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Resonant inductive coupling is commonly seen as an undesired fundamental phenomenon emergent in densely packed resonant structures, such as nuclear magnetic resonance phased array detectors. The need to mitigate coupling imposes rigid constraints on the detector design, impeding performance and limiting the scope of magnetic resonance experiments. Here we introduce a high impedance detector design, which can cloak itself from electrodynamic interactions with neighboring elements. We verify experimentally that the high impedance detectors do not suffer from signal-to-noise degradation mechanisms observed with traditional low impedance elements. Using this new-found robustness, we demonstrate an adaptive wearable detector array for magnetic resonance imaging of the hand. The unique properties of the detector glove reveal new pathways to study the biomechanics of soft tissues, and exemplify the enabling potential of high-impedance detectors for a wide range of demanding applications that are not well suited to traditional coil designs.
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Conventional readout of a superconducting nanowire single-photon detector (SNSPD) sets an upper bound on the output voltage to be the product of the bias current and the load impedance, $I_mathrm{B}times Z_mathrm{load}$, where $Z_mathrm{load}$ is limited to 50 $Omega$ in standard r.f. electronics. Here, we break this limit by interfacing the 50 $Omega$ load and the SNSPD using an integrated superconducting transmission line taper. The taper is a transformer that effectively loads the SNSPD with high impedance without latching. It increases the amplitude of the detector output while preserving the fast rising edge. Using a taper with a starting width of 500 nm, we experimentally observed a 3.6$times$ higher pulse amplitude, 3.7$times$ faster slew rate, and 25.1 ps smaller timing jitter. The results match our numerical simulation, which incorporates both the hotspot dynamics in the SNSPD and the distributed nature in the transmission line taper. The taper studied here may become a useful tool to interface high-impedance superconducting nanowire devices to conventional low-impedance circuits.
For the high luminosity upgrade of the LHC at CERN, ATLAS is considering the addition of a High Granularity Timing Detector (HGTD) in front of the end cap and forward calorimeters at |z| = 3.5 m and covering the region 2.4 < |{eta}| < 4 to help reducing the effect of pile-up. The chosen sensors are arrays of 50 {mu}m thin Low Gain Avalanche Detectors (LGAD). This paper presents results on single LGAD sensors with a surface area of 1.3x1.3 mm2 and arrays with 2x2 pads with a surface area of 2x2 mm^2 or 3x3 mm^2 each and different implant doses of the p+ multiplication layer. They are obtained from data collected during a beam test campaign in Autumn 2016 with a pion beam of 120 GeV energy at the CERN SPS. In addition to several quantities measured inclusively for each pad, the gain, efficiency and time resolution have been estimated as a function of the position of the incident particle inside the pad by using a beam telescope with a position resolution of few {mu}m. Different methods to measure the time resolution are compared, yielding consistent results. The sensors with a surface area of 1.3x1.3 mm^2 have a time resolution of about 40 ps for a gain of 20 and of about 27 ps for a gain of 50 and fulfill the HGTD requirements. Larger sensors have, as expected, a degraded time resolution. All sensors show very good efficiency and time resolution uniformity.
We present proof-of-operation for a new method of electron thermometry using microwave impedance of a hafnium micro-absorber. The new method leads to an ultimate THz-range detector suitable for microwave readout and frequency division multiplexing. The sensing part of the device is a hot-electron-gas absorber responding to the incident radiation by variation of its impedance measured at probing frequency about 1.5 GHz. The absorber is a microbridge made from hafnium (Tc = 375 mK, RN = 30 Ohm) sized 2.5 um by 2.5 um by 50 nm and integrated with a planar 600-700 GHz antenna placed near the open end of a quarter-wave CPW resonator (Q-factor about 10^4). All elements of the circuit, except the microbridge, are made from 100-nm thick Nb, including the resonator, which is weakly coupled to a throughput line. The device was tested at 50-350 mK smoothly responding with its transmission coefficient S21 to applied microwave power at the resonance frequency. We have found that the power absorbed by the bridge fits to the model of hot electron gas, P=k(Te^n-Tph^n) (n = 5...6). The idle NEP down to about 10^-18 W/Hz^(-1/2) and the corresponding cross-over temperature for photon background about 5 K are estimated from the measured data. The saturation power of about 1 pW and possibility of moderate gain are anticipated for a practicable device operating at temperature 200 mK. Since the optimum readout frequency is found exactly at the resonance, the detector is insensitive to most phase instabilities at the probing frequency.
We developed an impedance bridge that operates at cryogenic temperatures (down to 60 mK) and in perpendicular magnetic fields up to at least 12 T. This is achieved by mounting a GaAs HEMT amplifier perpendicular to a printed circuit board containing the device under test and thereby parallel to the magnetic field. The measured amplitude and phase of the output signal allows for the separation of the total impedance into an absolute capacitance and a resistance. Through a detailed noise characterization, we find that the best resolution is obtained when operating the HEMT amplifier at the highest gain. We obtained a resolution in the absolute capacitance of 6.4~aF$/sqrt{textrm{Hz}}$ at 77 K on a comb-drive actuator, while maintaining a small excitation amplitude of 15~$k_text{B} T/e$. We show the magnetic field functionality of our impedance bridge by measuring the quantum Hall plateaus of a top-gated hBN/graphene/hBN heterostructure at 60~mK with a probe signal of 12.8~$k_text{B} T/e$.
As part of an experiment to measure the spectrum of photons emitted in beta-decay of the free neutron, we developed and operated a detector consisting of 12 bismuth germanate (BGO) crystals coupled to avalanche photodiodes (APDs). The detector was operated near liquid nitrogen temperature in the bore of a superconducting magnet and registered photons with energies from 5 keV to 1000 keV. To enlarge the detection range, we also directly detected soft X-rays with energies between 0.2 keV and 20 keV with three large area APDs. The construction and operation of the detector is presented, as well as information on operation of APDs at cryogenic temperatures.
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