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arXiv:cond-mat/0601316

A Twisted Pair Cryogenic Filter

Lafe Spietz, John D. Teufel, R. J. Schoelkopf

Published Jan 15, 2006• 7 citations
arXivHTMLPDF

Abstract

In low temperature transport measurements, there is frequently a need to protect a device at cryogenic temperatures from thermal noise originating in warmer parts of the experiment. There are also a wide range of experiments, such as high precision transport measurements on low impedance devices, in which a twisted-pair wiring configuration is useful to eliminate magnetic pickup. Furthermore, with the rapid growth in complexity of cryogenic experiments, as in the field of quantum computing, there is a need for more filtered lines into a cryostat than are often available using the bulky low temperature filters in use today. We describe a low cost filter that provides the needed RF attenuation while allowing for tens of wires in a twisted pair configuration with an RF-tight connection to the sample holder. Our filter consists of manganin twisted pairs wrapped in copper tape with a light-tight connection to the shield of the sample holder. We demonstrate agreement of our filter with a theoretical model up to the noise floor of our measurement apparatus (90 dB). We describe operation of our filter in noise thermometry experiments down to 10 mK.

DOI
10.48550/arxiv.cond-mat/0601316

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In low temperature transport measurements, there is frequently a need to protect a device at cryogenic temperatures from thermal noise originating in warmer parts of the experiment. There are also a wide range of experiments, such as high precision transport measurements on low impedance devices, in which a twisted-pair wiring configuration is useful to eliminate magnetic pickup. Furthermore, with the rapid growth in complexity of cryogenic experiments, as in the field of quantum computing, there is a need for more filtered lines into a cryostat than are often available using the bulky low temperature filters in use today. We describe a low cost filter that provides the needed RF attenuation while allowing for tens of wires in a twisted pair configuration with an RF-tight connection to the sample holder. Our filter consists of manganin twisted pairs wrapped in copper tape with a light-tight connection to the shield of the sample holder. We demonstrate agreement of our filter with a theoretical model up to the noise floor of our measurement apparatus (90 dB). We describe operation of our filter in noise thermometry experiments down to 10 mK.
In experiments on single electron devices, the electromagnetic noise from parts of the apparatus at temperatures higher than that of the device can dramatically increase the tunnel rates out of the Coulomb-blocked state and therefore increase the device error rate. The electrical lines must therefore be filtered adequately. We derive simple expressions for calculating the required attenuation coefficient. We describe a wide-band miniature dissipative filter functioning at cryogenic temperatures. The effective thermalization of an experiment at 30 mK can be obtained by placing four of these filters in series at temperatures ranging from 4 K to 30 mK.
We developed a quantum noise measurement system in a dilution refrigerator by using three kinds of cryogenic low pass filters. One of them is a commercial low pass filter inserted into the noise measurement lines instead of the conventional powder filter, which assures well-defined circuit parameters necessary for the noise measurement at a finite frequency. We checked that this filter gives sufficiently large attenuation up to 20 GHz at room temperature, 77 and 4.2 K. The electron temperature of the mesoscopic device placed in the present system was confirmed to be down to around 20 mK by measuring the thermal noise of the device.
Minimizing microwave background of cryogenic environment is crucial for quantum devices where single microwave photon energy is sufficient to disturb the measured system. Microwave photons with sufficient energy originate from the warmer parts of the cryostat, for example from parts at liquid helium temperature (4.2 K), which can be about three orders of magnitude higher than the sample temperature. Reduction of the microwave background is possible with careful design of both shielding and filtering of the sample holder. Additionally, on-chip filtering of the actual quantum device can improve the device performance significantly.
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.
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^\mathrm{{TM}}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^\mathrm{{TM}}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 μsT_1, T_2^E \approx 100~\mu sT1​,T2E​≈100 μs ) using these filters and attenuators, thereby confirming their efficacy.