  A new type of low-pass filter based on a leaky coaxial waveguide is
presented. The filter has minimal insertion loss in the pass band, while at the
same time high attenuation in the stop band is achieved. Thanks to its
arrangement, the filter does not present parasitic leakage paths, so that,
unlike conventional resonant filters, the stop band extends to very high
frequencies. It is shown that a particular stop-band attenuation can be
obtained by adding or removing leaking sections. Coupling between the center
coaxial structure and the leaking holes is investigated. A prototype is
manufactured and scattering parameters are measured up to 145 GHz. The
prototype shows an insertion loss of less than 0.15 dB up to 10 GHz and an
attenuation in excess of 60 dB above 70 GHz. The proposed filter is suitable
for superconducting quantum computing applications where qubits are sensitive
to radiation with energy high enough to break Cooper-pairs, thereby destroying
superconductivity.
