Tomonori Arakawa

The operation of superconducting qubits requires a sensitive readout circuit at cryogenic temperatures, driving the demand for cryogenic non-reciprocal microwave components such as circulators. However, evaluating these components at low temperatures presents significant challenges for companies and institutions without specialized measurement systems. In the development of such cryogenic non-reciprocal components, the temperature dependence of ferrite’s magnetic properties is the most critical factor. Therefore, an evaluation technique for accurately assessing these properties at cryogenic temperatures is essential. In this study, we develop a measurement method to characterize low-loss ferrite materials over a temperature range of 300 K to 2 K. The use of the circularly polarized resonance mode TE_11n enables the direct estimation of circular complex permeability and the determination of key material parameters, including saturation magnetization and damping constant—both essential for assessing the performance of ferrite materials in circulator applications. To validate the reliability of our measurement method, we selected single-crystal YIG as the test material, as its magnetic properties at cryogenic temperatures are relatively well known. This demonstration confirms that our method is effective for characterizing various low-loss ferrite materials that are potential candidates for compact cryogenic non-reciprocal devices.

This paper presents broadband perpendicular permittivity measurements at frequencies up to 220 GHz by using a balanced-type circular disk resonator (BCDR). To achieve this, we develop a BCDR excited by 0.6-mm coaxial lines. The developed resonator is designed to be connectable to a commercially available ultra-broadband vector network analyzer system with 0.6-mm coaxial interfaces, and provides broadband characterization of low-loss substrate materials owing to its mode-selective behavior. Moreover, we propose a new sample configuration for the BCDR method involving dielectric sheets metallized on both sides to eliminate air gaps between the sample and the resonator, which lead to measurement inaccuracies. The developed BCDR and proposed sample configuration are experimentally demonstrated up to 220 GHz by measuring three ceramic samples: fused silica, aluminum nitride, and alumina. The measured perpendicular permittivity results are in good agreement with those obtained using the split-cylinder resonator method and previously reported literature values. We also observe that conventional BCDR measurements using bare samples, without metallization on either side, underestimate the relative permittivity by 1.5 %, 5.5 %, and 4.9 % for fused silica, aluminum nitride, and alumina samples, respectively, owing to the air gap errors. Accurate perpendicular permittivity measurements using the improved BCDR method are expected to contribute to the development of advanced materials for 6G applications.

Quantum information processing systems rely on cryogenic microwave electronics, and printed circuit board (PCB) laminates play an essential role, including integrating quantum chips and connecting microwave circuit elements. In this Letter, we report a method for accurately determining the microwave conductivity and complex permittivity of PCB laminates over a wide temperature range, from 4 to 300 K. The use of higher-order resonant modes of a balanced-type circular disk resonator (BCDR) enables broadband measurements ranging from below 5 GHz to above 20 GHz. Furthermore, a temperature-independent determination scheme is achieved by employing a pair of BCDRs and a cryogenic calibration technique. This method is demonstrated by measuring two commercially available PCB laminates. The results indicate that while dielectric loss is monotonically reduced at cryogenic temperatures, the reduction in conductor loss is strongly suppressed by the surface roughness of the copper foil. Additionally, the obtained conductivity as a function of frequency and temperature fits well with the Gradient Model, allowing for the evaluation of the root mean square roughness parameter.

We have developed a circularly polarized dielectric rutile (TiO2) cavity with a high quality-factor that can generate circularly polarized microwaves from two orthogonal linearly polarized microwaves with a phase difference of ±π/2 using a hybrid coupler. Using this cavity, we have established a new methodology to measure the microwave Hall conductivity of a small single crystal of metal in the skin-depth region. Based on the cavity perturbation technique, we have shown that all components of the surface impedance tensor can be extracted under the application of a magnetic field by comparing the right- and left-handed circularly polarized modes. To verify the validity of the developed method, we performed test measurements on tiny Bi single crystals at low temperatures. As a result, we have successfully obtained the surface impedance tensor components and confirmed that the characteristic field dependence of the ac Hall angle in the microwave region is consistent with the expectation from the dc transport measurements. These results demonstrate a significant improvement in sensitivity compared to previous methods. Thus, our developed technique allows for more accurate microwave Hall measurements, opening the way for new approaches to explore novel topological quantum phenomena, such as time-reversal symmetry breaking in superconductors.

To realize a quantum information processing system, it is necessary not only to increase the number of physical qubits but also to cooperatively operate a multitude of cryogenic microwave components such as cables, circulators, and amplifiers for controlling or reading them. Therefore, it is essential to develop metrological methods to precisely evaluate the microwave scattering parameters of each component at low temperatures. Here, we report a calibrated microwave measurement method for a wide temperature range of 4 K to 300 K. At each temperature, we performed a full two-port calibration, which enables us to precisely measure the scattering parameters of various cryogenic microwave components up to 26.5 GHz by using 3.5 mm connectors as the reference planes. We carefully verified the validity of the calibration impedance elements and demonstrated our method by evaluating the temperature-dependent characteristics of a coaxial cable and a circulator.

As an application of cryo-CMOS technology for use in future quantum computing, we have explored low-frequency noise reduction in a two-dimensional (2D) system consisting of a molybdenum disulfide (MoS2) channel sandwiched by hexagonal boron nitride (hBN) layers. Due to the passivation effect of hBN layers, low-frequency noise in hBN/MoS2/hBN channel devices exhibited substantial reduction compared to the case of MoS2 channel directly on a silicon dioxide substrate and also silicon devices, suggesting that the clean interface of substrate and gate dielectric layers, as well as the protected surface of the MoS2 channel by hBN passivation from the damage by the fabrication process, contribute to the strong reduction in low-frequency noise. The results indicated that 2D materials are suitable for cryo-CMOS technology in terms of low-frequency noise since they will bring about mitigation of one of the most serious causes of quantum phase decoherence of qubits in future large-scale integrated quantum computers.

To support the development of smart technology, such as the Internet of Things, smart devices, and smart manufacturing, 5G and beyond mobile communication systems will utilize higher frequencies. Accordingly, the number and types of network devices will increase, and how to suppress electromagnetic interference in such small wireless communication terminals will be an important issue. As one of the essential technologies, a noise suppression sheet needs to be developed to minimize electromagnetic interference problems in the new frequency bands. Here, we have developed a method to evaluate the transmission attenuation characteristics of noise suppression sheets up to 60 GHz, with reference to the IEC standard (IEC62333-2: below 6 GHz). To confirm its practicality, a systematic demonstration was performed by measuring commercially available noise suppression sheets under various conditions. The mechanism of the transmission attenuation effect in a wide frequency range, including an unexpected effect due to surface modes, is investigated based on material properties and electromagnetic simulations.

Dynamical conductivity contains information of dissipative and nondissipative processes induced by ac-electric fields. In the integer quantum Hall (QH) effect where the nondissipative Hall current is the most prominent feature, its robustness is assured by localized states within the Landau levels. We establish a noncontact method with a circular cavity resonator and detect the real and imaginary parts of the longitudinal and Hall conductivities at a microwave frequency in magnetic fields. The conventional Shubnikov–de Haas oscillations and QH plateaus are observed in the real parts of longitudinal and Hall conductivities, respectively, while periodic structures can be seen in the imaginary parts which are scaled by the QH filling factor. The latter originates from intra-Landau level transitions between different orbital angular momenta. The results demonstrate that the dynamical conductivity measurement provides microscopic information which is not accessible by conventional static methods. The present noncontact method would pave the way to reveal the electron dynamics in other two-dimensional systems such as twisted bilayer graphene.