The interconnection between cryogenic quantum chips and room temperature measurement and control systems is a critical issue for quantum computing. In this paper, integrated filtering cryogenic flexible cables are designed to suppress external noise for the transmission of weak and high-frequency signal between low temperature and room temperature. They are made of flexible substrate (PI or LCP) with good flexibility and high integration, and lower heat loss than typical cryogenic coaxial cables. The low-pass filter is integrated in the cable with changing the line width, and the high-frequency noise can be effectively suppressed.
The interconnection between cryogenic quantum chips and room temperature measurement and control systems is a critical issue for quantum computing. In this paper, integrated filtering cryogenic flexible cables are designed to suppress external noise for the transmission of weak and high-frequency signal between low temperature and room temperature. They are made of flexible substrate (PI or LCP) with good flexibility and high integration, and lower heat loss than typical cryogenic coaxial cables. The low-pass filter is integrated in the cable with changing the line width, and the high-frequency noise can be effectively suppressed.
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.
Electromagnetic noise is one of the main external factors decreasing superconducting qubit coherence. Matched coaxial filters can prevent microwave and IR photons' negative influence on superconducting quantum circuits. In this report, we describe the design and fabrication process of matched coaxial filters for noise-sensitive measurements at millikelvin temperatures. A robust transmission coefficient and ultralow reflection loss of −20 dB in the frequency range up to 20 GHz is achieved. Fabricated low-pass filters have linear transmission and reflection characteristics with 3 dB-cutoff frequency of 1.5–2.5 GHz. A method for extracting the propagation constant and filter impedance from scattering parameter measurements is demonstrated. This method is experimentally approved on a filter with a compound of Cu powder and Stycast epoxy resin and a filter filled with ECCOSORB CR-110 epoxy resin. The proposed design and assembly technology are versatile for various compounds and provide highly repeatable geometric and microwave characteristics. Furthermore, we demonstrate that these low-pass coaxial filters can be effectively utilized to improve superconducting qubit relaxation due to suppressing standing waves originating from reflections in control coaxial cables.
We developed thin semi-rigid coaxial cables for use as readout noise filters in low temperature experiments. The cables reported here had an outer diameter of 0.86 mm, and consisted of a standard cupronickel (CuNi) alloy as a seamless outer conductor and polytetrafluoroethylene (PTFE) as a dielectric material. The inner conductor was superconducting niobium-titanium (NbTi) covered by a CuNi cladding layer with a thickness of 0, 13, or 38 μm. The thermal conductance and transmission properties for the cables were measured at cryogenic temperatures and compared to those for a conventional CuNi semi-rigid cable. Between 1.4 and 8.4 K, the thermal conductance was similar to that for the conventional cable because the contribution from the cladding was estimated to be small. On the other hand, measured at about 3 K, the attenuation at high frequency up to 10 GHz differed in that the cutoff frequency increased with decreasing CuNi cladding thickness. For a cladding thickness of 38 μm, the 3 dB cutoff frequency for a 1-m-long cable was 100 MHz, whereas for a cladding thickness of 13 μm, it was 500 MHz. This type of semi-rigid cable can, therefore, be used as a low-pass filter whose attenuation characteristics can be tailored by changing the thickness of the cladding layer.
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.
We present recent work on design, fabrication, and characterization of thin-film, flexible superconducting stripline cables. Fabrication details, dc, and microwave performance (up to 14 GHz) of cables with Al/Nb/Al conductors and polyimide (PI) dielectric are discussed. Thin Al was used as a barrier layer between Nb and polyimide to protect the Nb superconductivity during subsequent PI curing steps, which were performed at a reduced temperature. The dc performance, including critical transition temperature and critical current, of the stripline transmission lines is presented. We observed Tcvalues of ~8.8 and ~8.6 K for signal traces and ground planes, respectively. Microwave characterization, including cross-talk, was performed on stripline transmission lines and resonators. We observed reasonable impedance match (S11better than -15 dB), low insertion loss (S21better than 0.04 dB/cm) and cross-talk below -60 dB for stripline transmission lines measured up to 10 GHz at 4.2 K. Furthermore, similarly fabricated stripline resonators exhibited quality factors ~7 000 at 10 GHz and 1.2 K for stripline resonators. The results of these experiments provide insight into material stack-ups for robust, multi-layer superconducting flexible cables that can find use in future cryogenic electronics systems.
Electromagnetic filtering is essential for the coherent control, operation and readout of superconducting quantum circuits at milliKelvin temperatures. The suppression of spurious modes around transition frequencies of a few GHz is well understood and mainly achieved by on-chip and package considerations. Noise photons of higher frequencies - beyond the pair-breaking energies - cause decoherence and require spectral engineering before reaching the packaged quantum chip. The external wires that pass into the refrigerator and go down to the quantum circuit provide a direct path for these photons. This article contains quantitative analysis and experimental data for the noise photon flux through coaxial, filtered wiring. The attenuation of the coaxial cable at room temperature and the noise photon flux estimates for typical wiring configurations are provided. Compact cryogenic microwave low-pass filters with CR-110 and Esorb-230 absorptive dielectric fillings are presented along with experimental data at room and cryogenic temperatures up to 70 GHz. Filter cut-off frequencies between 1 to 10 GHz are set by the filter length, and the roll-off is material dependent. The relative dielectric permittivity and magnetic permeability for the Esorb-230 material in the pair-breaking frequency range of 75 to 110 GHz are measured, and the filter properties in this frequency range are calculated. The estimated dramatic suppression of the noise photon flux due to the filter proves its usefulness for experiments with superconducting quantum systems.