Counting Near Infrared Photons with Microwave Kinetic Inductance Detectors

We demonstrate photon counting at 1550 nm wavelength using microwave kinetic inductance detectors (MKIDs) made from TiN/Ti/TiN trilayer films with superconducting transition temperature Tc ~ 1.4 K. The detectors have a lumped-element design with a large interdigitated capacitor (IDC) covered by aluminum and inductive photon absorbers whose volume ranges from 0.4 um^3 to 20 um^3. We find that the energy resolution improves as the absorber volume is reduced. We have achieved an energy resolution of 0.22 eV and resolved up to 7 photons per pulse, both greatly improved from previously reported results at 1550 nm wavelength using MKIDs. Further improvements are possible by optimizing the optical coupling to maximize photon absorption into the inductive absorber.