Improved powder filters for qubit measurements.

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.