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.
An ultra-wide stopband hairpin bandpass filter with integrated nonuniform transmission lines was designed and fabricated for highly sensitive measurements at cryogenic temperatures down to millikelvin and a frequency range of 10 Hz-10 GHz. The scattering matrices of the filter were characterized at T = 4.2 K. The filter provides a stopband from 10 Hz to 2.2 GHz and from 2.3 GHz to 10 GHz with more than 50 dB and 40 dB of amplitude suppression, respectively. The center frequency of the passband is f0 = 2.25 GHz with a bandwidth Δf = 80 MHz. The maximum insertion loss in the passband is 4 dB. The filter has a 50 Ω input and output impedance, SubMiniature version A connector termination, and significantly reduced form factor. The wide stopband frequency range and narrow passband in conjunction with small dimensions make the filter suitable to use it as a part of a high sensitive readout for superconducting quantum circuits, such as superconducting quantum bits and cryogenic parametric amplifiers.
Cryogenic filtering and attenuation play a crucial role in the control and readout of superconducting qubits, ensuring that unwanted noise does not degrade qubit coherence. These devices typically operate in the microwave C band (4–8 GHz) and use attenuators (input) and circulators/isolators (output) to control in-band noise, while reflective filters are used to reject out-of-band noise. However, these devices tend to become transparent to terahertz and infrared radiation. Such high-frequency radiation reaching the qubit destroys superconductivity and induces dissipation, degrading the coherence properties of these devices. Magnetically loaded epoxy (like Eccosorb
TM ) has been widely used as a dielectric to construct filters that absorb high-frequency radiation while minimizing loss in the operating band. Here, we demonstrate the construction and characterization of absorptive filters with variable in-band attenuation by combining Eccosorb
TM or iron powder mixed with Stycast in different ratios. We show that such filters can be made with sufficiently large in-band attenuation to replace the usual thin-film resistor-based attenuators and simplify cryogenic wiring by combining attenuation and IR filtering in a single device. We characterize the microwave performance at room temperature and at dilution fridge temperatures for devices with insertion losses ranging from 1 dB to 20 dB at 7 GHz. In a separate set of experiments, we also test the materials’ absorption capabilities at terahertz frequencies up to 450 GHz. Finally, we demonstrate high coherence in a 3D superconducting qubit (
T1,T2E≈100 μs ) using these filters and attenuators, thereby confirming their efficacy.