A transition edge sensor (TES) is one of superconducting photon detectors, which has a photon number resolving
ability in light pulses. The TES device is a kind of calorimeters operated at an extremely low temperature, and
the energy of the photons is measured as a resistance change in a superconducting transition region of the TES.
The advantages of the TESs are an excellent energy resolution and a high quantum efficiency. However a response
speed is limited due to slow thermal recovery time. To overcome this, we fabricated new TES devices which are
based on a titanium superconductor. The critical temperature of our titanium films is around 410 mK, which
greatly improves the thermal recovery time. The observed decay time constant of response signals to the light
pulses is around several hundreds of ns, that make it possible to operate the devices at a counting rate over 1
MHz. The photon number resolving power is 0.35 eV(FWHM) for a 5 μm size device even at the high operating
temperature. The system quantum efficiency is 65 % by embedding the TES films in an optical structure with
a high reflection dielectric mirror and an anti-reflection coatings fabricated by an ion beam assisted sputtering
method. These features are very promising for high speed photon number resolving applications in the quantum
information field.
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