To enable the practical deployment of hardware-based quantum computers employing laser-cooled atoms and ions, it is essential to develop optical source systems that can be operated stably over long periods with high uptime. In particular, when Yb atoms are used as qubits, their two-valence-electron nature provides a rich internal level structure and offers advantages for high-coherence operation and the implementation of error-correction-oriented schemes. Under such conditions, the required light sources must provide lower phase noise and better frequency stability than optical clocks typically require.
As candidate light sources, we selected external-cavity diode lasers (ECDLs), which provide fast response, wide wavelength tunability, compact size, and long operational lifetime. In addition, as the low-noise reference, we employed a 1550 nm laser stabilized to a highly stable optical cavity with an oscillation spectral linewidth below 1Hz, together with an optical frequency comb for transferring these characteristics to arbitrary wavelengths. On this basis, we are pursuing the realization of a compact and robust low-noise optical source system.

To date, we have demonstrated the long-term operation and low-noise performance of the laser system. In this fiscal year, we applied this system to the laser used for observing the clock transition in the optical lattice clock under development in our group, and confirmed its long-term continuous operation.

To reduce the error rate, it is necessary to excite and de-excite atoms with a high fidelity of more than 99 %. Achieving this requires precise control not only of phase noise, but also of the phase, pulse duration, and power. For practical deployment, these conditions must also be reproduced reliably at any time.
At present, this project has achieved an excitation probability slightly above 90 %, as shown in the figure below. Reaching a fidelity of 99 %, or even above 99.9 %, will likely require further noise reduction together with more precise control of the initial phase, power, and pulse duration. In addition, not only the light source but also the atomic system itself will likely need further refinement, such as optimization of the cooling scheme and potential distribution.
We believe that the methods we have developed so far will form part of the foundation for standard light-source technologies for quantum computers in the 2030–2050 time frame, thereby contributing to the realization and practical deployment of quantum computers.