Frequency multiplexed superconducting quantum interference device readout of large bolometer arrays for cosmic microwave background measurements

Photon noise correlations in millimeter-wave telescopes

Many modern millimeter and submillimeter ("mm-wave") telescopes for astronomy are deploying more detectors by increasing the detector pixel density and, with the rise of lithographed detector architectures and high-throughput readout techniques, it is becoming increasingly practical to overfill the focal plane. However, when the pixel pitch p p i x is small compared to the product of the wavelength λ and the focal ratio F, or p p i x ≲1.2F λ, the Bose term of the photon noise correlates between neighboring detector pixels due to the Hanbury Brown and Twiss (HBT) effect. When this HBT effect is non-negligible, the array-averaged sensitivity scales with the detector count N det less favorably than the uncorrelated limit of Ndet-1/2. In this paper, we present a general prescription to calculate this HBT correlation based on a quantum optics formalism and extend it to polarization-sensitive detectors. We then estimate the impact of HBT correlations on the sensitivity of a model mm-wave telescope and discuss the implications for a focal plane design.

Frequency division multiplexing readout of a transition edge sensor bolometer array with microstrip-type electrical bias lines

We demonstrate multiplexed readout of 43 transition edge sensor (TES) bolometers operating at 90 mK using a frequency division multiplexing (FDM) readout chain with bias frequencies ranging from 1 to 3.5 MHz and a typical frequency spacing of 32 kHz. We improve the previously reported performance of our FDM system by two important steps. First, we replace the coplanar wires with microstrip wires, which minimize the cross talk from mutual inductance. From the measured electrical cross talk (ECT) map, the ECT of all pixels is carrier leakage dominated. Only five pixels show an ECT level higher than 1%. Second, we reduce the thermal response speed of the TES detectors by a factor of 20 by increasing the heat capacity of the TES, which allows us to bias all TES detectors below 50% in transition without oscillations. We compare the current-voltage curves and noise spectra of the TESs measured in single-pixel mode and multiplexing mode. We also compare the noise equivalent power (NEP) and the saturation power of the bolometers in both modes, where 38 pixels show less than 10% difference in NEP and 5% difference in saturation power when measured in the two different modes. The measured noise spectrum is in good agreement with the simulated noise based on measured parameters from an impedance measurement, confirming that our TES is dominated by phonon noise.

Frequency shift algorithm: Design of a baseband phase locked loop for frequency-domain multiplexing readout of x-ray transition-edge sensor microcalorimeters

The Transition-Edge Sensor (TES) is an extremely sensitive device, which is used to measure the energy of individual x-ray photons. For astronomical spectrometry applications, SRON develops a frequency domain multiplexing readout system for kilopixel arrays of such TESs. Each TES is voltage biased at a specific frequency in the range of 1-5 MHz. Isolation between the individual pixels is obtained through very narrow-band (high-Q) lithographic LC resonators. To prevent energy resolution degradation due to intermodulation line noise, the bias frequencies are distributed on a regular grid. The requirements on the accuracy of the LC resonance frequency are very high. The deviation of the resonance frequencies due to production tolerances is significant with respect to the bandwidth, and a controller is necessary to compensate for the LC series impedance. We present two such controllers: a simple orthogonal proportional-integral controller and a more complex impedance estimator. Both controllers operate in baseband and try to make the TES current in-phase with the bias voltage, effectively operating as phase-locked loops. They allow off-LC-resonance operation of the TES pixels while preserving the TES thermal response and energy resolution. Extensive experimental results-published in a companion paper recently-with the proposed methods show that these controllers allow the preservation of single pixel energy resolution in multiplexed operation.

Electrical cross talk of a frequency division multiplexing readout for a transition edge sensor bolometer array

We have characterized and mapped the electrical cross talk (ECT) of a frequency division multiplexing (FDM) system with a transition edge sensor (TES) bolometer array, which is intended for space applications. By adding a small modulation at 120 Hz to the AC bias voltage of one bolometer and measuring the cross talk response in the current noise spectra of the others simultaneously, we have for the first time mapped the ECT level of 61 pixels with a nominal frequency spacing of 32 kHz in a 61 × 61 matrix and a carrier frequency ranging from 1 MHz to 4 MHz. We find that about 94% of the pixels show an ECT level of less than 0.4%. Only the adjacent pixels reach this level, and the ECT for the rest of the pixels is less than 0.1%. We also observe higher ECT levels, up to 10%, between some of the pixels, which have bundled long, parallel coplanar wires connecting TES bolometers to inductor-capacitor filters. In this case, the high mutual inductances dominate. To mitigate this source of ECT, the coplanar wires should be replaced by microstrip wires in the array. Our study suggests that an FDM system can have a relatively low ECT level, e.g., around 0.4% if the frequency spacing is 30 kHz. Our results successfully demonstrate a low electrical cross talk for a space FDM technology.

Direct-coupled micro-magnetometer with Y-Ba-Cu-O nano-slit SQUID fabricated with a focused helium ion beam

Direct write patterning of high-transition temperature (high-T C) superconducting oxide thin films with a focused helium ion beam is a formidable approach for the scaling of high-T C circuit feature sizes down to the nanoscale. In this letter, we report using this technique to create a sensitive micro superconducting quantum interference device (SQUID) magnetometer with a sensing area of about 100 × 100 μm2. The device is fabricated from a single 35-nm thick YBa2Cu3O7- δ film. A flux concentrating pick-up loop is directly coupled to a 10 nm × 20 μm nano-slit SQUID. The SQUID is defined entirely by helium ion irradiation from a gas field ion source. The irradiation converts the superconductor to an insulator, and no material is milled away or etched. In this manner, a very narrow non-superconducting nano-slit is created entirely within the plane of the film. The narrow slit dimension allows for maximization of the coupling to the field concentrator. Electrical measurements reveal a large 0.35 mV modulation with a magnetic field. We measure a white noise level of 2 μΦ0/Hz1∕2. The field noise of the magnetometer is 4 pT/Hz1∕2 at 4.2 K.

Neurodevelopment in children with intrauterine growth restriction: adverse effects and interventions

Intrauterine growth restriction (IUGR) is associated with higher rates of fetal, perinatal, and neonatal morbidity and mortality. The consequences of IUGR include short-term metabolic, hematological and thermal disturbances that lead to metabolic syndrome in children and adults. Additionally, IUGR severely affects short- and long-term fetal brain development and brain function (including motor, cognitive and executive function) and neurobehavior, especially neuropsychology. This review details the adverse effects of IUGR on fetal brain development and discusses intervention strategies.

Experimental measurements and noise analysis of a cryogenic radiometer

A cryogenic radiometer device, intended for use as part of an electrical-substitution radiometer, was measured at low temperature. The device consists of a receiver cavity mechanically and thermally connected to a temperature-controlled stage through a thin-walled polyimide tube which serves as a weak thermal link. With the temperature difference between the receiver and the stage measured in millikelvin and the electrical power measured in picowatts, the measured responsivity was 4700 K/mW and the measured thermal time constant was 14 s at a stage temperature of 1.885 K. Noise analysis in terms of Noise Equivalent Power (NEP) was used to quantify the various fundamental and technical noise contributions, including phonon noise and Johnson-Nyquist noise. The noise analysis clarifies the path toward a cryogenic radiometer with a noise floor limited by fundamental phonon noise, where the magnitude of the phonon NEP is 6.5 fW/√Hz for the measured experimental parameters.