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 covered by aluminum and inductive photon absorbers whose volume ranges from 0.4 µm 3 to 20 µm 3 . The energy resolution improves as the absorber volume is reduced. We achieved an energy resolution of 0.22 eV and resolved up to 7 photons per optical 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.Photon-number-resolving (PNR) detectors at near infrared wavelengths have important applications in a number of frontier fields, such as quantum secure communications [1], linear optical quantum computing [2] and optical quantum metrology [3]. Compared to more conventional detectors at this wavelength, such as siliconbased detectors [4], superconducting detectors have lower dark-count rate, higher sensitivity, and broadband response. They show great promise in serving as the basic building blocks for efficient PNR devices. For example, by spatial or temporal multiplexing of superconducting nanowire single-photon detectors (SNSPDs) [5][6][7][8], photons can be counted at high speed. But the singleelement nanowire has no intrinsic PNR and energyresolving capabilities. Alternatively, single-element transition edge sensors (TESs) [9] have demonstrated high quantum efficiency and multi-photon discrimination at telecommunication wavelengths [10][11][12]. Recently, counting up to 29 photons and intrinsic energy resolution ≈ 0.11 eV at 1550 nm wavelength have been achieved in Ti/Au TESs [13][14][15].Another type of superconducting detector possessing intrinsic photon-number-resolving and energy-resolving power is the microwave kinetic inductance detector (MKID) [16]. MKIDs are cooper pair breaking detectors based on high-quality factor (high-Q) superconducting resonators [17,18]. The absorption of a photon with energy higher than twice the gap energy (hν > 2∆) can break Cooper pairs into quasiparticles, changing the surface impedance of the resonator and resulting in a lower resonance frequency f r and higher internal dissipation (or lower quality factor Q i ). When applying a short optical pulse to the detector and probing the resonator with a * Electronic mail: qubit@home.swjtu.edu.cn † Electronic mail: weilianfu@gmail.com ‡ Contribution of the U.S. government, not subject to copyright microwave tone near the resonance frequency, one can obtain a pulse response in the complex forward transmission S 21 , as shown in Fig. 1(a). This photon response can be measured using a homodyne detection scheme ( Fig. 1(d)) and the signal can be decomposed into frequency and dissipation responses ( Fig. 1(a),(b)) for pulse analysis. Compared to TESs, MKIDs are easy to fabricate and multiplex into...

We present a wafer trimming technique for producing superconducting micro-resonator arrays with highly uniform frequency spacing. With the light-emitting diode (LED) mapper technique demonstrated previously, we first map the measured resonance frequencies to the physical resonators. Then, we fine-tune each resonator's frequency by lithographically trimming a small length, calculated from the deviation of the measured frequency from its design value, from the interdigitated capacitor. We demonstrate this technique on a 127-resonator array made of titanium-nitride (TiN) and show that the uniformity of frequency spacing is greatly improved. The array yield in terms of frequency collisions improves from 84 % to 97 %, while the quality factors and noise properties are unaffected. The wafer trimming technique provides an easy-to-implement tool to improve the yield and multiplexing density of large resonator arrays, which is important for various applications in photon detection and quantum computing.

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