MKIDs The First Decades Author Benjamin A Mazin

MKIDs: The First Decades Author: Benjamin A. Mazin Reader: Min Zhong 2020/8/21 1

Introduction Ø Microwave Kinetic Inductance Detectors Ø Work principle Ø Incident photons change the surface impedance of a superconductor through the kinetic inductance effect. Ø This change can be accurately measured using a thin film superconducting resonant circuit Ø For near-IR to X-ray photons: energy and arrival time For lower energy photons: total photon flux Ø Primary attraction Ø Easy to multiplex into large arrays 2

Introduction 3

Implementations - CPW Ø Coplanar waveguide resonators Ø The first type of MKIDs to be studied in detail Ø Can be easily fabricated with a single metal layer on a crystalline dielectric. Ø Successful materials: Nb, Nb. Ti. N, Ta, Re, Al. Mn, Mo, Pt. Si, Ti, and Ir Ø Drawbacks Ø The sensitivity to quasiparticles peaks in areas of high current Ø For quarter wave resonators: near the shorted end of the resonator Ø A separate structure, such as the antenna is required to absorb the photons Ø Noise and dielectric loss associated with amorphous thin film dielectrics 4

Implementations - CPW Ø Structure Ø Consists of a CPW transmission line which is two slots cut into a metal ground plane to form a center strip 5

Implementations - LEKID Ø Lumped element MKIDs Ø Structure Ø Use a separate inductor and capacitor to form a resonator Ø Example: An interdigitated capacitor can be attached to an inductive meander 6

Implementations - LEKID Ø Attractions Ø Simple to make since they do not require quasiparticle trapping Ø In the near IR through UV, the lack of quasiparticle trapping allows significantly higher theoretical energy resolution for a given operating temperature. 7

Implementations – Mixed Designs Ø Use aspects of both LEKID and CPW Ø Show that a resonator with an interdigitated capacitor and a CPW inductor can drastically reduce the observed excess phase noise 8

Two Level System Noise Ø Primary source of the observed excess phase noise in MKIDs Ø Two level systems (TLSs) on the surfaces of the metals and dielectrics of the resonator Ø By making the geometry of the capacitive section larger, TLS noise can be reduced significantly 9

Lifetimes Ø The quasiparticle lifetime in superconducting films Ø Does not conform well to theoretical predictions at temperatures well below Tc Ø Reducing the flux of stray blackbody radiation absorbed in the resonator both directly and through the coaxial cables increases the observed lifetimes Ø Experiments at Delft have shown that the quasiparticle lifetime is related to disorder of the film. 10

Readouts Ø Phase change & Increased dissipation Ø Phase change: caused by the kinetic inductance effect Ø Increased dissipation: caused by broken Cooper pairs Ø The phase response is usually ∼ 4 times greater than the dissipation response Ø In many cases the optimum readout method will be a function of signal frequency, so most readouts are designed to record both the phase and dissipation response. Ø Comparisons with TES Ø Much of the complexity is moved from the base temperature to room temperature, where the full power of modern microwave electronics is available. 11

Current Work Ø Millimeter and Submillimeter Arrays Ø The most active current area of MKID research Ø Democam, MKID Camera, NIKA, etc. Ø Near Infrared to Ultraviolet Arrays Ø Palomar and Keck telescopes (at ARCHONS) Ø Dark Matter Detectors Ø Conventional CPW design using large Al fins as quasiparticle collectors (CIT) Ø Vacuum dielectric microstrip design (Berkeley) 12

Outstanding Issues Ø Performance under optical loading Ø The measured dark NEPs often quoted will invariably degrade under optical loading Ø The interaction of the readout photons with quasiparticles in the resonator can lead to complex effects Ø Quasiparticle trapping in thin films Ø For photon counting MKIDs Ø Interaction of MKIDs with magnetic fields Ø MKIDs show significant frequency response to changes in the magnetic field normal to the metal surface. Ø Can be a significant problem in applications that move a dewar in Earth’s field. 13

Conclusions Ø Growth Ø MKIDs have been on a growth trajectory for the last decade Ø The complexity and number of pixels in millimeter and submillimeter instrumentation continues to grow Ø Potential Ø Lower cost Ø Complexity advantage 14