A new carbon nanotube-based filter for quantum computing applications

Superconducting quantum circuits are sensitive to electromagnetic interference and thermal radiation, and such interactions with the environment lead to the undesirable phenomenon of quantum decoherence.

Commercially available filters, such as lumped-element filters and waveguide-based filters, become leaky or simply stop working above certain frequencies. While lossy transmission line filters can successfully attenuate high frequency radiation, they are bulky in size. As the complexity of quantum circuits grow, packing and cooling down hundreds or thousands of these filters becomes a challenge.

A new article introduces a new type of lossy transmission line filter made with multiwalled carbon nanotubes that improves coherence times while also being more compact in size. The unique physical properties of carbon nanotubes — high optical absorption, excellent thermal conductivity and high aspect ratio — make them an ideal filter material for superconducting quantum computing applications.

The researchers mixed a solution of carbon nanotubes with stainless steel powder to reduce their attenuation to a desired level. Once the mixture dried in a vacuum, a copper block filter enclosure with a microwave transmission line was filled with the powder and sealed shut.

To test its performance in a quantum computing system, they used the filter to characterize the coherence times of a superconducting qubit. A microwave switch was used to alternate between a measurement line with a carbon nanotube filter and a measurement line without one. The filter significantly improved the energy relaxation time by more than 61 percent and the pure dephasing time by 291 percent.

The authors hope that their carbon nanotube-based filter will be adopted as the new standard for quantum computing applications and improve the quality of related experiments.

Source: “Carbon nanotube-based lossy transmission line filter for superconducting qubit measurements,” by Mehran Vahdani Moghaddam, C. W. Sandbo Chang, Ibrahim Nsanzineza, A. M. Vadiraj, and C. M. Wilson, Applied Physics Letters (2019). The article can be accessed at http://doi.org/10.1063/1.5116109 .