Demonstrating Long-Term Stable Operation of Qubit Control Device

[Overview]

A device development and research group, including Engineer Yoshinori Kurimoto, Research Scientist Ryutaro Ohira, and CTO Takefumi Miyoshi (Specially Appointed Professor at Osaka University's Quantum Information and Quantum Life Science Center (QIQB)) from QuEL Inc., along with Professor Makoto Negoro from Osaka University's QIQB, has developed the qubit control device "QuEL-1 SE", which achieves long-term stabilization of microwave (Note 2) signals used for controlling superconducting qubits (Note 1), and experimentally demonstrated its performance. In this study, by introducing individual temperature control mechanisms for analog devices such as Phase-Locked Loops (PLLs) and amplifiers, fluctuations in the amplitude and phase of microwave signals, which become problematic during long-term operation, were significantly suppressed. Simultaneous measurements of 15 channels of microwave output over 24 hours confirmed that the standard deviation (Note 3) of amplitude was suppressed to 0.09–0.22% (average 0.15%), and the standard deviation of phase was suppressed to 0.35–0.44° (average 0.39°). Errors in single-qubit gates caused by these amplitude and phase fluctuations are estimated to be sufficiently smaller than the typical fault-tolerant threshold required for quantum error correction. This achievement contributes to the realization of highly stable microwave control technology, which is crucial for the long-term operation and scaling up of quantum computers. This research result was published in Review of Scientific Instruments.

[Announcement Details]

In quantum computers, microwave signals are widely used to manipulate the state of qubits. Especially for superconducting qubits, precise control of the microwave's frequency, amplitude, and phase is essential for achieving high-fidelity quantum gates. However, in actual control devices, temperature changes in analog circuits such as Phase-Locked Loops (PLLs), amplifiers, and mixers cause the amplitude and phase of microwave signals to fluctuate over time. These fluctuations degrade the accuracy of quantum gates during long-term quantum computation and quantum error correction processes. To solve this problem, the research group introduced a device-level temperature control mechanism in the qubit control device QuEL-1 SE (Figure 1) to individually control the temperature of each analog device.

In this system, temperature-sensitive components such as amplifiers, PLLs, and mixers are subjected to feedback control combining temperature detection by thermistors and heating by heaters, achieving stable operation unaffected by ambient temperature fluctuations.

In the experiment, a total of 15 channels of microwave signals output from three QuEL-1 SE devices were simultaneously measured with a single ADC to evaluate amplitude and phase fluctuations over 24 hours.

As a result, it was confirmed that the standard deviation of amplitude was suppressed to a very small value of 0.09–0.22%, and the standard deviation of phase to 0.35–0.44°. These fluctuations were less than half compared to cases without temperature control, demonstrating the effectiveness of temperature control (Figure 2).

Regarding the impact of this stability on quantum gates, single-qubit gate errors due to amplitude and phase errors were estimated to be approximately 2×10⁻⁶ and approximately 2×10⁻⁵, respectively, which are well below the fault-tolerant threshold assumed for quantum error correction.

These results indicate that QuEL-1 SE can provide stable microwave control even during long-term quantum computations, and it is expected to become an important foundational technology for the realization of future large-scale quantum computers.