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Chinese Physics B

An Integrated Broadband Low-Noise Microwave Source for Superconducting Quantum Circuits

Tingjin Chen, Jiawei Zhang, Zhiyi Wu, Rui Wang, Zhili Zhong, Peisheng 培生 Huang 黄, Ruyi Kang, Zechen Guo, Yongqi Liang, Daxiong Sun, Xuandong Sun, Zimu Yang, Xiaohan Yang, Yubin Zhang, Jin Zhou, Jiawei Qiu, Ji Chu, Ji Jiang, Xiayu Linpeng, Jiajian Zhang, Yuefeng Yuan, Song Liu, Junhua Zhang, Jingjing Niu, Dapeng Yu

Published Oct 9, 2026• 0 citations
Publisher

Abstract

Abstract Scaling superconducting quantum processors increasingly relies on custom control electronics that can simultaneously satisfy spectral purity, multi-channel isolation, and long-term stability within a compact and integrated platform. Here we present Quantum microwave source (QMW), a classical three-channel broadband low-noise microwave source that consolidates qubit XY drive, readout excitation, and Josephson parametric amplifier (JPA) pump generation onto a single hardware module operating from 4 to 14 GHz with an output power range of -20 to +10 dBm. Electronic characterization yields a spurious-free dynamic range greater than 55 dB, equivalently a largest spur below -55 dBc, a phase noise as low as -110 dBc/Hz at 10 kHz offset for a 5 GHz carrier and below -103 dBc/Hz at the same offset across the full band, and output power stability within 0.1 dB over 24 hours. Integrated into the control chain of a 36-qubit superconducting processor, QMW pumps a JPA to 19.2 dB gain to enable high-SNR single-shot readout, while sustaining qubit coherence times of T 1 = 60.1 ± 0.8 μs, T 2,Ramsey = 24.2 ± 1.5 μs, and T 2,echo = 58.6 ± 2.1 μs, and an average single-qubit gate fidelity of 99.95 ± 0.05% measured by randomized benchmarking. The compact and modular nature of QMW provides a scalable microwave generation solution for largescale superconducting quantum processors, with potential extensions to other quantum hardware platforms and precision measurement applications.

DOI
10.1088/1674-1056/aeb23d

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Abstract Scaling superconducting quantum processors increasingly relies on custom control electronics that can simultaneously satisfy spectral purity, multi-channel isolation, and long-term stability within a compact and integrated platform. Here we present Quantum microwave source (QMW), a classical three-channel broadband low-noise microwave source that consolidates qubit XY drive, readout excitation, and Josephson parametric amplifier (JPA) pump generation onto a single hardware module operating from 4 to 14 GHz with an output power range of -20 to +10 dBm. Electronic characterization yields a spurious-free dynamic range greater than 55 dB, equivalently a largest spur below -55 dBc, a phase noise as low as -110 dBc/Hz at 10 kHz offset for a 5 GHz carrier and below -103 dBc/Hz at the same offset across the full band, and output power stability within 0.1 dB over 24 hours. Integrated into the control chain of a 36-qubit superconducting processor, QMW pumps a JPA to 19.2 dB gain to enable high-SNR single-shot readout, while sustaining qubit coherence times of T 1 = 60.1 ± 0.8 μs, T 2,Ramsey = 24.2 ± 1.5 μs, and T 2,echo = 58.6 ± 2.1 μs, and an average single-qubit gate fidelity of 99.95 ± 0.05% measured by randomized benchmarking. The compact and modular nature of QMW provides a scalable microwave generation solution for largescale superconducting quantum processors, with potential extensions to other quantum hardware platforms and precision measurement applications.
Abstract With the rapid scaling of superconducting quantum processors, electronic control systems relying on commercial off-the-shelf instruments face critical bottlenecks in signal density, power consumption, and crosstalk mitigation. Here we present a custom dual-channel direct current (DC) source module (QPower) dedicated to large-scale superconducting quantum processors. The module delivers a voltage range of ±7 V with 200 mA maximum current per channel, while achieving the following key performance benchmarks: noise spectral density of 20 nV/ Hz at 10 kHz, output ripple <500 μVpp within 20 MHz bandwidth, and long-term voltage drift <5 μVpp over 12 hours. Integrated into the control electronics of a 66-qubit quantum processor, QPower enables qubit coherence time of T 1 = 87.6 μs and Ramsey dephasing time of T 2 = 5.1 μs, with qubit resonance frequency drift constrained to ±40 kHz during 12-hour operation. This modular design is compact in size and efficient in energy consumption, providing a scalable DC source solution for intermediate-scale quantum processors with stringent noise and stability requirements, with potential extensions to other quantum hardware platforms and precision measurement systems.
Nonreciprocal microwave devices, such as circulators and isolators, are needed in high-fidelity qubit readout schemes to unidirectionally route the readout signals and protect the qubits against noise coming from the output chain.However, cryogenic circulators and isolators are prohibitive in scalable superconducting architectures because they rely on magnetic materials.Here we report a fast (750 ns) high-fidelity (95%) quantum nondemolition readout of a coherent superconducting qubit (T 1 = 52 μs, T 2E = 35 μs) without any nonreciprocal magnetic devices.We use in our readout chain a microwave-controlled qubit readout multichip module (QRMCM) that integrates interferometric directional Josephson devices consisting of an isolator and a reconfigurable isolator or amplifier device, and an off-chip low-pass filter.Using the QRMCM, we demonstrate isolation up to 45 dB within 13 MHz, when both directional devices are operated as isolators, and low-noise amplification in excess of 10 dB within a dynamical bandwidth of 10 MHz, when the reconfigurable device is operated as an amplifier.We also investigate the dependence of the qubit coherence times T ϕ and T 2E on the isolation response of the QRMCM, which we control in situ using the microwave drives feeding the isolators.Furthermore, by directly comparing the QRMCM performance with that of a state-of-art configuration (with T 2E ≈ 2T 1 ) that uses a pair of wideband magnetic isolators, we find that the excess pure dephasing measured with the QRMCM (for which T 2E ≈ T 1 ) is likely limited by the residual thermal photon population in the readout resonator.Improved versions of the QRMCM could replace magnetic circulators and isolators in large superconducting quantum processors.
Fast, high-fidelity, single-shot readout of superconducting qubits in quantum processors demands quantum-limited amplifiers to preserve the optimal signal-to-noise ratio. Typically, quantum-limited amplification is achieved with parametric down-conversion of a strong pump tone, which imposes significant hardware overhead and severely limits scalability. Here, we demonstrate the first DC-powered broadband amplifier operating within 0.2 photons of the quantum limit. Our impedance-engineered Inelastic Cooper-pair Tunneling Amplifier (ICTA)-a voltage-biased SQUID in which Cooper pairs tunnel inelastically by emitting signal-idler photon pairs-operates in reflection, delivering 13 dB of average gain across a 3.5 GHz bandwidth in a single stage. Semiclassical simulations accurately predict the gain and saturation power, enabling further design improvements. By eliminating the pump-tone infrastructure, the broadband ICTA promises to dramatically reduce the hardware complexity of quantum-limited amplification in superconducting quantum processors.
High-fidelity qubit measurement is a critical element of all quantum computing architectures. In superconducting systems, qubits are typically measured by probing a readout resonator with a weak microwave tone that must be amplified before reaching the room temperature electronics. Superconducting parametric amplifiers have been widely adopted as the first amplifier in the chain, primarily because of their low noise performance, approaching the quantum limit. However, they require isolators and circulators to route signals up the measurement chain and to protect qubits from amplified noise. While these commercial components are wideband and simple to use, their intrinsic loss, size, and magnetic shielding requirements impact overall measurement efficiency and scalability. Here we report a parametric amplifier that achieves both broadband forward amplification and backward isolation in a single, compact, non-magnetic circuit that could be integrated on chip with superconducting qubits. The approach relies on a nonlinear transmission line that supports traveling-wave parametric amplification of forward propagating signals, and isolation via frequency conversion of backward propagating signals. This traveling-wave parametric amplifier and converter has the potential to reduce the readout hardware overhead when scaling up the size of superconducting quantum computers.
Control electronics for superconducting quantum processors have strict requirements for accurate command of the sensitive quantum states of their qubits. Hinging on the purity of ultra-phase-stable oscillators to upconvert very-low-noise baseband pulses, conventional control systems can become prohibitively complex and expensive when scaling to larger quantum devices, especially as high sampling rates become desirable for fine-grained pulse shaping. Few-gigahertz radio-frequency (RF) digital-to-analog converters (DACs) present a more economical avenue for high-fidelity control while simultaneously providing greater command over the spectrum of the synthesized signal. Modern RF DACs with extra-wide bandwidths are able to directly synthesize tones above their sampling rates, thereby keeping the system clock rate at a level compatible with modern digital logic systems while still being able to generate high-frequency pulses with arbitrary profiles. We have incorporated custom superconducting qubit control logic into off-the-shelf hardware capable of low-noise pulse synthesis up to 7.5 GHz using an RF DAC clocked at 5 GHz. Our approach enables highly linear and stable microwave synthesis over a wide bandwidth, giving rise to high-resolution control and a reduced number of required signal sources per qubit. We characterize the performance of the hardware using a five-transmon superconducting device and demonstrate consistently reduced two-qubit gate error (as low as 1.8%), which we show results from superior control chain linearity compared to traditional configurations. The exceptional flexibility and stability further establish a foundation for scalable quantum control beyond intermediate-scale devices.
Feedback control in classical systems is an indispensable, ubiquitous tool. The theoretical basis for achieving optimal classical control is well understood, and crucially relies on a very classical assumption: that measurements of the state of a system under control need not perturb that state. In a quantum context this assumption is fundamentally invalid. Although many aspects of the theory of quantum feedback control are relatively well developed, the technological basis for feedback control of a single quantum system has only very recently matured. We demonstrate the experimental realization of a quantum feedback control protocol, perpetually stabilizing the coherent Rabi oscillations of a superconducting qubit. This is the first utilization of quantum feedback control for stabilizing a dynamical process, and the first application of quantum feedback in a solid-state system of any kind. This demonstration comprises the first half of this thesis. The feedback protocol is predicated on the ability to make high-fidelity quantum measurements, which are enabled by quantum-limited Josephson parametric amplifiers (JPAs). The design and realization of the novel Josephson traveling-wave parametric amplifier (JTWPA) comprises the second half of this thesis. The JTWPA achieves order-of-magnitude improvements over state of the art JPAs in bandwidth and signal power handling while providing quantum-limited noise performance, potentially enabling the simultaneous readout of dozens of superconducting qubits and the generation of broadband multi-mode squeezing in the microwave domain.