Microwave Output Stabilization of a Qubit Controller via Device-Level Temperature Control

TL;DR

This work tackles long-term amplitude and phase drift in microwave qubit controllers by implementing device-level temperature stabilization for critical analog blocks in the QuEL-1 SE platform. The authors demonstrate subpercent amplitude stability ($\sim$0.15% on average) and sub-degree phase stability ($\sim$0.39$^{\circ}$) across 15 channels over 24 hours, yielding gate infidelities for an $X_{\pi/2}$ operation on the order of $2\times 10^{-6}$ (amplitude) and $2\times 10^{-5}$ (phase). The two-tier thermal management combines global enclosure cooling with local heater/thermistor loops around PLLs, amplifiers, and mixers, significantly reducing drift compared with uncontrolled operation. The results support the scalability of QuEL-1 SE as a reliable, multi-channel microwave control platform for superconducting qubits and potentially other modalities, enabling long-duration quantum operations with high fidelity.

Abstract

We present the design and performance of QuEL-1 SE, which is a multichannel qubit controller developed for superconducting qubits. The system incorporates the active thermal stabilization of critical analog integrated circuits, such as phase-locked loops, amplifiers, and mixers, to suppress the long-term amplitude and phase drift. To evaluate the amplitude and phase stability, we simultaneously monitor 15 microwave output channels over 24 h using a common analog-to-digital converter. Across the channels, the normalized amplitude exhibits standard deviations of 0.09\%--0.22\% (mean: 0.15\%), and the phase deviations are 0.35$^\circ$--0.44$^\circ$ (mean: 0.39$^\circ$). We further assess the impact of these deviations on quantum gate operations by estimating the average fidelity of an $X_{π/2}$ gate under the coherent errors corresponding to the deviations. The resulting gate infidelities are $2\times 10^{-6}$ for amplitude errors and $2\times 10^{-5}$ for phase errors, which are significantly lower than typical fault-tolerance thresholds such as those of the surface code. These results demonstrate that the amplitude and phase stability of QuEL-1 SE enables reliable long-duration quantum operations, thus highlighting its utility as a scalable control platform for superconducting and other qubit modalities.

Figures (11)

• Figure 1: (a) Photograph of interior of QuEL-1 SE unit. The unit is designed as a 19-inch rack-mount chassis with a height of 3U. The height is perpendicular to the plane of the paper. Twelve brown rectangles visible in the "Filter, Path Selector" section are heaters used for temperature stabilization of the RF amplifiers. Note that additional heaters for the PLL ICs and mixers are present but not visible. Fans are placed to the left of the "Power Regulation" section. (b) Simplified block diagram of QuEL-1 SE unit. Two AD9082 devices (labeled "DAC ADC") are used asymmetrically. The upper device handles the four CTRL ports without upconversion, whereas the lower device is used for ROUT and PUMP ports that require upconversion. The QuEL-1 SE unit is optimized for four-multiplex readout of superconducting qubits. For this application, one CTRL port of the lower device serves as an auxiliary channel.
• Figure 2: Clock distribution network of the QuEL-1 SE system. To synchronize all derived clocks with the Rubidium oscillator, an oven-controlled crystal oscillator (OCXO) is employed in the primary clock distributor so that its frequency is stabilized by the temperature control of the OCXO. A compensator for fine frequency adjustment is implemented in the FPGA.
• Figure 3: Step response of the output microwave to a temperature change. (a) Normalized amplitude and phase. (b) Temperatures around an amplifier and PLL IC. The abrupt changes in device temperatures are induced by individual heaters placed close to the devices. The temperatures are measured by thermistors placed close to the devices. As the thermistors are mounted on the printed circuit board (PCB), these measurements do not directly represent the die temperatures.
• Figure 4: Active thermal management system of the QuEL-1 SE. All measured temperatures are digitized using ADCs (Texas Instruments AD7490). The processor compares each digitized temperature with its reference value and applies a proportional–-integral compensator to the difference to determine the PWM duty, which controls the power of the heaters and fans. The heaters and fans are driven by individual MOSFET bridge circuits using PWM gate pulses.
• Figure 5: Component temperatures (PLL IC, amplifier, and mixer) and room temperature (a) with and (b) without active thermal control. The room temperature is shown for reference. When using typical air conditioners, the room temperature fluctuates within a range of 1--2 ℃ with a period of several tens of minutes.