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Superconductor Science and Technology

Design and performance of multiloop and washer SQUIDs intended for sub-kelvin operation

M. Kiviranta, Jari S Penttilä, Leif Grönberg, Juha Hassel, Antti Virtanen, Heikki Seppä

Published Mar 30, 2004• 16 citations
Publisher

Abstract

We have developed a set of dc SQUIDs intended for frequency-domain multiplexed readout of transition edge sensor (TES) arrays. The first design is based on the Ketchen-style washer, and has exhibited a 1.2 × 10 −7 Φ 0 Hz 1/2 flux noise both at 4.2 and 0.43 K. The second design is based on the multiloop structure and utilizes the spoke-terminating resistors which double as junction shunts in order to minimize the excess noise due to the resonance damping. The multiloop device has exhibited a 3.5 × 10 −7 Φ 0 Hz 1/2 flux noise level at 4.2 K. The mutual inductances are chosen such that a typical ac-biased TES-based x-ray calorimeter would need only a moderate amount of negative feedback in order to handle the dynamic range of the signal.

DOI
10.1088/0953-2048/17/5/038

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TEION KOGAKU (Journal of Cryogenics and Superconductivity Society of Japan)•Jan 1, 2004•0 citations
We have developed a set of dc SQUIDs intended for frequency-domain multiplexed readout of transition edge sensor (TES) arrays. The first design is based on the Ketchen-style washer, and has exhibited a 1.2 × 10 −7 Φ 0 Hz 1/2 flux noise both at 4.2 and 0.43 K. The second design is based on the multiloop structure and utilizes the spoke-terminating resistors which double as junction shunts in order to minimize the excess noise due to the resonance damping. The multiloop device has exhibited a 3.5 × 10 −7 Φ 0 Hz 1/2 flux noise level at 4.2 K. The mutual inductances are chosen such that a typical ac-biased TES-based x-ray calorimeter would need only a moderate amount of negative feedback in order to handle the dynamic range of the signal.
We have developed a set of SQUIDs optimized for readout of ac-biased transition edge sensors. Junction shunts are made of Pd and attached to cooling fins to facilitate low-noise operation at sub-kelvin temperatures. SQUID's have a low loop inductance in order to reach a large natural dynamic range even without negative feedback. The SQUIDs are intended to be used for frequency-domain multiplexing of X-ray calorimeter arrays. Both traditional dc-SQUIDs and novel un-SQUIDs are manufactured. The fabrication process and SQUID design criteria are reviewed.
We describe our frequency-domain readout multiplexer for transition-edge sensor (TES) bolometers and present measurements of an eight-channel multiplexer. Each sensor is biased with a sinusoidal bias at a distinct frequency. As the sensor absorbs power, it amplitude-modulates its sinusoidal bias. Sensor currents are summed and measured with a single superconducting quantum interference device (SQUID) array. The SQUID array consists of 100 dc-SQUID's in series and is operated with shunt feedback electronics which have a slew rate of 1.210/sup 7/ /spl Phi//sub 0//s. A tuned filter consisting of an inductor and capacitor are placed in series with each sensor to both limit the bandwidth of the Nyquist noise from each sensor and to allow us to bias all multiplexed sensors with a common wire. We place an upper limit on crosstalk between adjacent channels of 0.004, well below our design requirements. Demodulated noise spectra from multiplexed sensors show the expected white noise levels at frequencies above 200 mHz.
Superconducting transition-edge sensors (TESs) are highly sensitive detectors, can detect electromagnetic wave radiations from millimeter/submillimeter, optical to x/γ rays, suitable for astrophysics, quantum information, and biosensing. Direct current superconducting quantum interference devices (DC SQUIDs) are the detector of choice for TES readout, thanks to their low noise, low impedance, and low power dissipation. In this work, we designed the DC SQUID with a second-order gradient structure, which can effectively mitigate the effect of external magnetic flux. We optimized the fabrication process and successfully fabricated the high-performance DC SQUID with Nb/Al-AlOx/Nb Josephson junctions (critical current density: JC~1μA/μm2). The properties of the fabricated DC SQUID were in good agreement with the simulation results. Meanwhile, the measured flux noise was 6 μΦ0/√Hz, corresponding to a current noise of 100 pA/√Hz. Furthermore, we demonstrated the capability of the DC SQUID to read out the TES detector, including I-V curves, photon pulse response, and noise performance.
A SQUID tandem is a two-stage amplifier consisting of the front end SQUID atTT∼ 4 K. We give an overview of the dc SQUID design for the tandem used in the recent 37-pixel frequency domain multiplexed (FDM) readout demonstration of X-ray calorimeters. The SQUID devices were fabricated with pillar-type Josephson junctions at the nominalJC= 500 A/cm2critical current density. We further discuss possible improvements to the FDM tandem configuration, including (i) the class-B operation for better power efficiency, (ii) the input power combiner as an improvement over the simple T-junction summing, with the promise for larger multiplexing factors, and (iii) the new booster SQUID device. The new improved booster is a 8-parallel 66-series SQUID array fabricated with SWAPS Josephson junctions at the nominal JC= 1.5 kA/cm2 critical current density, and it shows dV/dΦ > 30 mV/Φ0gain in the symmetric-slope operation, as well as ΦN≈ 0.05 μΦ0/ Hz1/2flux noise.
Abstract Direct-current superconducting quantum interference devices (dc-SQUIDs) are one of the most sensitive magnetic detectors. These sensors are extensively used in the readout of superconducting transition edge sensors (TESs), which are used for the detection of weak signals. A cosmic microwave background (CMB) polarization telescope operating in 22–48 GHz is currently under developing. The TESs calorimeter of the telescope will be readout by a time-division multiplexer SQUID readout system. We develop a two-stage dc-SQUID amplifier circuit, comprising an input-stage SQUID with 4 SQUID cells and a series SQUID array with 100 SQUID cells. This configuration has been shown to achieve extremely high signal gain while effectively controlling system noise. We assess the system noise at 300 mK in an adiabatic demagnetization refrigerator. The measured magnetic flux noise of the two-stage SQUID circuit system is approximately 0.3 μ Φ 0 / Hz at 10 kHz. The current noise equivalent to the input coil of input SQUID is about 2.4 pA / Hz . This result meets the low-noise readout requirements of the CMB TES and other applications with TES detectors.
Dissipationless microwave radio frequency superconducting interference device (RF-SQUID) multiplexers are attractive for readouts of large format arrays of transition edge sensors (TES). This multiplexer has the potential for readout bandwidth of several GHz and power consumption of ≈0.1 pW per pixel, which are ≈103 and ≈10-4 times as large as those of DC-SQUID-based conventional multiplexers. To evaluate their readout performance for TES photon counters ranging from near infrared to gamma ray, white noise of this multiplexer was experimentally studied at 4 K. The reason for this was, firstly, to avoid the low-frequency fluctuation obvious at around 0.1 K but trivial for the energy resolution of photon counters, and secondly for a feasibility study of readout operation at 4 K for extended applications. To increase resonant Q at 4 K and maintain low-noise SQUID operation, multiplexer chips consisting of niobium nitride (NbN)-based coplanar-waveguide resonators and niobium (Nb)-based RF-SQUIDs have been developed. This hybrid multiplexer exhibited 1×104≤QI≤2×104 and the square root of spectral density of current noise referred to the SQUID input √SI =31 pA/√Hz, where QI is the internal Q of resonators. The former and the latter are factor-of-five and seven improvements from our previous results on Nb-based resonators, respectively. Systematic noise measurement with various microwave readout power PMR and two-directional readout on the complex plane of transmission component of scattering matrix S21 makes the contribution of each noise source obvious. By decreasing these noises to the degree achievable by current technology, we predicted the microwave RF-SQUID multiplexer would exhibit √SI ≤5 pA/√Hz; i.e., close to √SI of state-of-the-art DC-SQUID-based multiplexers.