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Chinese Science Bulletin

Cascaded low-pass filters for the measurements of DC characteristics of superconducting qubits

Tie Li

Published Jan 1, 2010• 0 citations
OpenAlex

Abstract

Cascaded low-pass filters are designed and fabricated for the measurements of dc characteristics of superconducting qubits. The filters are cascaded by LCR filter and copper powder filter. At 77 K the cutoff frequency is 120 kHz and the roll-off is -40 dB/decade. We achieved an attenuation -60 dB when the frequency is above 10 MHz. The frequency response of the cascaded filters is better than single LCR filter or copper powder filter and high frequency noise can be suppressed effectively at low temperature.

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Cascaded low-pass filters are designed and fabricated for the measurements of dc characteristics of superconducting qubits. The filters are cascaded by LCR filter and copper powder filter. At 77 K the cutoff frequency is 120 kHz and the roll-off is -40 dB/decade. We achieved an attenuation -60 dB when the frequency is above 10 MHz. The frequency response of the cascaded filters is better than single LCR filter or copper powder filter and high frequency noise can be suppressed effectively at low temperature.
We designed and fabricated miniature low-pass metal powder filters suitable for noise-sensitive measurements at cryogenic temperatures. In comparison with previous powder filters, our filters have a much better frequency response and significantly smaller dimensions (0.7 cm(3) including the plugs) and can also be used as hermetic feedthroughs at low temperatures. Their transmission characteristics are smooth, contain no ripples, and have a steep decay above the cutoff frequency. At 4.2 K the cutoff frequency of a single filter is f(c)=1 MHz and the roll-off is -50 dB per decade. All of the fabricated filters have identical frequency responses at 4.2 K and their characteristics are reliably reproducible.
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 fabricated and tested a squelch circuit consisting of a copper powder filter with an embedded Josephson junction connected to ground. For small signals (squelch ON), the small junction inductance attenuates strongly from dc to at least 1 GHz, while for higher frequencies dissipation in the copper powder increases the attenuation exponentially with frequency. For large signals (squelch OFF), the circuit behaves as a regular metal powder filter. The measured ON/OFF ratio is larger than 50 dB up to 50 MHz. This squelch can be applied in low temperature measurement and control circuitry for quantum nanostructures, such as superconducting qubits and quantum dots.
Electromagnetic noise is one of the key external factors decreasing superconducting qubits coherence. Matched coaxial filters can prevent microwave and IR photons negative influence on superconducting quantum circuits. Here, we report on design and fabrication route of matched low-pass coaxial filters for noise-sensitive measurements at milliKelvin temperatures. A robust transmission coefficient with designed linear absorption (-1dB/GHz) and ultralow reflection losses less than -20 dB up to 20 GHz is achieved. We present a mathematical model for evaluating and predicting filters transmission parameters depending on their dimensions. It is experimentally approved on two filters prototypes different lengths with compound of Cu powder and Stycast commercial resin demonstrating excellent matching. The presented design and assembly route are universal for various compounds and provide high repeatability of geometrical and microwave characteristics. Finally, we demonstrate three filters with almost equal reflection and transmission characteristics in the range from 0 to 20 GHz, which is quite useful to control multiple channel superconducting quantum circuits.
Protecting superconducting quantum circuits from nonideal return loss, including out-of-band circulator behavior, and enhancing the performance of broadband quantum-limited amplifiers can be accomplished using a superconducting version of a special class of microwave filters known as reflectionless filters. These filters can simultaneously permit low passband loss to preserve quantum efficiency and broadband reflectionless characteristics in the stop and pass bands. The filter also suppresses thermal photons emitted in its passband from the termination resistors by the nature of the dual-network topology. This work will review the application, theory, design, and modeling of a superconducting reflectionless filter, followed by fabrication details and the presentation of cryogenic performance measurements of a monolithic device. The filter was fabricated using Al on Si and incorporated NiCr resistors, allowing simple integration with other superconducting quantum devices. The filter with an area of 0.6 mm2achieves insertion loss of−0.75 dB-0.75{\,}\text {dB}−0.75dB(σ=0.24 dB\sigma = 0.24{\,}\text {dB}σ=0.24dB), including its connectorized package over a 80% fractional bandwidth centered at 8 GHz, and 10-dB packaged return loss from dc to above 14.5 GHz.