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arXiv:2408.11671 · Applied Physics Letters

In situ mixer calibration for superconducting quantum circuits

Nan Wu, Jing Lin, Changrong Xie, Zechen Guo, Wenhui Huang, Libo Zhang, Yuxuan Zhou, Xuandong Sun, Jiawei Zhang, Weijie Guo, Xiayu Linpeng, Song Liu, Yang Liu, Wenhui Ren, Ziyu Tao, Ji Jiang, Ji Chu, Jingjing Niu, Youpeng Zhong, Dapeng Yu

Published Nov 11, 2024• 5 citations
arXivHTMLPDF

Abstract

Mixers play a crucial role in superconducting quantum computing, primarily by facilitating frequency conversion of signals to enable precise control and readout of quantum states. However, imperfections, particularly local oscillator leakage and unwanted sideband signal, can significantly compromise control fidelity. To mitigate these defects, regular and precise mixer calibrations are indispensable, yet they pose a formidable challenge in large-scale quantum control. Here, we introduce an in situ and scalable mixer calibration scheme using superconducting qubits. Our method leverages the qubit's response to imperfect signals, allowing for calibration without modifying the wiring configuration. We experimentally validate the efficacy of this technique by benchmarking single-qubit gate error and qubit coherence time.

DOI
10.1063/5.0234579

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Mixers play a crucial role in superconducting quantum computing, primarily by facilitating frequency conversion of signals to enable precise control and readout of quantum states. However, imperfections, particularly local oscillator leakage and unwanted sideband signal, can significantly compromise control fidelity. To mitigate these defects, regular and precise mixer calibrations are indispensable, yet they pose a formidable challenge in large-scale quantum control. Here, we introduce an in situ and scalable mixer calibration scheme using superconducting qubits. Our method leverages the qubit's response to imperfect signals, allowing for calibration without modifying the wiring configuration. We experimentally validate the efficacy of this technique by benchmarking single-qubit gate error and qubit coherence time.
High-fidelity control of superconducting qubits requires the generation of microwave-frequency pulses precisely tailored on nanosecond timescales. These pulses are most commonly synthesized by up-converting and superimposing two narrow-band intermediate-frequency signals referred to as the in-phase (I) and quadrature (Q) components. While the calibration of their DC-offsets, relative amplitude and phase allows one to cancel unwanted sideband and carrier leakage, this IQ mixing approach suffers from the presence of additional spurious frequency components. Here, we experimentally study an alternative approach based on double frequency conversion, which overcomes this challenge and circumvents the need for IQ-calibration. We find a spurious-free dynamic range of more than 70 \,dB and compare the quality of pulse generation against a state-of-the-art IQ mixing scheme by performing repeated single-qubit randomized benchmarking on a superconducting qubit.
In the last decade, quantum information processing has transformed from a field of mostly academic research to an applied engineering subfield with many commercial companies announcing strategies to achieve quantum advantage and construct a useful universal quantum computer. Continuing efforts to improve qubit lifetime, control techniques, materials and fabrication methods together with exploring ways to scale up the architecture have culminated in the recent achievement of quantum supremacy using a programmable superconducting processor -- a major milestone in quantum computing en route to useful devices. Marking the point when for the first time a quantum processor can outperform the best classical supercomputer, it heralds a new era in computer science, technology and information processing.One of the key developments enabling this transition to happen is the ability to exert more precise control over quantum bits and the ability to detect and mitigate control errors and imperfections. In this thesis, ways to efficiently control superconducting qubits are explored from the experimental viewpoint.We introduce a state-of-the-art experimental machinery enabling one to perform one- and two-qubit gates focusing on the technical aspect and outlining some guidelines for its efficient operation. We describe the software stack from the time alignment of control pulses and triggers to the data processing organisation. We then bring in the standard qubit manipulation and readout methods and proceed to describe some of the more advanced optimal control and calibration techniques.The described toolbox together with some newly developed advances such as multi-level single-shot readout are applied to a practical problem of generating high-quality random numbers. Following the recent advances in the field, we certify our generator based on the Kochen-Specker theorem utilizing contextuality of quantum mechanics as an underlying resource of randomness. Our implementation based on the solid-state circuit quantum electrodynamics (QED) platform and the new certification scheme allows us to greatly enhance the bit rate of certified quantum random number generators (QRNGs) and approach the requirements for real-world applications.Addressing some of the control and state preparation errors, we develop a set of techniques to use a qubit as an in situ probe of distortions of its control lines and its effective temperature. Considering a qubit simultaneously driven by a longitudinal and a transverse MW-drives under certain resonant conditions, we show that in the rotating dressed-state picture (second rotating frame) one can get information about both the amplitude and the phase of the drive signals. This allows us to employ a qubit as an on-chip vector network analyzer and precisely measure the transfer functions of its control lines. As no other method with comparable frequency domain coverage exists, we verify our technique by cancelling out the distortions and increasing the fidelity of entangling two-qubit CPHASE gate.Finally, we show that the decay of the correlations between two sequential measurements of a quantum bit is a manifestation of its nonzero effective temperature. We use it to develop a precise method of evaluating the qubits population allowing for virtually unlimited suppression of absolute errors. We realize the method and demonstrate its applicability to the calibration of a quantum processor and evaluation of active reset techniques. As the actual mechanism of the increased excited-state population is currently unknown, we use our method's precision to show the qubit’s coupling to two separate independent thermal baths. We also introduce the ‘temperature spectroscopy’ as a tool to acquire more information about the qubits spurious excitation sources.
High-quality two-qubit gate operations are crucial for scalable quantum information processing. Often, the gate fidelity is compromised when the system becomes more integrated. Therefore, a low-error-rate, easy-to-scale two-qubit gate scheme is highly desirable. Here, we experimentally demonstrate a new two-qubit gate scheme that exploits fixed-frequency qubits and a tunable coupler in a superconducting quantum circuit. The scheme requires less control lines, reduces cross talk effect, and simplifies calibration procedures, yet produces a controlled-Z gate in 30 ns with a high fidelity of 99.5%, derived from the interleaved randomized benchmarking method. Error analysis shows that gate errors are mostly coherence limited. Our demonstration paves the way for large-scale implementation of high-fidelity quantum operations.
As superconducting quantum circuits are scaling up rapidly towards the noisy intermediate-scale quantum (NISQ) era, the demand for electronic control equipment has increased significantly. To fully control a quantum chip of N qubits, the common method based on up-conversion technology costs at least 2 × N digital-to-analog converters (DACs) and N IQ mixers. The expenses and complicate mixer calibration have become a hinderance for intermediate-scale quantum control. Here we propose a universal control scheme for superconducting circuits, fully based on parametric modulation. To control N qubits on a chip, our scheme only requires N DACs and no IQ mixer, which significantly reduces the expenses. One key idea in the control scheme is to introduce a global pump signal for single-qubit gates. We theoretically explain how the universal gates are constructed using parametric modulation. The fidelity analysis shows that parametric single-qubit (two-qubit) gates in the proposed scheme can achieve low error rates of 10−4, with a gate time of about 60 ns (100 ns).
A challenge in building large-scale superconducting quantum processors is the precise control and manipulation of the qubit state. However, the crosstalk between the microwave control lines impedes the parallel execution of high-fidelity digital and analog quantum operations. Here, we propose and demonstrate a universal compensation protocol for calibrating the microwave signal crosstalk. We also introduce amplified error sequences to optimize accuracy. Furthermore, we show a definitive improvement in parallel gate operations with crosstalk cancellation, demonstrating the technique's effectiveness. This work paves the way for superconducting hardware that features the automated calibration of microwave crosstalk, leading to enhanced fidelities in multiqubit circuits.
In the pursuit of scalable superconducting quantum computing, tunable couplers have emerged as a pivotal component, offering the flexibility required for complex quantum operations of high performance. In most current architectures of superconducting quantum chips, such couplers are not equipped with dedicated readout circuits to reduce complexity in both design and operation. However, this strategy poses challenges in precise characterization, calibration, and control of the couplers. In this work, we develop a hardware-efficient and robust technique based on adiabatic control to address this issue. The critical ingredient of this technique is adiabatic swap operation between a tunable coupler and nearby qubits. Using this technique, we have characterized and calibrated tunable couplers in our chips and achieved straightforward and precise control over these couplers. For example, we have demonstrated the calibration and correction of the flux distortion of couplers. In addition, we have also expanded this technique to tune the dispersive shift between a frequency-fixed qubit and its readout resonator over a wide range.