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Bolometer operating at the threshold for circuit quantum electrodynamics

Roope Kokkoniemi, J.-P. Girard, Dibyendu Hazra, Antti Laitinen, Joonas Govenius, Russell E. Lake, Iiro Sallinen, Visa Vesterinen, Matti Partanen, J. Y. Tan, K. W. Chan, Kuan Yen Tan, Pertti Juhani Hakonen, Mikko Möttönen

Published Jan 1, 2020• 96 citations
OpenAlex

Abstract

Abstract Radiation sensors based on the heating effect of absorbed radiation are typically simple to operate and flexible in terms of input frequency, so they are widely used in gas detection1, security2, terahertz imaging3, astrophysical observations4 and medical applications5. Several important applications are currently emerging from quantum technology and especially from electrical circuits that behave quantum mechanically, that is, circuit quantum electrodynamics6. This field has given rise to single-photon microwave detectors7–9 and a quantum computer that is superior to classical supercomputers for certain tasks10. Thermal sensors hold potential for enhancing such devices because they do not add quantum noise and they are smaller, simpler and consume about six orders of magnitude less power than the frequently used travelling-wave parametric amplifiers11. However, despite great progress in the speed12 and noise levels13 of thermal sensors, no bolometer has previously met the threshold for circuit quantum electrodynamics, which lies at a time constant of a few hundred nanoseconds and a simultaneous energy resolution of the order of 10h gigahertz (where h is the Planck constant). Here we experimentally demonstrate a bolometer that operates at this threshold, with a noise-equivalent power of 30 zeptowatts per square-root hertz, comparable to the lowest value reported so far13, at a thermal time constant two orders of magnitude shorter, at 500 nanoseconds. Both of these values are measured directly on the same device, giving an accurate estimation of 30h gigahertz for the calorimetric energy resolution. These improvements stem from the use of a graphene monolayer with extremely low specific heat14 as the active material. The minimum observed time constant of 200 nanoseconds is well below the dephasing times of roughly 100 microseconds reported for superconducting qubits15 and matches the timescales of currently used readout schemes16, 17, thus enabling circuit quantum electrodynamics applications for bolometers.

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Sensitive and reliable photon detectors are ubiquitous tools in countless fields of engineering and the sciences. One of the most mature and widely applied approaches to photodetection is bolometry, where the absorption of radiation causes a measurable change in the temperature of the absorber material. Applications of bolometry include fields such as astrophysics, medicine, particle physics, and metrology, all of which greatly benefit from the development of more versatile and efficient detectors. In the field of superconducting quantum circuits, considerable progress has recently been made in developing ultrasensitive superconducting bolometers. Operating in the single-photon regime, these devices hold great potential for several applications within quantum technology, including qubit state measurement and characterization of microwave components and cryogenic environments. State-of-the-art bolometers based on graphene have been shown to reach sufficient energy resolution and operation speed to be applicable in the context of circuit quantum electrodynamics. In this thesis, a superconducting bolometer based on superconductor-normal-metal-superconductor Josephson junctions is studied. In particular, a detection protocol based on optimal filtering of the noisy detector output signal is implemented to detect microwave pulses during continuous operation of the device. The fidelity of the implemented real-time detection scheme is experimentally tested and compared to the theoretical energy resolution of the studied bolometer, computed from noise equivalent power. The device is found to reach 50% detection probability with minimal dark-count rate at pulse energies of approximately 2.2 zJ, exceeding the predicted resolution of 0.75 zJ by only a factor of three. In addition, the characteristic energy of the detector is determined to be approximately 1.5 zJ based on the total energy of pulses detected with fidelity F=0.5. The results give hope of future applications of the detection scheme to more sensitive bolometer samples.

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Superconducting hot electron bolometers (HEBs) are widely used as terahertz (THz) heterodyne detectors (mixers) and becoming the most advantageous candidate for low noise receiver at frequencies above 1.2 THz. However, there is still lack of accurate theoretical modeling which explains the complex mechanism of different aspects of superconducting HEBs. These difficulties do not prevent researchers around the world from making experimental progress. In the last few years, superconducting HEBs are also used as THz direct detectors to develop the array for the applications in biology, medicine and security systems. Recently, a noise equivalent power (NEP) of 0.2 pW/√Hz at 2.5 THz is reported for the superconducting HEBs with thermal biasing. Instead of thermal one, microwave (MW) which was first used to stabilize the HEB mixer is also applied to bias the device as a direct detector. Furthermore, THz can be used to bias the HEBs, if THz sources can be provided easy. Although MW biasing can improve sensitivity of the superconducting HEB direct detectors, little work has been done to compare the responsivity and NEP among the THz direct detectors with thermal, THz and MW biasing. To find a best biasing method, we have studied the similarities and differences of the direct detectors based on the superconducting HEBs with three different biasing methods. The fabricated NbN HEB detectors consist of a complementary logarithmic-spiral antenna made of gold and an NbN film (bridge) connecting across the antenna’s inner terminals. We have fabricated the niobium nitride (NbN) superconducting HEB mixers with the system double sideband (DSB) noise temperature better than 8 times of the quantum limit: hf/kB (where h is the Planck constant, kB is the Boltzmann constant and f is the operating frequency) at the frequencies higher than 1 THz. Here, the mixer chips with the system DSB noise temperature of about 400 K and the intermediate frequency (IF) gain bandwidth (GBW) of larger than 5 GHz at 4.2 K and 0.65 THz have been used. As the direct detectors, the current responsivity of 290 A/W and 105 A/W are obtained for MW and THz biasing, which about one order higher than that of thermal biasing. The NEP of 2.7 pW/√Hz is obtained with MW biasing, which can be expected to be improved in the future.

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We introduce a microwave bolometer aimed at high-quantum-efficiency detection of wave packet energy within the framework of circuit quantum electrodynamics, the ultimate goal being single microwave photon detection. We measure the differential thermal conductance between the detector and its heat bath, obtaining values as low as $5\phantom{\rule{4.pt}{0ex}}\text{fW}/\mathrm{K}$ at $50\phantom{\rule{4.pt}{0ex}}\text{mK}$. This is one tenth of the thermal conductance quantum and corresponds to a theoretical lower bound on noise-equivalent power of order ${10}^{\ensuremath{-}20}\phantom{\rule{4.pt}{0ex}}\text{W}/\sqrt{\text{Hz}}$ at $50\phantom{\rule{4.pt}{0ex}}\text{mK}$. By measuring the differential thermal conductance of the same bolometer design in substantially different environments and materials, we determine that electron-photon coupling dominates the thermalization of our nanobolometer.

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Abstract Radiation sensors based on the heating effect of absorbed radiation are typically simple to operate and flexible in terms of input frequency, so they are widely used in gas detection1, security2, terahertz imaging3, astrophysical observations4 and medical applications5. Several important applications are currently emerging from quantum technology and especially from electrical circuits that behave quantum mechanically, that is, circuit quantum electrodynamics6. This field has given rise to single-photon microwave detectors7–9 and a quantum computer that is superior to classical supercomputers for certain tasks10. Thermal sensors hold potential for enhancing such devices because they do not add quantum noise and they are smaller, simpler and consume about six orders of magnitude less power than the frequently used travelling-wave parametric amplifiers11. However, despite great progress in the speed12 and noise levels13 of thermal sensors, no bolometer has previously met the threshold for circuit quantum electrodynamics, which lies at a time constant of a few hundred nanoseconds and a simultaneous energy resolution of the order of 10h gigahertz (where h is the Planck constant). Here we experimentally demonstrate a bolometer that operates at this threshold, with a noise-equivalent power of 30 zeptowatts per square-root hertz, comparable to the lowest value reported so far13, at a thermal time constant two orders of magnitude shorter, at 500 nanoseconds. Both of these values are measured directly on the same device, giving an accurate estimation of 30h gigahertz for the calorimetric energy resolution. These improvements stem from the use of a graphene monolayer with extremely low specific heat14 as the active material. The minimum observed time constant of 200 nanoseconds is well below the dephasing times of roughly 100 microseconds reported for superconducting qubits15 and matches the timescales of currently used readout schemes16, 17, thus enabling circuit quantum electrodynamics applications for bolometers.
Sensitive and reliable photon detectors are ubiquitous tools in countless fields of engineering and the sciences. One of the most mature and widely applied approaches to photodetection is bolometry, where the absorption of radiation causes a measurable change in the temperature of the absorber material. Applications of bolometry include fields such as astrophysics, medicine, particle physics, and metrology, all of which greatly benefit from the development of more versatile and efficient detectors. In the field of superconducting quantum circuits, considerable progress has recently been made in developing ultrasensitive superconducting bolometers. Operating in the single-photon regime, these devices hold great potential for several applications within quantum technology, including qubit state measurement and characterization of microwave components and cryogenic environments. State-of-the-art bolometers based on graphene have been shown to reach sufficient energy resolution and operation speed to be applicable in the context of circuit quantum electrodynamics. In this thesis, a superconducting bolometer based on superconductor-normal-metal-superconductor Josephson junctions is studied. In particular, a detection protocol based on optimal filtering of the noisy detector output signal is implemented to detect microwave pulses during continuous operation of the device. The fidelity of the implemented real-time detection scheme is experimentally tested and compared to the theoretical energy resolution of the studied bolometer, computed from noise equivalent power. The device is found to reach 50% detection probability with minimal dark-count rate at pulse energies of approximately 2.2 zJ, exceeding the predicted resolution of 0.75 zJ by only a factor of three. In addition, the characteristic energy of the detector is determined to be approximately 1.5 zJ based on the total energy of pulses detected with fidelity F=0.5. The results give hope of future applications of the detection scheme to more sensitive bolometer samples.
Bolometers are radiation sensors that are central to wide areas such as dark matter search, radio astronomy, material science, and qubit readouts, among others. There have been different kinds of bolometer realizations in the recent past. The challenge is to have a single device that combines high sensitivity, broad bandwidth, and a fast readout scheme. Here we demonstrate the usage of Josephson parametric amplifiers (JPA) as highly sensitive bolometers. Our key finding is that the Kerr non-linearity of the JPA boosts the device’s sensitivity. When the bolometer is biased in the non-linear regime, it enhances the up-converted signals (~100 times), resulting in an order of magnitude improvement in sensitivity compared to the linear regime. In the non-linear biasing, we achieve a NEP ~ 500 aW/ Hz\sqrt{{{{\rm{Hz}}}}}Hz​ . Our device offers a fast detection scheme with a thermal time constant of 4.26 μs and an intrinsic JPA time constant of 70 ns. Our work integrates a JPA into a bolometer, enabling a fast and sensitive operation compared to previously studied graphene-based bolometers. Our study demonstrates a way forward to improve the quantum sensors based on 2D materials by leveraging the inherent non-linear response. Graphene-based Josephson junction bolometers hold promise as sensitive single-photon detectors, but they are normally limited by slow readout schemes and narrow operational bandwidths. Here, the authors report the application of Josephson parametric amplifiers as fast, tunable bolometers with sensitivity enhanced by the intrinsic Kerr non-linearity.
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.
We present a theoretical analysis and experimental evaluation of a transition-edge superconducting bolometer for detecting infrared and millimeter waves. The superconducting film is voltage biased and the current is read by a superconducting quantum interference device ammeter. Strong electrothermal feedback maintains the sensor temperature within the transition, gives a current responsivity that is simply the inverse of the bias voltage, and reduces the response time by several orders of magnitude below the intrinsic time constant C/G. We evaluated a voltage-biased bolometer that operates on the Tc∼95 mK transition of a tungsten film with a thermal conductance of G∼1.2×10−9 W/K. As expected, the electrical noise equivalent power of 3.3×10−17/W√Hz is close to the thermal fluctuation noise limit and is lower than that of other technologies for these values of G and temperature. The measured time constant of 10 μs is ∼100 times faster than the intrinsic time constant.
Superconducting hot electron bolometers (HEBs) are widely used as terahertz (THz) heterodyne detectors (mixers) and becoming the most advantageous candidate for low noise receiver at frequencies above 1.2 THz. However, there is still lack of accurate theoretical modeling which explains the complex mechanism of different aspects of superconducting HEBs. These difficulties do not prevent researchers around the world from making experimental progress. In the last few years, superconducting HEBs are also used as THz direct detectors to develop the array for the applications in biology, medicine and security systems. Recently, a noise equivalent power (NEP) of 0.2 pW/√Hz at 2.5 THz is reported for the superconducting HEBs with thermal biasing. Instead of thermal one, microwave (MW) which was first used to stabilize the HEB mixer is also applied to bias the device as a direct detector. Furthermore, THz can be used to bias the HEBs, if THz sources can be provided easy. Although MW biasing can improve sensitivity of the superconducting HEB direct detectors, little work has been done to compare the responsivity and NEP among the THz direct detectors with thermal, THz and MW biasing. To find a best biasing method, we have studied the similarities and differences of the direct detectors based on the superconducting HEBs with three different biasing methods. The fabricated NbN HEB detectors consist of a complementary logarithmic-spiral antenna made of gold and an NbN film (bridge) connecting across the antenna’s inner terminals. We have fabricated the niobium nitride (NbN) superconducting HEB mixers with the system double sideband (DSB) noise temperature better than 8 times of the quantum limit: hf/kB (where h is the Planck constant, kB is the Boltzmann constant and f is the operating frequency) at the frequencies higher than 1 THz. Here, the mixer chips with the system DSB noise temperature of about 400 K and the intermediate frequency (IF) gain bandwidth (GBW) of larger than 5 GHz at 4.2 K and 0.65 THz have been used. As the direct detectors, the current responsivity of 290 A/W and 105 A/W are obtained for MW and THz biasing, which about one order higher than that of thermal biasing. The NEP of 2.7 pW/√Hz is obtained with MW biasing, which can be expected to be improved in the future.
We introduce a microwave bolometer aimed at high-quantum-efficiency detection of wave packet energy within the framework of circuit quantum electrodynamics, the ultimate goal being single microwave photon detection. We measure the differential thermal conductance between the detector and its heat bath, obtaining values as low as 5fW/K5\phantom{\rule{4.pt}{0ex}}\text{fW}/\mathrm{K}5fW/K at 50mK50\phantom{\rule{4.pt}{0ex}}\text{mK}50mK. This is one tenth of the thermal conductance quantum and corresponds to a theoretical lower bound on noise-equivalent power of order ${10}^{\ensuremath{-}20}\phantom{\rule{4.pt}{0ex}}\text{W}/\sqrt{\text{Hz}}$ at 50mK50\phantom{\rule{4.pt}{0ex}}\text{mK}50mK. By measuring the differential thermal conductance of the same bolometer design in substantially different environments and materials, we determine that electron-photon coupling dominates the thermalization of our nanobolometer.