Since superconducting quantum computers use microwave signals to control and readout their qubits, microwave transmission lines and circuits that can be used under cryogenic conditions are required. For microwave transmission lines, thermal insulation is essential to enhance wiring density and to prevent parasitic heating that can lead to increased computational errors. Thus, here we design and characterize a scalable, thermally insulated microwave transmission line. The transmission line designed for superconducting qubit readout at 5 GHz band achieves both thermal insulation and low loss, by employing resonator-based impedance matching in a microstripline structure. A flexible cable including eight planar transmission lines was designed, fabricated and characterized using S-parameter measurement, demonstrating low crosstalk below 40dB and signal attenuation of -23.5±0.07dB; this coincides with electromagnetic simulations. These microwave properties remained stable across a wide temperature range from room temperature down to 4 K, including steep gradients. The integration of chip attenuators further reduces thermal noise and protects sensitive cryogenic components. Our approach offers a promising solution for compact, high-density microwave wiring using flat cables under thermal gradients, necessary for large scale quantum computing.
Since superconducting quantum computers use microwave signals to control and readout their qubits, microwave transmission lines and circuits that can be used under cryogenic conditions are required. For microwave transmission lines, thermal insulation is essential to enhance wiring density and to prevent parasitic heating that can lead to increased computational errors. Thus, here we design and characterize a scalable, thermally insulated microwave transmission line. The transmission line designed for superconducting qubit readout at 5 GHz band achieves both thermal insulation and low loss, by employing resonator-based impedance matching in a microstripline structure. A flexible cable including eight planar transmission lines was designed, fabricated and characterized using S-parameter measurement, demonstrating low crosstalk below 40dB and signal attenuation of -23.5±0.07dB; this coincides with electromagnetic simulations. These microwave properties remained stable across a wide temperature range from room temperature down to 4 K, including steep gradients. The integration of chip attenuators further reduces thermal noise and protects sensitive cryogenic components. Our approach offers a promising solution for compact, high-density microwave wiring using flat cables under thermal gradients, necessary for large scale quantum computing.
Quantum computers are expected to surpass current modern computers in certain types of computations by utilizing quantum mechanical properties of atoms, molecules, and photons. Among the various systems being explored for realizing such computers, superconducting qubits have enabled many pioneering experimental demonstrations due to their ability to perform nonlinear operations and be integrated using semiconductor fabrication technologies. These quantum computers require operation at extremely low temperatures, around 10 mK, using dilution refrigerators. A major challenge at these temperatures is the heat inflow caused by wiring used to control and read out the qubits. This study reports on the development of a transmission line that can operate at low temperatures and achieves both low thermal inflow and low transmission loss in the 5 GHz band, which is used for qubit readout. Here we focus on the scalability of the transmission line and design a parallel set of transmission lines with low crosstalk.
Integration and low power consumption of cryogenic amplifiers, which are essential for reading out quantum qubits, are inevitable issues for scaling up quantum information processing systems. Although cryogenic probers play an important role in this development, measurement solutions using impedance tuners are not available due to the long high frequency wiring for thermal insulation. In this paper we propose a new probe head structure with high thermal insulation. We eliminate metal heat conduction by using electromagnetic coupling between planar resonant circuits. In addition to evaluating the transmission characteristics of the prototype probe head at room temperature, we experimentally assess its thermal stability under conditions with a significant thermal gradient. This technique enables shorter high-frequency lines in cryogenic probers by providing the probe head itself with thermal insulating properties.
We describe progress and initial results achieved towards the goal of developing integrated multi-conductor arrays of shielded controlled-impedance flexible superconducting transmission lines with ultra-miniature cross sections and wide bandwidths (dc to >10 GHz) over meter-scale lengths. Intended primarily for use in future scaled-up quantum computing systems, such flexible thin-film niobium/polyimide ribbon cables could provide a physically compact and ultra-low thermal conductance alternative to the rapidly increasing number of discrete coaxial cables that are currently used by quantum computing experimentalists to transmit signals between the several low-temperature stages (from ∼4 K down to ∼20 mK) of a dilution refrigerator. We have concluded that these structures are technically feasible to fabricate, and so far they have exhibited acceptable thermo-mechanical reliability. S -parameter results are presented for individual 2-metal layer Nb microstrip structures having 50 Ω characteristic impedance; lengths ranging from 50 to 550 mm were successfully fabricated. Solderable pads at the end terminations allowed testing using conventional rf connectors. Weakly coupled open-circuit microstrip resonators provided a sensitive measure of the overall transmission line loss as a function of frequency, temperature, and power. Two common microelectronic-grade polyimide dielectrics, one conventional and the other photo-definable (PI-2611 and HD-4100, respectively) were compared. Our most striking result, not previously reported to our knowledge, was that the dielectric loss tangents of both polyimides, over frequencies from 1 to 20 GHz, are remarkably low at deep cryogenic temperatures, typically 100 × smaller than corresponding room temperature values. This enables fairly long-distance (meter-scale) transmission of microwave signals without excessive attenuation, and also permits usefully high rf power levels to be transmitted without creating excessive dielectric heating. We observed loss tangents as low as 2.2 × 10 −5 at 20 mK, although losses increased somewhat at very low rf power levels, similar to the well-known behavior of amorphous inorganic dielectrics such as SiO 2 . Our fabrication techniques could be extended to more complex structures such as multiconductor cables, embedded microstrip, 3-metal layer stripline or rectangular coax, and integrated attenuators and thermalization structures.
A robust cryogenic infrastructure in form of a wired, thermally optimized dilution refrigerator is essential for solid-state based quantum processors. Here, we engineer a cryogenic setup, which minimizes passive and active heat loads, while guaranteeing rapid qubit control and readout. We review design criteria for qubit drive lines, flux lines, and output lines used in typical experiments with superconducting circuits and describe each type of line in detail. The passive heat load of stainless steel and NbTi coaxial cables and the active load due to signal dissipation are measured, validating our robust and extensible concept for thermal anchoring of attenuators, cables, and other microwave components. Our results are important for managing the heat budget of future large-scale quantum computers based on superconducting circuits.
Abstract Superconducting quantum computers require microwave control lines running from room temperature to the mixing chamber of a dilution refrigerator. Adding more lines without preliminary thermal modeling to make predictions risks overwhelming the cooling power at each thermal stage. In this paper, we investigate the thermal load of SC-086/50-SCN-CN semi-rigid coaxial cable, which is commonly used for the control and readout lines of a superconducting quantum computer, as we increase the number of lines to a quantum processor. We investigate the makeup of the coaxial cables, verify the materials and dimensions, and experimentally measure the total thermal conductivity of a single cable as a function of the temperature from cryogenic to room temperature values. We also measure the cryogenic DC electrical resistance of the inner conductor as a function of temperature, allowing for the calculation of active thermal loads due to Ohmic heating. Fitting this data produces a numerical thermal conductivity function used to calculate the static heat loads due to thermal transfer within the wires resulting from a temperature gradient. The resistivity data is used to calculate active heat loads, and we use these fits in a cryogenic model of a superconducting quantum processor in a typical Bluefors XLD1000-SL dilution refrigerator, investigating how the thermal load increases with processor sizes ranging from 100 to 225 qubits. We conclude that the theoretical upper limit of the described architecture is approximately 200 qubits. However, including an engineering margin in the cooling power and the available space for microwave readout circuitry at the mixing chamber, the practical limit is approximately 140 qubits.
As quantum computing technology advances, the demand for efficient, scalable, and thermally optimized solutions for qubit control and readout under cryogenic conditions has become increasingly critical. Currently, cryogenic qubit systems rely on numerous coaxial cables and DC wires for readout and control—typically two to three per qubit. Such dense wiring introduces significant engineering challenges, particularly regarding heat dissipation and spatial limitations [1]. To address these issues, wireless approaches have been proposed, offering promising advantages in scalability, thermal load reduction, and efficient quantum state transfer. Although wireless systems eliminate the need for cables, dilution refrigerators (DRs) impose strict spatial constraints, limiting the use of conventional microwave-range wireless components such as horn antennas and dielectric lenses. In addition, the sensitive and enclosed cryogenic environment demands precise beam shaping to prevent unwanted scattering and reflections. Hence, there is a pressing need for a compact and effective wireless solution tailored to such environments. In this study, we present a next-generation wireless communication system specifically designed for cryogenic quantum environments, enabling qubit control and readout at millikelvin temperatures. To achieve optimal signal transmission, we explored and simulated multiple design strategies, addressing challenges such as signal degradation, noise interference, and limited spatial volume within the refrigerator. Our system employs metasurface lens antennas to enhance transmission efficiency while minimizing signal-to-noise ratio (SNR) degradation. Considering the confined geometry of cryogenic setups, we developed two types of compact metalens antennas operating in both the microwave and millimeterwave regimes. Experimental measurements demonstrate more than 20 dB enhancement in wireless transmission, consistent with simulation results. These findings confirm that the proposed system satisfies the stringent requirements of quantum control and readout, making it a promising and practical solution for future quantum computing applications. We will share further details and insights on the metasurface-enabled wireless quantum computing platform at the upcoming conference presentation.