Abstract
Properties of superconducting nanowires set the performance level for superconducting nanowire single photon detectors (SNSPDs). Reset time in commonly employed large area SNSPDs, 1–10 ns, is known to be limited by the nanowire’s kinetic inductance to the load impedance ratio. On the other hand, reduction of the kinetic inductance in small area (waveguide integrated) SNSPDs prevents biasing them close to the critical current due to latching into a permanent resistive state. In order to reduce the reset time in SNSPDs, superconducting nanowires with both low kinetic inductance and fast electron energy relaxation are required. In this paper, we report on a study of kinetic inductance in narrow (15–100 nm) and long (up to 120 μm) superconducting MgB2 nanowires made from 5 nm thick films, offering such combination of properties. Such films were grown using hybrid physical chemical vapor deposition, resulting in a critical temperature of ∼32 K, and a switch current density of 5 × 107 A cm−2 (at 4.8 K). Using microwave reflectometry, we measured a kinetic inductance of L k0(4.8 K) = 1.3–1.6 pH/□ regardless of the nanowire width, which results in a magnetic field penetration depth of ∼90 nm. These values are very close to those in pristine MgB2. We showed that after excitations by a 50 fs pulsed laser the reset time in 35 nm × 120 μm MgB2 nanowires is 130 ps, which is more than a factor of 10 shorter than in NbN nanowires of similar length-to-width ratios. Depending on the bias current, such MgB2 nanowires function as single-, double, or triple-photon detectors for both visible (λ = 630 nm) and infrared (λ = 1550 nm) photons, with a dark count rate of <10 cps. Although the apparent photon detection efficiency seems so far to be low, further technological advances (uniform nanowire width, smaller thickness, increasing the switching current closer to the pair-breaking current) may improve this figure of merit.
Highlights
During the last two decades we have seen enormous progress with superconducting nanowire single photon detectors (SNSPDs), from first demonstration of principles [1] to integrated circuits and a wide range of applications in nanophysics [2], high speed communication [3], quantum information [4], quantum key distribution [5], laser-ranging [6], deep space communication [7], etc
We investigated ultra-thin MgB2 films in an attempt to create a new material basis for SNSPDs
hybrid physical chemical vapor deposition (HPCVD) was known to be a way of making clean MgB2 films, which so far been limited to thicknesses >10–15 nm
Summary
During the last two decades we have seen enormous progress with superconducting nanowire single photon detectors (SNSPDs), from first demonstration of principles [1] to integrated circuits and a wide range of applications in nanophysics [2], high speed communication [3], quantum information [4], quantum key distribution [5], laser-ranging [6], deep space communication [7], etc. Because the superconducting energy gap ∆ is in the meV-range, a large number of non-equilibrium quasiparticles are generated at visible or infrared photon (∼eV) impacts Both the small width of the nanowire, w and the critical current, Ic being close to the pair-breaking current ensure high detection efficiency [10, 11]. In long (l) and narrow (w) NbN nanowires, large kinetic inductivity (Lk0(4.8 K) = 80–90 pH/□) (inductance per square of the nanowire, l = w) leads to a significantly slower current return (restoration of the superconducting state), resulting in a much larger reset time τ [14]: τ = Lk/RL ≫ τ 0, where Lk = Lk0 × l/w is the total kinetic inductance of the nanowire and RL is the impedance of the read-out circuit (∼50 Ω). When biased close to the critical (switch) current, such devices show a linear dependence of the number of response pulses vs the incident laser power, corresponding to the single-photon detection regime
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