We have developed a kinetic inductance travelling-wave amplifier toward the THz-band operation. In this paper, we discuss the nonlinear kinetic inductance of single-crystal niobium titanium nitride (NbTiN) thin film in response to the application of current to the film. In our calculation, it is found that single-crystal NbTiN films have a large current variation in the kinetic inductance compared with that of polycrystalline NbTiN films. To evaluate the nonlinearity, we fabricated a superconducting coplanar waveguide with a 0.2-m line length and measured the transmittance. We observed a variation of the electrical length of the line caused by the modulation of the kinetic inductance as a function of the value of the current applied to the strip.
We have developed a kinetic inductance travelling-wave amplifier toward the THz-band operation. In this paper, we discuss the nonlinear kinetic inductance of single-crystal niobium titanium nitride (NbTiN) thin film in response to the application of current to the film. In our calculation, it is found that single-crystal NbTiN films have a large current variation in the kinetic inductance compared with that of polycrystalline NbTiN films. To evaluate the nonlinearity, we fabricated a superconducting coplanar waveguide with a 0.2-m line length and measured the transmittance. We observed a variation of the electrical length of the line caused by the modulation of the kinetic inductance as a function of the value of the current applied to the strip.
Nonlinear resonators of superconducting thin-film kinetic inductance have attracted considerable research interest in the fields of detectors, qubits, parametric amplifiers, and more. By tuning the deposition parameters, niobium titanium nitride (NbTiN) films (∼12 nm in thickness) present high resistivity(∼2000 μΩ cm) and large sheet kinetic inductance (∼0.7 nH/□). We designed resonators with NbTiN micronwire as the inductor and aluminum as the capacitor, which result in a high internal quality factor and a Kerr nonlinearity of a few hertz. The radio frequency response of the resonators demonstrates nonlinear behavior similar to that of the cubic Duffing oscillator, including frequency shift and hysteresis region. The kinetic inductance resonator is a promising candidate for high saturation power parametric amplifiers.
Background Kinetic inductance travelling-wave parametric amplifiers (KITWPAs) have proven effective in achieving high gain over broad bandwidths whilst achieving near quantum-limited noise performance, properties which are extremely important for many ultra-sensitive experiments such as radio/millimetre/sub-millimetre astronomy, neutrino mass experiments, and axion dark matter searches, as well as commercial applications like quantum computational and communication systems. Methods This paper introduces the preliminary experimental exploration of an alternative KITWPA utilising thin titanium nitride (TiN) films as opposed to the conventionally used niobium titanium nitride (NbTiN) films. TiN films exhibit properties that could be advantageous for KITWPA operation, owing to its controllable critical temperature (T c) and physical robustness, hence KITWPAs fabricated from TiN films have the potential to replace NbTiN-based TWPAs for various applications, such as reducing the high pump power required to drive a NbTiN TWPA. Our TiN-based KITWPA devices were designed and fabricated with a 42 cm long coplanar waveguide transmission line patterned into a 100 nm thick TiN film, wound in a double spiral fashion to fit into a 2.2×2.2 cm chip. Results We successfully achieved broadband performance, with the best device displaying a peak averaged gain of ~5 dB with a bandwidth of ~3–13 GHz, making this one of the first broadband gain measurements of a TiN KITWPA reported in the literature. Despite this, the measured gain in all batches was lower than simulated due unexpected additional loss mechanisms. Conclusions The preliminary results presented here suggest that TiN films are suitable for KITWPA operation, although further investigation is required to improve the gain performance.
Abstract A Kinetic Inductance Traveling Wave Amplifier (KIT) utilizes the nonlinear kinetic inductance of superconducting films, particularly niobium titanium nitride (NbTiN), for parametric amplification. These amplifiers achieve remarkable performance in terms of gain, bandwidth, and compression power and frequently approach the quantum limit for noise. However, most KIT demonstrations have been isolated from practical device readout systems. Using a KIT as the first amplifier in the readout chain of an unoptimized microwave SQUID multiplexer coupled to a transition-edge sensor microcalorimeter, we see an initial improvement in the flux noise [1]. One challenge in KIT integration is the considerable microwave pump power required to drive the non-linearity. To address this, we have initiated efforts to reduce the pump power by using thinner NbTiN films and an inverted microstrip transmission line design. In this article, we present the new transmission line design, fabrication procedure, and initial device characterization—including gain and added noise. These devices exhibit over 10 dB of gain with a 3 dB bandwidth of approximately 5.5–7.25 GHz, a maximum practical gain of 12 dB, and typical gain ripple under 4 dB peak to peak. We observe an appreciable impedance mismatch in the NbTiN transmission line, which is likely the source of the majority of the gain ripple. Finally, we perform an initial noise characterization and demonstrate system-added noise of three quanta or less over nearly the entire 3 dB bandwidth.
Abstract This study predicts the performance of millimeter-wave kinetic-inductance traveling-wave parametric amplifiers (KITWPAs) designed using single-crystal NbTiN thin films that exhibit lower transmission losses. For comparison, polycrystalline NbTiN films, which are commonly been used for KITWPA fabrication, were also considered. Half-wavelength resonators were fabricated from both types of films, and their sheet inductances and resistances were measured. Using these values, KITWPAs with an amplification band of 35–50 GHz were designed, and their gain characteristics were analyzed. The results suggest that KITWPAs using single-crystal NbTiN films provide higher gain than those using polycrystalline NbTiN films.
Background: Kinetic Inductance Travelling Wave Parametric Amplifiers (KITWPAs) are a variant of superconducting amplifier that can potentially achieve high gain with quantum-limited noise performance over broad bandwidth, which is important for many ultra-sensitive experiments. In this paper, we present a novel modelling technique that can better capture the electromagnetic behaviour of a KITWPA without the translation symmetry assumption, allowing us to flexibly explore the use of more complex transmission line structures and better predict their performance. Methods: In order to design a KITWPA with optimal performance, we investigate the use of different superconducting thin film materials, and compare their pros and cons in forming a high-gain low-loss medium feasible for amplification. We establish that if the film thickness can be controlled precisely, the material used has less impact on the performance of the device, as long as it is topologically defect-free and operating within its superconducting regime. With this insight, we propose the use of Titanium Nitride (TiN) film for our KITWPA as its critical temperature can be easily altered to suit our applications. We further investigate the topological effect of different commonly used superconducting transmission line structures with the TiN film, including the effect of various non-conducting materials required to form the amplifier. Results: Both of these comprehensive studies led us to two configurations of the KITWPA: 1) A low-loss 100 nm thick TiN coplanar waveguide amplifier, and 2) A compact 50 nm TiN inverted microstrip amplifier. We utilise the novel modelling technique described in the first part of the paper to explore and investigate the optimal design and operational setup required to achieve high gain with the broadest bandwidth for both KITWPAs, including the effect of loss. Conclusions: Finally, we conclude the paper with the actual layout and the predicted gain-bandwidth product of our KITWPAs.
We report on the fabrication and characterization of superconducting magnesium diboride (MgB2) thin films intended for quantum-limited devices based on non-linear kinetic inductance (NLKI) such as parametric amplifiers with either elevated operating temperatures or expanded frequency ranges. In order to characterize the MgB2 material properties, we have fabricated coplanar waveguide (CPW) transmission lines and microwave resonators using ≈40 nm thick MgB2 films with a measured kinetic inductance of ∼5.5 pH/□ and internal quality factors Qi≈3×104 at 4.2 K. We measure the NLKI in MgB2 by applying a DC bias to a 6 cm long by 4 μm wide CPW transmission line and measuring the resulting phase delay caused by the current dependent NLKI. We also measure the current dependent NLKI through CPW resonators that shift down in frequency with increased power applied through the CPW feedline. Using these measurements, we calculate the characteristic non-linear current parameter, I*, for multiple CPW geometries. We find values for corresponding current density, J*=12–22 MA/cm2, and a ratio of the critical current to the non-linear current parameter, IC/I*=0.14–0.26, similar to or higher than values for other superconductors such as NbTiN and TiN.