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.
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 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.
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.
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.
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.
We have developed a new type of x-ray detector based on a superconducting transition-edge thermometer operated near 100 mK. A superconducting quantum interference device is used to measure the current through the thermometer, and negative electrothermal feedback is used to improve the energy resolution and shorten the thermal time constant. We have used a detector mounted on a scanning electron microscope to measure the energy of titanium Kα (4.5 keV) fluorescence x rays with a resolution better than 14 eV full width at half-maximum. Using two other devices, we have measured an energy resolution for Joule heat pulses of 2.6 eV at 1 keV and 0.2 eV at 4 eV, the best reported for any calorimeter. An electrical noise equivalent power of 3×10−18 W/√Hz was also measured, suggesting the use of these detectors as infrared bolometers.
This dissertation focuses on the development of a new type of thermal microwave photodetector based on superconductor–normal-metal–superconductor (SNS) junctions.
We motivate the development of the detector mainly by microwave quantum optics applications in the context of superconducting qubits coupled to microwave transmission lines and resonators, i.e., in the context of circuit quantum electrodynamics (cQED). In cQED, single-photon microwave pulses naturally arise as a results of a qubit exchanging its excitation with a transmission line. While immense progress has been achieved in cQED in general, and in linear microwave amplification in particular, the challenge of developing an efficient and practical detector for single itinerant photons remains open, mostly due to the exceedingly small energy of individual microwave photons. This prevents microwave implementations of a certain class of quantum optical protocols, including the parity measurement protocol proposed in this dissertation.
The core of this dissertation is dedicated to introducing our detector design, discussing the thermal properties of the detector in detail, and demonstrating the operation of the detector in a time-gated threshold detection mode. In particular, we demonstrate threshold detection of coherent 8.4 GHz microwave pulses containing roughly 200 photons, or 1.1 zJ of energy. Compared to other thermal detectors, this is an order of magnitude improvement in the energy of the detected pulses.
In addition, we characterize the linear response of the SNS junctions as separate components. That is, we embed the junctions in a microwave circuit that is specifically designed to allow determining the electrical admittance of the SNS junctions from the response of the circuit as a whole.; Tama vaitoskirja keskittyy uudentyyppisen mikroaaltofotoni-ilmaisimen kehitykseen. Ilmaisin perustuu suprajohde–normaalimetalli–suprajohde-liitoksiin (SNS).
Ilmaisin on suunnattu lahinna mikroaaltotaajuuksilla tapahtuviin kvanttioptiikan sovellutuksiin, joissa suprajohtavat kvanttibitit (eli kubitit) kytketaan mikroaaltosiirtolinjoihin. Tallaisissa niin sanotuissa piirikvanttisahkodynamiikan kokeissa yhden fotonin mikroaaltopulsseja syntyy luonnollisesti virittyneen kubitin luovuttaessa energiansa mikroaaltosiirtolinjaan. Yhden fotonin liikkuvia mikroaaltopulsseja ei kuitenkaan kyeta havaitsemaan tehokkaasti nykyaan kaytossa olevalla tekniikalla, lahinna yksittaisten mikroaaltofotonien haviavan pienen energian vuoksi. Tama siitakin huolimatta, etta lineaariset vahvistimet, joita useimmiten kaytetaan mikroaaltomittauksissa, ovat kehittyneet viime vuosina vahvasti. Ilman yksifotoni-ilmaisia eraita kvanttioptiikan menetelmia ei voida kuitenkaan hyodyntaa, mukaan lukien pariteettimittausmenetelma, jota ehdotetaan tassa vaitoskirjassa.
Vaitoskirjan keskeisimmassa osassa esitetaan ilmaisimen toimintaperiaate ja tarkeimmat termodynaamiset ominaisuudet seka tutkitaan kuinka heikkoja mikroaaltopulsseja ilmaisin kykenee havaitsemaan. Jos pulssin…