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arXiv:2610.33414

Zeptojoule Microwave Calorimetry with Nanosecond Recovery in Graphene

Seunghan Lee, Woochan Jung, Jayeon Kim, Dongsung T. Park, Seong Jang, Seongeun Jeon, Kenji Watanabe, Takashi Taniguchi, Jong-Chul Park, Philip Kim, Kin Chung Fong, Gil-Ho Lee

cond-mat.mes-hallquant-phSubmitted Oct 24, 2026• Announced Oct 27, 2026
arXivHTMLPDF

Abstract

Graphene calorimeters are predicted to combine extreme energy sensitivity with rapid thermal recovery, yet experiments remain far from ideal estimates. Here, we directly calibrate a graphene Josephson-junction microwave calorimeter using nanosecond pulses and independently determine the thermal parameters of the same operating device. At 110 mK, calibrated pulse measurements yield a characteristic switching-energy scale of 18.1 meV (2.90 zJ), while time-resolved two-pulse thermometry gives an intrinsic relaxation time of 19.5 ns. Pulse-width-dependent calorimetry and bolometric thermometry show that the electronic heat capacity is 12.4 times and the electron-phonon coupling is approximately 50 times larger than ideal estimates. A thermal model using these measured parameters predicts 21.2 meV, quantitatively reproducing the observed switching response. These results identify the microscopic origin of the sensitivity gap in graphene calorimeters and establish a self-consistent calibration framework for ultrafast nanoscale thermal detectors.

AI Summary

Graphene microwave calorimeters are expected to combine low detectable energies with fast thermal recovery, but measured sensitivities have remained above ideal estimates. This study asks whether independently measured thermal properties can account for that discrepancy in a graphene Josephson-junction calorimeter. Nanosecond microwave pulses directly calibrate the switching response of the device. At $110\,\mathrm{mK}$, the measured characteristic switching-energy scale is $18.1\,\mathrm{meV}$, or $2.90\,\mathrm{zJ}$. Time-resolved two-pulse thermometry measures an intrinsic thermal relaxation time of $19.5\,\mathrm{ns}$. Pulse-width-dependent calorimetry and bolometric thermometry determine the thermal parameters of the same operating device: the electronic heat capacity is $12.4$ times the ideal estimate, and the electron–phonon coupling is approximately $50$ times the ideal estimate. A thermal model incorporating these measured parameters predicts a switching-energy scale of $21.2\,\mathrm{meV}$, compared with the observed $18.1\,\mathrm{meV}$. The comparison connects the sensitivity gap to enhanced heat capacity and thermal coupling, while demonstrating a calibration procedure that combines pulse response and independent thermal characterization. Only the abstract is available. It reports a characteristic switching-energy scale, not an energy-resolution distribution or single-photon detection efficiency. It does not specify measurement uncertainties, device-to-device reproducibility, or a microscopic mechanism responsible for the enhanced thermal parameters, so those aspects cannot be assessed from the supplied material.

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Written by gpt-6.1-sol from the abstract only. Automated: it can be wrong.

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Graphene calorimeters are predicted to combine extreme energy sensitivity with rapid thermal recovery, yet experiments remain far from ideal estimates. Here, we directly calibrate a graphene Josephson-junction microwave calorimeter using nanosecond pulses and independently determine the thermal parameters of the same operating device. At 110 mK, calibrated pulse measurements yield a characteristic switching-energy scale of 18.1 meV (2.90 zJ), while time-resolved two-pulse thermometry gives an intrinsic relaxation time of 19.5 ns. Pulse-width-dependent calorimetry and bolometric thermometry show that the electronic heat capacity is 12.4 times and the electron-phonon coupling is approximately 50 times larger than ideal estimates. A thermal model using these measured parameters predicts 21.2 meV, quantitatively reproducing the observed switching response. These results identify the microscopic origin of the sensitivity gap in graphene calorimeters and establish a self-consistent calibration framework for ultrafast nanoscale thermal detectors.
Graphene microwave calorimeters are expected to combine low detectable energies with fast thermal recovery, but measured sensitivities have remained above ideal estimates. This study asks whether independently measured thermal properties can account for that discrepancy in a graphene Josephson-junction calorimeter.
Nanosecond microwave pulses directly calibrate the switching response of the device. At 110 mK110\,\mathrm{mK}110mK, the measured characteristic switching-energy scale is 18.1 meV18.1\,\mathrm{meV}18.1meV, or 2.90 zJ2.90\,\mathrm{zJ}2.90zJ. Time-resolved two-pulse thermometry measures an intrinsic thermal relaxation time of 19.5 ns19.5\,\mathrm{ns}19.5ns. Pulse-width-dependent calorimetry and bolometric thermometry determine the thermal parameters of the same operating device: the electronic heat capacity is 12.412.412.4 times the ideal estimate, and the electron–phonon coupling is approximately 505050 times the ideal estimate.
A thermal model incorporating these measured parameters predicts a switching-energy scale of 21.2 meV21.2\,\mathrm{meV}21.2meV, compared with the observed 18.1 meV18.1\,\mathrm{meV}18.1meV. The comparison connects the sensitivity gap to enhanced heat capacity and thermal coupling, while demonstrating a calibration procedure that combines pulse response and independent thermal characterization.
Only the abstract is available. It reports a characteristic switching-energy scale, not an energy-resolution distribution or single-photon detection efficiency. It does not specify measurement uncertainties, device-to-device reproducibility, or a microscopic mechanism responsible for the enhanced thermal parameters, so those aspects cannot be assessed from the supplied material.
The measurement of energy is a fundamental tool used in quantum technology and computing. Some of the most sensitive energy detectors-bolometers and calorimeters-are thermal, meaning that they operate by absorbing incoming energy, converting it into heat and reading out the resulting temperature change electrically using a thermometer. Recently, superconductor-normal-conductor-superconductor radiation sensors with metallic and graphene absorbers haven been predicted to be capable of full-width at half-maximum energy resolutions of 0.75 zJ and 0.05 zJ, respectively. However, these estimates are only mathematically extracted from steady-state noise and responsivity measurements. Here we show that a metallic superconductor-normal-conductor-superconductor sensor can be used for zeptojoule calorimetry. With the approach, we measure the energy of 1-μs-long 8.4-GHz microwave pulses with a full-width at half-maximum energy resolution finer than 0.95 ± 0.02 zJ (=5.9 ± 0.12 meV) corresponding to 170 photons at 8.4 GHz. The technique provides a potential path to real-time calorimetric detection of single photons in the 10-GHz range.
Graphene Josephson junctions (JJs) are promising platforms for broadband quantum sensing because graphene combines frequency-independent absorption, ultralow electronic heat capacity, and weak electron–phonon coupling. While previous studies focused on microwave and infrared regimes, the terahertz (THz) range─where highly sensitive quantum detectors remain scarce─has largely remained unexplored. Here, we demonstrate a gate-tunable THz photoresponse in graphene JJs. Low-intensity THz illumination strongly suppresses the critical current, generating a pronounced photovoltage under current bias. From photovoltage measurements and independent electron thermometry, we extract a responsivity of 88 kV W –1 and a noise-equivalent power of 45 aW Hz –1/2 at 1.7 K. In addition, the hysteretic regime that persists up to 0.9 K suggests a possible route toward single-photon THz detection above millikelvin temperatures. Our results establish graphene JJs as promising candidates for cryogenic THz quantum sensing.
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.
Quantum calorimetry, the thermal measurement of quanta, is a method of choice for ultrasensitive radiation detection ranging from microwaves to gamma rays. The fundamental temperature fluctuations of the calorimeter, dictated by the coupling of it to the heat bath, set the ultimate lower bound of its energy resolution. Here we reach this limit of fundamental equilibrium fluctuations of temperature in a nanoscale electron calorimeter, exchanging energy with the phonon bath at very low temperatures. The approach allows noninvasive measurement of energy transport in superconducting quantum circuits in the microwave regime with high efficiency, opening the way, for instance, to observe quantum jumps, detecting their energy to tackle central questions in quantum thermodynamics.
We propose to use graphene-based Josephson junctions (GJJs) to detect single photons in a wide electromagnetic spectrum from visible to radio frequencies. Our approach takes advantage of the exceptionally low electronic heat capacity of monolayer graphene and its constricted thermal conductance to its phonon degrees of freedom. Such a system could provide high-sensitivity photon detection required for research areas including quantum information processing and radio astronomy. As an example, we present our device concepts for GJJ single-photon detectors in both the microwave and infrared regimes. The dark count rate and intrinsic quantum efficiency are computed based on parameters from a measured GJJ, demonstrating feasibility within existing technologies.
We report measurements of the energy resolution of ultrasensitive superconducting bolometric detectors. The device is a superconducting titanium nanobridge with niobium contacts. A fast microwave pulse is used to simulate a single higher-frequency photon, where the absorbed energy of the pulse is equal to the photon energy. This technique allows precise calibration of the input coupling and avoids problems with unwanted background photons. Present devices have an intrinsic full-width at half-maximum energy resolution of approximately 23 THz, near the predicted value due to intrinsic thermal fluctuation noise.