MKID

MKID, or Microwave Kinetic Inductance Detector, is a superconducting detector technology that uses the kinetic inductance of Cooper pairs in a superconductor to sense photons. When photons are absorbed, they break Cooper pairs and generate quasiparticles. This increases the kinetic inductance and shifts the resonant frequency and dissipation of a superconducting microwave resonator. The detector is typically implemented as a lithographically patterned resonator coupled to a common microwave feedline.

Each MKID is designed to have a unique resonance frequency within the microwave range, enabling large arrays to be read out over a single pair of cables through frequency-division multiplexing. The measured changes in the complex transmission—the amplitude and phase of the response—provide information about when a photon arrived and, with appropriate calibration, the energy of the photon.

MKIDs are fabricated from superconducting materials such as aluminum, titanium nitride, or niobium nitride, and require cryogenic cooling to temperatures of tens to hundreds of millikelvin. They offer advantages including straightforward fabrication, natural multiplexing, and low readout power, but performance depends on material properties and device design. Noise sources include quasiparticle generation-recombination fluctuations and dielectric two-level-system noise.

Applications and development: MKIDs have been demonstrated for optical, near-infrared, submillimeter, and X-ray photon counting and spectroscopy. They are used in astronomy for wide-field imaging, time-domain studies, and spectrographic modes. Ongoing work aims to improve energy and time resolution, uniformity across large arrays, and scalable readout schemes toward kilopixel and larger detectors.