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(Color online) Top panel (a) shows the superconducting gap of the structure, including a quasiparticle diffusing into the aluminium MKID and being trapped by phonon emission. Middle panel (b) contains a drawing of the central region of a MKID strip detector. A , thick tantalum strip [residual resistivity ratio )] is fabricated on -plane sapphire and has MKIDs attached to both ends. The center strip of the thick aluminum CPW resonator that composes the MKID is flared out where it contacts the tantalum strip to allow lateral trapping of quasiparticles. Bottom panel (c) shows a scanning electron microscopy of the Al–Ta interface from the wafer tested in this letter. A patch of aluminum patterned with a lift-off process is used to bridge the Al–Ta interface to avert a step coverage problem. In this device the tantalum is nicely sloped and the aluminum resonator climbs smoothly over the step.
(Color online) Optimally filtered maximum phase pulse height in degrees observed in aluminum MKIDs attached to a tantalum strip is shown in (a). The pulse height in the left MKID is shown on the axis, while the right MKID is shown on the axis. The and lines from the source are clearly visible. These data are fit to determine the diffusion length, and these are used to compute the energy spectrum shown in the inset (b). We calculate a FWHM energy width at when we restrict our data to all pulses that show greater than 22° of phase shift in both MKIDs.
Summary of the noise sources present in our resonator. The noise due to quasiparticle creation and recombination (G-R noise) (Ref. 22) in the aluminum MKID is negligible. The intrinsic noise of the device from quasiparticle creation statistics (Fano noise) in tantalum is . The dielectric in our resonators adds phase noise to the measurement, (Ref. 17), increasing our expected energy width to . The excess dielectric noise displayed by this batch of resonators was significantly worse than expected from previous measurements due to the use of a sapphire wafer of poor quality. The best sapphire resonators we have tested which have the dynamic range to measure x rays would have given an expected substrate noise contribution of .
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