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Electronics Letters

Validating S‐parameter measurements of RF integrated circuits at milli‐Kelvin temperatures

Manoj Stanley, S. J. Park, Sebastian E. de Graaf, T. Lindström, J. E. Cunningham, Nick Ridler

Published Jun 18, 2022• 17 citations
Publisher

Abstract

Abstract Techniques to precisely characterise RF components at milli‐Kelvin temperatures support the development of quantum computing systems utilising these components. In this work, an S‐parameter measurement setup to characterise RF integrated circuits at milli‐Kelvin temperatures has been proposed and for the first time, the S‐parameter measurements at milli‐Kelvin temperatures have been validated using two independent calibration techniques, thereby providing more confidence in measurements. The techniques are demonstrated experimentally by comparing and validating calibrated S‐parameter measurements of a cryogenic attenuator integrated circuits at milli‐Kelvin temperatures.

DOI
10.1049/ell2.12545

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Abstract Techniques to precisely characterise RF components at milli‐Kelvin temperatures support the development of quantum computing systems utilising these components. In this work, an S‐parameter measurement setup to characterise RF integrated circuits at milli‐Kelvin temperatures has been proposed and for the first time, the S‐parameter measurements at milli‐Kelvin temperatures have been validated using two independent calibration techniques, thereby providing more confidence in measurements. The techniques are demonstrated experimentally by comparing and validating calibrated S‐parameter measurements of a cryogenic attenuator integrated circuits at milli‐Kelvin temperatures.
Quantum computing technologies utilise microwave components and interfaces at cryogenic temperatures during operation. This paper describes an S-parameter measurement system to characterise coaxial microwave devices at temperatures down to tens of milli-kelvin. The measurement system is calibrated using a Thru-reflect-line technique utilising a coaxial air-dielectric transmission line as the reference impedance standard. Various coaxial microwave devices having a wide range of reflection and transmission coefficient values are measured at both room temperature and mK temperature to investigate the calibration performance and understand the various error sources in the measurements.
Designing large-scale superconducting quantum circuits involves complex microwave engineering to minimize loss, spurious reflections, and signal crosstalk. Essential for engineering more complex circuits is precise knowledge of devices’ microwave performance, which can be characterized by measuring its calibrated scattering parameters (S-parameters) at mK temperatures. In this work, we introduce a full 2-port calibrated S-parameter measurement setup which can characterize superconducting quantum integrated circuits and other RF integrated circuits operating at mK temperatures. The design and architecture of the measurement system consisting of in-house developed microwave calibration unit (MCU) housing the newly designed cryogenic calibration standards and the device under test (DUT) is discussed in detail. The measurement setup is then used to demonstrate the first calibrated S-parameter measurements of an in-house developed superconducting qubit integrated circuit at mK temperature.
In the field of quantum information science, superconducting quantum integrated circuits are operated at millikelvin temperatures inside dilution refrigerators. The microwave performance of such non-connectorised devices can be characterised by measuring their calibrated S-parameters. The design of high-quality microwave standards is essential to achieve accurate calibration. In this work we describe the architecture of a full 2-port S-parameter measurement set-up for use at millikelvin temperatures along with the microwave calibration unit. The design of the standards for use in the calibration unit at millikelvin temperatures is described and verified using electromagnetic simulation and measurements. The suitability of the standards is verified by characterising a commercial attenuator as a test device.
Abstract Development of large-scale quantum computing systems will require radio frequency (RF) and microwave technologies operating reliably at cryogenic temperatures down to tens of milli-Kelvin (mK). The quantum bits in the most promising quantum computing technologies such as the superconducting quantum computing are designed using principles of microwave engineering and operated using microwave signals. The control, readout, and coupling of qubits are implemented using a network of microwave components operating at various temperature stages. To ensure reliable operation of quantum computing systems, it is critical to ensure optimal performance of these microwave components and qubits at their respective operating temperatures, which can be as low as mK temperatures. It is, therefore, critical to understand the microwave characteristics of waveforms, components, circuits, networks, and systems at cryogenic temperatures. The UK’s National Physical Laboratory (NPL) is focussed on developing new microwave measurement capabilities through the UK’s National Quantum Technologies Programme to address various microwave test and measurement challenges in quantum computing. This includes the development of various measurement capabilities to characterize the microwave performance of quantum and microwave devices and substrate materials at cryogenic temperatures. This paper summarizes the roadmap of activities at NPL to address these microwave metrology challenges in quantum computing.
Development of RF and microwave metrology capabilities at cryogenic temperatures is critical for the development of high-performance microwave devices to facilitate commercialization of cryogenic quantum technologies. This article presents a broadband microwave S-parameter calibration scheme suitable for cryogenic environments operating at temperatures down to tens of milli-kelvin (mK). The technique is based on a weighted multi-line thru-reflect-line (TRL) calibration approach and is demonstrated using coaxial air line calibration standards. One-and two-port microwave devices commonly used in cryogenic quantum computing applications, a cryogenic 50 Ω~\Omega Ωmatched load and a cryogenic 6 dB attenuator, were measured. The measured results at mK temperatures indicate that when combined with calibration standards of appropriate electrical length, the weighted multi-line TRL calibration scheme allows broadband frequency coverage compared to conventional TRL calibration schemes utilizing a single line standard. The mechanical and electrical properties of the line standards at mK temperatures were investigated and discussed. These findings establish the feasibility of utilizing multiple off-the-shelf coaxial air lines to enhance the frequency range of calibrations at mK temperatures.
Accurate S-parameter characterization of coaxial microwave devices and interfaces at milli-kelvin (mK) temperatures is essential for building high performance quantum computing systems. Coaxial air-dielectric transmission lines (air lines) are used routinely as primary reference standards for S-parameter measurements at room temperature. For the first time, the feasibility of utilizing such air lines as primary reference standards at mK temperatures is investigated. However, for air lines to be used at such temperatures, they must first be characterized accurately in terms of their characteristic impedance and propagation constant. This paper describes techniques for characterizing air lines at both room temperature (taken here to be 296 K) and mK temperature (around 25 mK). The measurement results demonstrate the feasibility of using air lines as primary reference standards at mK temperatures.