Progress Report
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Development of Large-scale Fault-tolerant Universal Optical Quantum Computers[2] Research and development on superconducting photon number discriminator
Progress until FY2025
1. Outline of the project
Quantum entanglement is generated by squeezed light and a beam splitter, and GKP qubits (qubits encoded by an error correcting code for continuous variables proposed by Gottesman, Kitaev and Preskill) are generated by detecting a predetermined number of photons. The GKP qubit appears at the moment when a predetermined number of photons is detected. We are developing a high-speed photon number resolving detector for GHz clock optical quantum computers. A superconducting transition edge sensor (TES) in the communication wavelength band can resolve the number of photons. TES is operated at the superconducting transition edge, by setting a proper operation temperature.
The temperature rise triggered by the absorption of near-infrared incident photons is read out as a current drop which corresponds to an absorbed energy. In order to realize high-speed TES operation, we will fabricate a device that minimizes the size of the sensor so that the temperature rise can propagate immediately over the entire sensor and the current change occurs instantaneously. The sensor size is to be minimized, thus the corresponding heat capacity is minimized, which contributes to the higher temperature rise. Therefore, higher S/N ratio, faster signal, and better identification of the number of photons are expected.
2. Outcome so far
A photograph of the fabricated 8 μm square TES is shown in Fig.1 (left). The TES was prepared on a silicon substrate, and was processed to have an outer shape that matches the size of the optical component. It is then self-aligned so that it can be aligned with the optical fiber. It was confirmed that the fabricated TES showed a rise time of 16.2 ns and a corresponding bandwidth of ~20 MHz.
Furthermore, to expand the operating bandwidth of the TES, we first focused on the fact that the signal generation speed is limited by thermal conduction within the thin film.
We developed a device in which the active area is divided into parallel segments to ensure high-speed response from each individual segment, and we have confirmed that photon-number resolution is still achievable with this design.
On the other hand, the bottleneck caused by readout technology remained an ongoing challenge. Typically, TES readout is performed using a SQUID (Superconducting Quantum Interference Device); however, due to limitations in its operating bandwidth, it was not possible to obtain high-speed signals. To address this, we considered a method to measure high-speed timing signals using a HEMT (High Electron Mobility Transistor) amplifier operated at cryogenic temperatures and successfully demonstrated this through experiments. Although it was still under high-photon-number irradiation, we obtained high-speed timing signals with sub-nanosecond regime.
Furthermore, to enhance the absorption efficiency, we developed an optical cavity structure for the iridium TES, consisting of a dielectric multilayer anti-reflective (AR) coating and a mirror structure.
3. Future plans
During the project period, we successfully achieved a breakthrough in enhancing the operating bandwidth, which was the most significant challenge. The future is to achieve high-speed timing characteristics even in the measurement of single to few-photon levels.