Long quasiparticle lifetime in Aluminum Microwave Kinetic Inductance Detectors using coaxial stray light filters

To achieve the high sensitivity requirements of future space based sub-mm spectroscopy missions is highly challenging. Microwave Kinetic Inductance Detectors are a promising candidate since they can theoretically achieve the performance required. However, they have yet to be shown to reach this performance. We will show that this is due in part to stray light saturating the device's sensitivity in aluminium KIDs in our experimental setup. We discuss a set of experiments that show that thermal radiation from the 3 K stage of our cooler is limiting the quasiparticle lifetime in our aluminium KIDs. Furthermore we discuss a new filtering method based upon coaxial cable filters to reduce this effect. Using the best filtering achievable we have measured a quasiparticle lifetime of 2.8 msec in a 100 nm thick Al KID.