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AIP Advances

High-resolution microwave frequency identification via bias-tuned superconducting transmission lines

Masato Naruse, Kohta Yamaguchi, Hiroaki Myoren

Published Nov 1, 2025• 0 citations
Publisher

Abstract

We develop a microwave frequency counter based on the complex conductivity modulation of a DC-biased superconducting transmission line while maintaining the bias below the critical current. A resonator-free platform is proposed for microwave frequency estimation. It utilizes a bias-controlled phase delay in a superconducting coplanar waveguide structure, which imprints frequency-dependent signatures on the transmitted microwave tone. Instead of relying on intentionally designed resonators, our method leverages the complex transmission spectra arising from numerous weak resonance structures due to subtle fabrication imperfections. However, even in the absence of such features, the method remains effective because the DC bias induces frequency-dependent phase shifts that modulate the microwave signal’s propagation velocity. Mixed signals are externally generated via in-phase-quadrature mixing with a reference signal, which manifests as interference-like patterns enabling high-resolution spectral identification. We measure the frequency-dependent changes in the interference-like patterns under this bias to catalog characteristic transmission loss profiles across a wide frequency range (2–8 GHz). These profiles serve as supervised training data for a machine learning algorithm that identifies unknown input microwave frequencies with high-resolution and accuracy. Our method achieves a frequency resolution (F/ΔF) of over 200 000 and a relative error in identification of below 0.01%. This scalable approach to distributed microwave sensing may be useful for quantum measurement and signal classification and is particularly well-suited for dark matter searches, such as hidden-photon detection, where broadband frequency identification without prior tuning is essential. The measured frequency range is primarily limited by the bandwidth of external microwave components, such as low-noise amplifiers and bias-tees. The superconducting device itself may be used up to its superconducting gap frequency (∼280 GHz for TiN at 3.8 K).

DOI
10.1063/5.0299865

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We develop a microwave frequency counter based on the complex conductivity modulation of a DC-biased superconducting transmission line while maintaining the bias below the critical current. A resonator-free platform is proposed for microwave frequency estimation. It utilizes a bias-controlled phase delay in a superconducting coplanar waveguide structure, which imprints frequency-dependent signatures on the transmitted microwave tone. Instead of relying on intentionally designed resonators, our method leverages the complex transmission spectra arising from numerous weak resonance structures due to subtle fabrication imperfections. However, even in the absence of such features, the method remains effective because the DC bias induces frequency-dependent phase shifts that modulate the microwave signal’s propagation velocity. Mixed signals are externally generated via in-phase-quadrature mixing with a reference signal, which manifests as interference-like patterns enabling high-resolution spectral identification. We measure the frequency-dependent changes in the interference-like patterns under this bias to catalog characteristic transmission loss profiles across a wide frequency range (2–8 GHz). These profiles serve as supervised training data for a machine learning algorithm that identifies unknown input microwave frequencies with high-resolution and accuracy. Our method achieves a frequency resolution (F/ΔF) of over 200 000 and a relative error in identification of below 0.01%. This scalable approach to distributed microwave sensing may be useful for quantum measurement and signal classification and is particularly well-suited for dark matter searches, such as hidden-photon detection, where broadband frequency identification without prior tuning is essential. The measured frequency range is primarily limited by the bandwidth of external microwave components, such as low-noise amplifiers and bias-tees. The superconducting device itself may be used up to its superconducting gap frequency (∼280 GHz for TiN at 3.8 K).
Scaling up a superconducting quantum computer will likely require quantum communication between remote chips, which can be implemented using an itinerant microwave photon in a transmission line. To realize high-fidelity communication, it is essential to control the frequency and temporal shape of the microwave photon. In this work, we demonstrate the generation of frequency-tunable shaped microwave photons without resorting to any frequency-tunable circuit element. We develop a framework that treats a microwave resonator as a band-pass filter mediating the interaction between a superconducting qubit and the modes in the transmission line. This interpretation allows us to stimulate the photon emission by an off-resonant drive signal. We characterize how the frequency and temporal shape of the generated photon depends on the frequency and amplitude of the drive signal. By modulating the drive amplitude and frequency, we achieve a frequency tunability of 40 MHz while maintaining the photon mode shape time symmetric. Through measurements of the quadrature amplitudes of the emitted photons, we demonstrate a consistently high state and process fidelities around 95% across the tunable frequency range. Our hardware-efficient approach eliminates the need for additional biasing lines typically required for frequency tuning, offering a simplified architecture for scalable quantum communication.
This paper reports the design, fabrication, and measurements of optically tunable superconducting microwave resonators and delay lines. The devices are comprised of superconducting coplanar waveguide (CPW) transmission lines fabricated from a 100 nm-thick YBCO film, interrupted by 3- and 5-m YBCO meander structures of the same thickness on top of an LAO substrate. The CPW is tested at 77 K up to a frequency of 50 GHz, where it exhibits a transmission loss of about 0.12 dB/mm. The measurements are done in a cryogenic microwave probe station, where all the calibrations have been performed at 77 K. It has been observed that the resonance frequencies of the superconducting resonators can be changed by more than 2 GHz under the optical illumination at a wavelength of 1550 nm. The delay lines also exhibit a phase retardation of up to 90 at the same operating conditions.
Superconducting nanowires are widely used as sensitive single photon detectors with wide spectral coverage and high timing resolution. We describe a demonstration of an array of dc-biased superconducting nanowire single photon detectors read out with a microwave multiplexing circuit. In this design, each individual nanowire is part of a resonant LC circuit where the inductance is dominated by the kinetic inductance of the nanowire. The circuit also contains two parallel plate capacitors, one of them is in parallel with the inductor and the other is coupled to a microwave transmission line that carries the signals to a cryogenic low-noise amplifier. All of the nanowires are connected via resistors to a single dc bias line that enables the nanowires to be current biased close to their critical current. When a photon hits a nanowire, it creates a normal hotspot that produces a voltage pulse across the LC circuit. This pulse rings down at the resonant frequency of the LC circuit over a time period that is fixed by the quality factor. We present measurements of an array of these devices and an evaluation of their performance in terms of frequency and time response.
© 2002-2011 IEEE. Superconducting nanowires are widely used as sensitive single photon detectors with wide spectral coverage and high timing resolution. We describe a demonstration of an array of dc-biased superconducting nanowire single photon detectors read out with a microwave multiplexing circuit. In this design, each individual nanowire is part of a resonant LC circuit where the inductance is dominated by the kinetic inductance of the nanowire. The circuit also contains two parallel plate capacitors, one of them is in parallel with the inductor and the other is coupled to a microwave transmission line that carries the signals to a cryogenic low-noise amplifier. All of the nanowires are connected via resistors to a single dc bias line that enables the nanowires to be current biased close to their critical current. When a photon hits a nanowire, it creates a normal hotspot that produces a voltage pulse across the LC circuit. This pulse rings down at the resonant frequency of the LC circuit over a time period that is fixed by the quality factor. We present measurements of an array of these devices and an evaluation of their performance in terms of frequency and time response.
Quantum-limited Josephson parametric amplifiers play a pivotal role in advancing the field of circuit quantum electrodynamics by enabling the fast and high-fidelity measurement of weak microwave signals. Therefore, it is necessary to develop robust parametric amplifiers with low noise, broad bandwidth, and reduced design complexity for microwave detection. However, current broadband parametric amplifiers either have degraded noise performance or rely on complex designs. Here, we present a device based on the broadband impedance-transformed Josephson parametric amplifier that integrates a hornlike coplanar waveguide transmission line, which significantly decreases the design and fabrication complexity while keeping comparable performance. The device shows an instantaneous bandwidth of 700 (200) MHz for 15 (20) dB gain with an average input saturation power of −110 dBm and near quantum-limited added noise. The operating frequency can be tuned over 1.4 GHz using an external flux bias. We further demonstrate the negligible backaction from our device on a transmon qubit. The amplification performance and simplicity of our device promise its wide adaptation in quantum metrology, quantum communication, and quantum information processing. Published by the American Physical Society 2024
Coplanar waveguide resonators are central to the thriving field of circuit quantum electrodynamics. Recently, we have demonstrated the generation of a broadband microwave-frequency comb spectrum using a superconducting quantum interference device (SQUID) driven by a time-dependent magnetic field. Here, the frequency comb is used to spectroscopically probe a bank of coplanar microwave resonators, inductively coupled to a common transmission line, a standard circuit with a variety of applications. We compare the resonator line shape obtained from signals synthesized at room temperature using conventional electronics with the radiation produced in the cryogenic environment by our source, showing substantial equivalence in the estimation of the resonator quality factors. To measure non-uniformly spaced resonant frequencies, we drive the generator with a bi-chromatic tone to generate intermodulation products. Such a dense frequency comb spectrum enables simultaneous addressing of a few resonators via frequency multiplexing. Finally, we discuss the criteria for achieving effective spectroscopic coverage of a given frequency bandwidth.