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MRS Advances

Electrodynamic simulation of an antenna-coupled Pb–graphene–Pb Josephson junction at millimeter wavelengths

Emily Frederick, Swastik Ballav, Javier González, R. E. Peale, Masa Ishigami

Published May 19, 2026• 0 citations
Publisher

Abstract

Potential for single-photon mm-wave detection by antenna-coupled Pb–graphene–Pb Josephson junctions (JJ) is evaluated. We consider the high-speed “video” (non-thermal) detection based on Shapiro steps induced by antenna-sourced AC currents across the JJ. Electrodynamic simulation informs the bowtie antenna design and estimates the AC current through JJ. Standard JJ models are used to estimate the corresponding AC voltage and the downward shift of the maximum zero-voltage DC current Ic, where a sharp transition occurs in the DC voltage–current curve. Calculated voltage responsivity for current biasing near Ic is compared with Johnson noise to estimate a lower limit for noise equivalent photon flux. Estimated responsivity is 0.9 $$MV/W$$ M V / W at 300 $$GHz$$ GHz , and the lower bound on the noise equivalent power is 0.11 fW/ $$\sqrt {{\text{Hz}}}$$ Hz giving a 2 photons per microsecond. Graphical abstract

DOI
10.1557/s43580-026-01676-7

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Potential for single-photon mm-wave detection by antenna-coupled Pb–graphene–Pb Josephson junctions (JJ) is evaluated. We consider the high-speed “video” (non-thermal) detection based on Shapiro steps induced by antenna-sourced AC currents across the JJ. Electrodynamic simulation informs the bowtie antenna design and estimates the AC current through JJ. Standard JJ models are used to estimate the corresponding AC voltage and the downward shift of the maximum zero-voltage DC current Ic, where a sharp transition occurs in the DC voltage–current curve. Calculated voltage responsivity for current biasing near Ic is compared with Johnson noise to estimate a lower limit for noise equivalent photon flux. Estimated responsivity is 0.9 MV/WMV/WMV/W M V / W at 300 GHzGHzGHz GHz , and the lower bound on the noise equivalent power is 0.11 fW/ Hz\sqrt {{\text{Hz}}}Hz​ Hz giving a 2 photons per microsecond. Graphical abstract
Millimeter-wave detection was investigated using a step-edge microbridge Josephson junction fabricated by a YBaCuO thin film for both the video detection and harmonic mixing detection. Current–voltage (I–V) characteristics, which could be described by the resistively shunted junction (RSJ) model, were obtained up to 70 K with the IcR product of 150 μV at 70 K. The responsivity and noise equivalent input power were calculated as 2400 W/V and 4.4×10−14 W/√Hz, respectively, in the video detection assuming the RSJ model approximation. Clear dependence of the intermediate frequency output on the applied dc bias voltage was observed in the harmonic mixing detection with the harmonic numbers of 5–16, reflecting the change of dynamic resistance at the constant voltage steps induced by the local oscillation signal on the I–V curve. A minimum detection power of −59.4 dB m and a conversion efficiency (η) of −49.6 dB were obtained for the harmonic number (N) of 5 at 60 K. The relationship between the η and N was expressed as η∝N−2.2.
In a quasi-optical system, the high temperature superconducting terahertz detector often suffers from a fundamental problem of low coupling efficiency with the terahertz signal, especially for the detector based on YBa2Cu3O7-δ (YBCO) bicrystal Josephson junction (JJ) due to a small normal-state resistance. Here, we developed a bowtie loaded meander antenna to enhance coupling efficiency. Differing from the conventional characterization confining on vector network analyzers, we applied three methods to evaluate the antenna, including the measurements of the maximal size of the first order Shapiro step under per incident power, the coupling efficiency between the antenna and the junction, and voltage responsivity. Furthermore, with simulation analysis, we propose that the inductive reactance of the YBCO bicrystal JJ is around 60 ohms under terahertz irradiation at 210 GHz, thus, the reactance is comparable as that of the antenna.
Abstract An experiment in video signal detection at 70 GHz was conducted with a Pb micro‐bridge prepared by electron‐beam exposure and the lift‐off method. It was found that for the dc Josephson effect the external noise is dominant over the internal noise of the specimen and that the equivalent noise becomes large near the critical temperature. The response to 100 kHz amplitude‐modulated signals was found to be ten times more rapid than that of the superconductor bolometer. Theoretically the maximum responsivity is obtained near the first Shapiro step, which, however, does not give a stable bias; maximum responsivity was actually observed near the zeroth Shapiro step, where it exceeded 6.3 × 104 V/W. The responsivity was found to increase with increasing IcRn.
Graphene Josephson junctions (JJ) provide a promising platform for ultra-broadband quantum sensing of light owing to graphene's frequency-independent absorption, vanishing electronic heat capacity, and weak electron-phonon coupling, which enable rapid suppression of the critical current through radiation-induced electron heating. Existing investigations have been confined to the microwave and infrared regimes, where competing detector technologies are already established; by contrast, the terahertz (THz) band - where sensitivity is most urgently lacking and no mature quantum sensor exists - has remained largerly unexplored. Here we demonstrate a strong photoresponse of graphene JJs at THz frequencies, establishing a first experimental step towards graphene-based THz quantum sensors. Under low-intensity illumination, we observe a pronounced suppression of the critical current that generates a strong photovoltage (Vph) under current bias. By tracking this Vph and independently measuring the electron temperature as a function of absorbed power, we extract a responsivity of 88 kV W^-1 and a noise-equivalent power of 45 aW Hz^-1/2 at 1.7 K. Furthermore, gate tunability of our JJ enables access to a regime where hysteretic current-voltage characteristics persist up to 0.9 K, offering a potential route toward single-photon THz detection beyond millikelvin (mK) temperatures. These findings establish graphene JJ as a versatile platform for broadband cryogenic radiation sensing and point towards their use as quantum sensors at THz frequencies.
A Josephson junction (JJ) has been under intensive study ever since 1960's. Yet even in the present era of building quantum information processing devices based on many JJs, open questions regarding a single junction remain unsolved, such as quantum phase transitions, coupling of the JJ to an environment and improving coherence of a superconducting qubit. Here we design and build an engineered on-chip reservoir that acts as an efficient bolometer for detecting the Josephson radiation under non-equilibrium (biased) conditions. The bolometer converts ac Josephson current at microwave frequencies, up to about 100 100\,100GHz, into a measurable dc temperature rise. The present experiment demonstrates an efficient, wide-band, thermal detection scheme of microwave photons and provides a sensitive detector of Josephson dynamics beyond the standard conductance measurements. Using a circuit model, we capture both the current-voltage characteristics and the measured power quantitatively.