest response characteristics reported for TES-based photon-number-resolving detectors. This fast response enabled photon-number discrimination under repetition rates of up to 10 MHz.
Using a national-standard-traceable calibration system, the system detection efficiency was evaluated to be 99.0 ± 0.60 % in the C-band, which exceeds the original project milestone and represents a world-class level of performance for photon-number-resolving detectors.
Furthermore, quantitative evaluation based on POVM reconstruction demonstrated high-fidelity photon-number discrimination, achieving an average fidelity of 0.991 for photon-number states from 0 to 16 photons, even under high repetition-rate operation.
(c) Signal processing architecture for high-count-rate operation
In parallel with device development, we established a real-time signal processing architecture implemented on FPGA to address signal pile-up and noise degradation under high count-rate conditions. A trapezoidal filter optimized for TES pulse shapes was implemented, enabling stable pulse-height extraction in continuous operation.
This signal processing framework, combined with the fast TES devices, forms a robust photon-number-resolving measurement system capable of reliable operation at high repetition rates.
3. Future perspective
Through this research, we have established a mature experimental platform for high-efficiency, high-speed photon-number-resolving detection that directly supports non-Gaussian resource generation such as multi-photon projective measurements required for fault-tolerant optical quantum computing.
While operation in the GHz regime remains a target for subsequent development phases, the present results clearly define a technically sound and quantitatively validated intermediate milestone, bridging conventional TES technology and future ultra-high-speed photon-number-resolving detectors. In parallel, a new photon-detection principle beyond conventional TES operation has been conceptually verified, and its intellectual property strategy is being prepared for future development.