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Conference on Precision Electromagnetic Measurements

Methods for characterization and minimization of microwave background in cryogenic environment

J. S. Lehtinen, E. Mykkänen, A. Kemppinen, Antti J. Manninen

Published Jul 1, 2016• 0 citations
Publisher

Abstract

Minimizing microwave background of cryogenic environment is crucial for quantum devices where single microwave photon energy is sufficient to disturb the measured system. Microwave photons with sufficient energy originate from the warmer parts of the cryostat, for example from parts at liquid helium temperature (4.2 K), which can be about three orders of magnitude higher than the sample temperature. Reduction of the microwave background is possible with careful design of both shielding and filtering of the sample holder. Additionally, on-chip filtering of the actual quantum device can improve the device performance significantly.

DOI
10.1109/cpem.2016.7540789

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Minimizing microwave background of cryogenic environment is crucial for quantum devices where single microwave photon energy is sufficient to disturb the measured system. Microwave photons with sufficient energy originate from the warmer parts of the cryostat, for example from parts at liquid helium temperature (4.2 K), which can be about three orders of magnitude higher than the sample temperature. Reduction of the microwave background is possible with careful design of both shielding and filtering of the sample holder. Additionally, on-chip filtering of the actual quantum device can improve the device performance significantly.
Abstract A normal metal–superconductor hybrid single-electron trap with tunable barrier is utilized as a tool for spectrum analysis at the extremely low signal levels, using only well filtered cryogenic microwave background as a photon source in the frequency range from about 50 to 210 GHz. We probe millimeter wave propagation in two superconducting systems: a Josephson junction array around its plasma frequency, and a superconducting titanium film in the limit when the photon energies are larger than the superconducting energy gap. This regime is relevant for improving the performance of cryogenic quantum devices but is hard to access with conventional techniques. We show that relatively simple models can be used to describe the essential properties of the studied components.
Quantum state tomography of weak microwave signals is an important part of many protocols in the field of quantum information processing with superconducting circuits. This step typically relies on an accurate estimation of signal losses in the experimental setup and requires a careful photon-number calibration. Here, we present an improved method for the microwave loss estimation inside of a closed cryogenic system. Our approach is based on Planck’s law and makes use of independent temperature sweeps of individual parts of the cryogenic setup. Using this technique, we can experimentally resolve changes in microwave losses of less than 0.1 dB in the cryogenic environment. We discuss potential applications of this approach for the precise characterization of quantum limited superconducting amplifiers and in other prominent experimental settings.
The careful filtering of microwave electromagnetic radiation is critical for controlling the electromagnetic environment for experiments in solid-state quantum information processing and quantum metrology at millikelvin temperatures. We describe the design and fabrication of a coaxial filter assembly and demonstrate that its performance is in excellent agreement with theoretical modelling. We further perform an indicative test of the operation of the filters by making current-voltage measurements of small, underdamped Josephson junctions at 15 mK.
Recent developments in microwave-based quantum computing technologies have motivated the need for the development of new and improved cryogenic microwave devices packaged in coaxial connectorized technology, and for their performance to be accurately characterized through S-parameter measurements made at cryogenic operating temperatures. To facilitate such accurate characterization, S-parameter measurement systems utilizing in-situ calibration techniques at cryogenic temperatures have been developed. This required use of cryogenic RF switches inside cryostats to select between calibration standards and devices to perform calibration and measurement in a single cooling cycle. However, this caused significant measurement errors due to non-identical signal paths due to differences in switch paths limiting the measurement accuracy of such measurement systems. In this work, a new technique is proposed to significantly improve the accuracy of cryogenic S-parameter measurements of coaxial connectorized devices by accounting for non-ideal effects in such switch-based cryogenic calibration units. The technique is demonstrated at room temperature (296 K) and cryogenic temperature (1.5 K) through S-parameter measurements of coaxial devices with up to two ports.
Cryogenic systems are essential to many modern scientific and technological applications, including MRI superconducting magnets, particle accelerators, astrophysical detectors and quantum computing platforms, all of which operate at temperatures well below 120 K. In this framework, accurate temperature monitoring is, therefore, critical. However, conventional Resistance Temperature Detectors (RTDs) suffer from significant limitations in cryogenic environments, such as wiring complexity, power dissipation, limited low-temperature operation and large footprint, which restrict their integration in increasingly complex cryogenic infrastructures. Superconducting devices offer a promising alternative due to their high sensitivity at low temperatures, low power dissipation and compatibility with integrated quantum technologies. Inspired by Microwave Kinetic Inductance Detectors (MKIDs), this work investigates superconducting microwave resonators as minimally invasive, frequency-multiplexed cryogenic thermometers operating below 12 K. In particular, strategies to tune and enhance temperature sensitivity are explored by studying the effects of the superconducting thin-film thickness and surface nanostructuring. The first part of the study examines NbTi(N) planar resonators with thicknesses ranging from 20 nm to 150 nm, where temperature sensing is based on the temperature dependence of the kinetic inductance. S-shaped split-ring resonators resonating near 1.1 GHz are inductively coupled to a coplanar transmission line and characterized under cryogenic conditions. A clear trade-off between temperature sensitivity and quality factor is observed: thinner films achieve higher sensitivity, up to approximately 19 MHz/K for 20 nm thick devices, while the quality factor, typically exceeding 10^4, is found to degrade when the thickness is reduced. This trade-off leads to an optimal temperature resolution, with sub-microkelvin noise-equivalent temperature (NET) values. In particular, a minimum NET of (0.53±0.09) µK/Hz^(1/2) at 4.2 K is achieved for a 100 nm thick resonator, which is also exploited to study low-frequency temperature fluctuations in liquid helium. In the second part, a possible approach to enhance the temperature sensitivity, through surface nanostructuring, is investigated. Arrays of nanogaps are patterned in high-current regions of a 1.3 GHz resonator, in order to locally suppress superconductivity, reduce the critical temperature of the film and, therefore, accentuate the slope of the temperature-sensitive curve. These weak-links are realized by partially etching a 100 nm NbTi film, to leave 20-30 nm thick remaining nanogaps. Measurements reveal a significant increase in temperature sensitivity, reaching values as high as up to (62±1) MHz/K, which correspond to one order-of-magnitude improvement over non-nanostructured devices. Although this enhancement comes with increased noise-equivalent temperature, the results demonstrate the feasibility of engineering superconducting weak links to improve RF cryogenic thermometry and provide a solid foundation for future optimization. In reason of the achieved results, such an approach could find successful applications for a sub-µK high-resolution cryogenic temperature monitoring, specifically for integrated quantum architectures, further benefiting from the frequency-multiplexing that naturally arises from the multiple allocation of different superconducting microwave resonators.
The performance and scalability of superconducting quantum circuits depends critically on the microwave environment. Minimizing signal reflections and suppressing thermal noise are essential for achieving high-fidelity readout and preserving qubit coherence. A significant challenge arises from the use of conventional cryogenic components such as isolators and circulators, which exhibit nonideal out-of-band reflection characteristics. Reflections degrade impedance matching and limit the performance of broadband quantum limited amplifiers. Superconducting implementations of reflectionless microwave filters offer a promising solution to mitigate these issues. The focus of this work is the fabrication and cryogenic characterization of reflectionless filters compatible with superconducting qubit fabrication flows. Devices were implemented on high resistivity silicon substrates using aluminum ground planes, integrated nichrome resistors, and crossovers formed with SiO2 interlayer dielectric. Cryogenic measurements at 20 mK demonstrate high return loss, confirming the viability of these filters for co-fabrication with traveling-wave parametric amplifiers (TWPAs) and circuit quantum electrodynamics (cQED) architectures. The filters exhibit low insertion loss in the passband to maintain quantum measurement efficiency and provide broadband reflection suppression across frequencies relevant to superconducting qubits, offering a scalable way to manage microwave noise in superconducting quantum processors.