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Photon imaging technology is applied in various research fields such as quantum information communication and biological imaging. We have been developing photon-counting devices using optical transition edge sensors (TES). We demonstrated a single-photon spectroscopic imaging system comprising an optical TES and a scanning microscope. However, a great number of TES pixels are required to increase the field of view in the imaging system. To read out the TES array, output signals need to be multiplexed. Microwave SQUID multiplexer (MW-Mux) is a kind of frequency multiplexing method with a carrier wave having a frequency of several GHz, and it can be used to widen the frequency band. In this paper, we report the first demonstration on readout of an optical TES with MW-Mux.

We are developing an x-ray imaging spectrometer for Super DIOS satellite mission, a future x-ray observatory, planned by JAXA, to be launched in 2030’s. Super DIOS will reveal the nature of the missing baryon in the warm-hot intergalactic medium because of its fine energy and angular resolution, large effective area and large field of view. A main detector on-board Super DIOS consists of a transition-edge sensor (TES) microcalorimeter array of over 30,000 pixels working at a temperature below 100 mK and it poses a considerable technical difficulty to the readout. A microwave superconducting quantum interference device (SQUID) multiplexing is promising technique and expected to achieve a large scale readout of more than 30,000 pixels. We describe our development of a 40-channel microwave SQUID multiplexer with low-noise characteristics∗ and a demonstration of simultaneously reading out 40-pixel TESs. Finally, we discuss a future prospect and a feasibility of reading out an array of more than 30,000 pixels.

We present the development of a frequency-domain multiplexing readout of kinetic inductance detectors (KIDs) for pulse signals with a self-trigger system. The KIDs consist of an array of superconducting resonators that have different resonant frequencies individually, allowing us to read out multiple channels in the frequency domain with a single wire using a microwave-frequency comb. The energy deposited to the resonators break Cooper pairs, changing the kinetic inductance and, hence, the amplitude and the phase of the probing microwaves. For some applications such as X-ray detections, the deposited energy is detected as a pulse signal shaped by the time constants of the quasiparticle lifetime, the resonator quality factor, and the ballistic phonon lifetime in the substrate, ranging from microseconds to milliseconds. A readout system commonly used converts the frequency-domain data to the time-domain data. For the short pulse signals, the data rate may exceed the data transfer bandwidth, as the short time constant pulses require us to have a high sampling rate. In order to overcome this circumstance, we have developed a KID readout system that contains a self-trigger system to extract relevant signal data and reduces the total data rate with a commercial off-the-shelf FPGA board. We have demonstrated that the system can read out pulse signals of 15 resonators simultaneously with about 10 Hz event rate by irradiating \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\alpha $$\end{document}α particles from \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^{241}$$\end{document}241Am to the silicon substrate on whose surface aluminum KID resonators are formed.

The quality factors and noise performances of superconducting resonators play an important role in microwave SQUID multiplexer (MW-MUX). Since we develop microwave SQUID multiplexer for transition edge sensor arrays, the operation temperature is around 100 mK where the effects of two level system (TLS) can be significant. In this paper, we have investigated the electrode-material or wafer dependence of noise performance of MW-MUX by characterizing four types of resonators. Although the temperature dependence of resonance frequency and the power dependence of ${{\boldsymbol{Q}}_{\boldsymbol{u}}}$ show clear difference among the materials, frequency noises which contribute the readout noise of MW-MUX are similar. Theoretical model predicts that not TLS but other loss dominates ${{\boldsymbol{Q}}_{\boldsymbol{u}}}$ in the power region suitable for the low-noise operation of our MW-MUX. Further, the estimated flux noise calculated from the measured frequency noise and the conversion coefficient from frequency to flux were 1–5 $({{{\mathbf \mu }}{{{\mathbf \Phi }}_0}/\sqrt {{{\mathbf Hz}}} })$ @ 1 Hz, 0.2–0.8 $({{{\mathbf \mu }}{{{\mathbf \Phi }}_0}/\sqrt {{{\mathbf Hz}}} })$ @ 3 kHz. These results indicate the noise of our MW-MUX has small material dependence and the compatible low noise operation to typical dc SQUIDs would be possible. Therefore, the MW-MUX with our materials would not suffer from the low frequency noise due to TLS.

In order to read out current signals from large-scale arrays of transition edge sensors (TES), microwave SQUID multiplexers (MW-MUX) have been developed by several groups. Reduction of crosstalk induced in the multiplexer is important in order to maintain the energy resolution or the noise equivalent power of detector arrays. In the present study, crosstalk between channels in MW-MUX with five different designs is experimentally evaluated. The resonance frequency separation is ten times as large as the resonator bandwidth. Without flux-ramp modulation, crosstalk between two adjacent channels neighboring in position decreases to ≍1 × 10–3 with increasing resonance frequency separation to ≍100 MHz. Crosstalk between SQUID channels neighboring in resonance frequency also decreases to 4 × 10–3 with increasing the distance to 2.5 mm. We show that nonlinear error can occur due to the crosstalk with flux-ramp modulation. Flux readings from a well-designed multiplexer exhibit crosstalk of less than 1 mΦ0 peak-to-peak and nonlinear errors of less than 2 mΦ0 peak-to-peak, which is sufficient for the readout of the gamma-ray TES arrays being developed. These results can provide guidelines for suppressing both the crosstalk and the nonlinear error to less than the criterion of TES spectrometers with typical energy-resolving ability.

We report very-low-noise, fast-response, middle-scale multiplexing in a microwave superconducting quantum interference device multiplexer (MW-Mux) as a transition-edge sensor (TES) readout. Our MW-Mux is able to read 40 channels with 500 kHz sampling and has a low readout noise of 0.9 μ Φ 0 / Hz (where Φ 0 is the magnetic flux quantum), equivalent to 9 pA / Hz. By contrast, a multiplexer of less than 10 pixels with 500 kHz sampling and ∼ 2 μ Φ 0 / Hz readout noise has so far been reported in the literature. Owing to the 500 kHz sampling, our MW-Mux exhibits a fast response to detect a TES pulse with a rise time around 12 μ s. We demonstrated simultaneous readout of 38 pixels from an array of x-ray TES microcalorimeters. The measured full-width values at half-maximum spectral resolution ranged from 2.79 to 4.56 eV, with a median value of 3.30 eV at 5.9 keV, including a ∼ 10 % contribution of readout noise, i.e., 0.9–1.7 eV.

Accepted: 2017-05-02, 資料番号: SA1170002000

Rapid single flux quantum digital-to-analog (D/A) converters which synthesize arbitrary waveforms with metrological accuracy are under development. We propose a 1-bit RSFQ D/A converter which is expected to operate at higher sampling frequencies than the multi-bit converter and is suitable for multi-chip operation to achieve the output voltages exceeding 100 mV. Calculations of the noise power and the attenuation of the signal suggested that the rms error in a 10 kHz sine wave synthesized by the 1-bit converter with a third-order low-pass filter can be smaller than 10 −7 at the sampling frequency of 100 MHz. A prototype 1-bit D/A converter was fabricated and the generation of dc voltages was confirmed as expected.

We propose a simple method of measuring bit error rates (BERs) for rapid single flux quantum (RSFQ) circuits operating at high frequencies. An error detector based on a SQUID current comparator, which operates without a timing clock, is employed to simplify the test system and measurements. We successfully investigated high-frequency operation of RSFQ cells that were utilized to implement our RSFQ digital-to-analogue converter for metrology applications. Sufficiently low BERs were demonstrated in the circuit operation at several tens of gigahertz.

The pulse-driven Josephson junction array is a promising candidate for an accurate waveform synthesizer. The accuracy of the synthesized waveform is chiefly limited by the in-band frequency components of the driving current because they couple to the output signal through parasitic circuit elements and are difficult to filter out. In this study, we propose a more accurate waveform synthesis method, in which the array is driven by current pulses which have the same waveform as the impulse response of the ideal bandpass filter. Simulations have shown that the current amplitude of the constant-voltage step of a junction driven by the filtered pulses exceeds 70% of the Josephson critical current in a wide range of output signals. Synthesis of a sine wave using this method was also demonstrated by simulation.

A digital-to-analog (D/A) converter based on rapid single flux quantum (RSFQ) circuits is a potential device for establishing a new generation of ac voltage standards. We are developing RSFQ D/A converters that can generate an ac waveform with an accurately defined rms value. Our goal in the near future is the synthesis of an ac waveform with a 100-mV peak voltage that will allow accurate characterization of the thermal voltage converters currently used for setting ac–dc transfer standards. We have designed, fabricated and successfully tested key subsystems of the RSFQ D/A converter including a 10-stage pulse-number multiplier, a 1-bit slice of a pulse distributor and a 9.5-bit voltage multiplier.

The voltage multiplier (VM) is a principal component in high-precision digital-to-analog (D/A) converters based on rapid single flux quantum (RSFQ) circuits. Experimental confirmation of correct operation of the VM is important to guarantee the accuracy of the D/A converter. In this study, a circuit for direct comparison of the output voltages of VMs was designed and demonstrated. An on-chip SQUID sensor was utilized to detect the VM errors. Bit error rates of a single-stage VM were successfully measured down to 10/sup -12/ in the constant-voltage regions. A method of detecting errors during transients was also proposed and carried out.

A pulse-number multiplier (PNM) is a principal subsystem of our rapid single flux quantum digital-to-analog converter for metrology applications. The PNM circuit generates a high-speed pulse train from an input of a precise low-frequency clock signal. We designed, fabricated, and successfully tested a PNM test circuit. The PNM test circuit was implemented by using our standard cell library on 1.6 kA/cm/sup 2/ Nb trilayer technology. Fully functional operation was confirmed by low-speed testing. Experimental bias margins were wider than /spl plusmn/20%, and these results agreed with a simulation. We also tested the circuit by average-voltage measurements for estimating the frequencies of output pulse trains. Results of average-voltage measurements implied that the output frequency of the PNM circuit was tunable from 5 GHz to 17 GHz. The bit error rate (BER) of the PNM circuit was measured, and extrapolation of the error-function fits suggested a sufficiently small BER at the optimum bias point.

Rapid single flux quantum (RSFQ) digital-to-analog (D/A) converters that synthesize arbitrary waveforms with metrological accuracy are under development. In the D/A converter, a magnetically coupled voltage multiplier (VM) is utilized as a precise frequency-to-voltage converter that generates a voltage proportional to the input frequency. To increase the output voltage of the VM, parameters that determine the cycle time of the VM cell were optimized. Experimental results obtained with 16-stage VMs showed that the maximum operating frequency of the new VM was 50 GHz, which is twice that of the previous type.

We present the development of a rapid single flux quantum (RSFQ) standard cell library for middle-scale applications. The library includes elementary cells frequently used to build large circuit blocks. The standard cells were designed for a simple process with 8 mask levels. The circuits were optimized and laid out to minimize effects of high parasitic inductance associated with a single ground plane layer. Circuit parameters on the wafer were determined by electrical measurements on parametric test circuits, which gave feedback on the design and process. All cells were individually fabricated and tested, and the design and layout were confirmed.

We investigated the effects of magnetic fields induced by bias currents on rapid single flux quantum (RSFQ) circuits. We measured the mutual inductance between a signal line and a bias line in the circuit. The mutual inductance values obtained were in the range of 5–50 fH. We simulated operation of a 2-stage shift register under the magnetic fields induced by an additional control current. The calculated operating regions were significantly reduced by the magnetic fields induced by the control currents on the order of 10 mA. The experimental results obtained for the shift register agreed with the simulation. These results suggest that it is important to take the effects of current-induced magnetic fields into account in the design of large-scale RSFQ circuits.

We present design and experimental results of a voltage multiplier (VM) that is a main component of a Josephson D/A converter based on single flux quantum pulses. We have analyzed the VM operation through detailed simulations and have re-designed the circuit. The amplitude of the constant-voltage step for a 64-stage VM with the new design is 80 /spl mu/A at 10 GHz of an input frequency, which is sufficiently large for D/A converter applications.

We are developing an X-ray microcalorimeter with high energy resolution and position sensitivity by using an Iridium transition edge sensor. The Ir-TES with a new pixel-type geometry has ten small pixel TES array. Each pixel size is 80 μm ×200 μm . This device has shown a good energy resolution of 26 eV (FWHM) at 5.9 keV . Furthermore, the Ir-TES film was scanned along each pixel by a collimated X-ray microbeam and signal pulses were measured at each incident position. The pulse height and rise time map has shown ten different regions where pulses are concentrated which correspond to each pixel. This might be useful for an imaging TES array.

A novel approach, frequency-domain cascading microwave multiplexers (MW-Mux), has been proposed and its basic operation has been demonstrated to increase the number of pixels multiplexed in a readout line U of MW-Mux for superconducting detector arrays. This method is an alternative to the challenging development of wideband, large power, and spurious-free room-temperature (300 K) electronics. The readout system for U pixels consists of four main parts: (1) multiplexer chips connected in series those contain U superconducting resonators in total. (2) A cryogenic high-electron-mobility transistor amplifier (HEMT). (3) A 300 K microwave frequency comb generator based on N(≡U/M) parallel units of digital-to-analog converters (DAC). (4) N parallel units of 300 K analog-to-digital converters (ADC). Here, M is the number of tones each DAC produces and each ADC handles. The output signal of U detectors multiplexed at the cryogenic stage is transmitted through a cable to the room temperature and divided into N processors where each handles M pixels. Due to the reduction factor of 1/N, U is not anymore dominated by the 300 K electronics but can be increased up to the potential value determined by either the bandwidth or the spurious-free power of the HEMT. Based on experimental results on the prototype system with N = 2 and M = 3, neither excess inter-pixel crosstalk nor excess noise has been observed in comparison with conventional MW-Mux. This indicates that the frequency-domain cascading MW-Mux provides the full (100%) usage of the HEMT band by assigning N 300 K bands on the frequency axis without inter-band gaps.

We are developing a digital-to-analog (D/A) converter based on rapid single flux quantum (RSFQ) logic circuits. Since the RSFQ D/A converter operates according to the Josephson frequency–voltage relationship, arbitrary waveforms can be synthesized with an accuracy equivalent to that of the Josephson dc voltage standard. We have examined the characteristics of magnetically coupled voltage multipliers (VMs). The VM is a key component in the RSFQ D/A converter for use in metrological applications.

To realize imaging X-ray spectroscopy with high-energy resolution, we proposed a new pixellated array of Ir-TES. In this type of device, the bias point of each pixel is slightly modified. Therefore the analysis of the pulse shape parameter, such as the pulse height, the rise time and the fall time of a signal pulse, can identify the incident pixel position. We have obtained 13 eV (FWHM) at 3 keV energy resolution and 80 μm position resolution for 10 pixel devices. At this moment we could successfully operate a 20 pixel device.

Digital-to-analog (D/A) converters based on the Josephson effect are promising for ac voltage standards because they produce voltage steps with ultimate precision and stability. We are currently developing a single-flux-quantum (SFQ)-based D/A converter, designed for synthesizing sinusoidal waveforms for metrological applications. In this paper, we examine feasibility of an ac-dc transfer difference measurement for a thermal converter using the SFQ-based D/A converter.

To develop high energy resolution x-ray microcalorimeters, we have fabricated single element superconductor TES detectors with W and Ir. In the W-TES, the detector is operated in the saturation mode because of high sensitivity of the W film. Energy resolution of 34.2eV (FWHM) at 5.9 keV is obtained by integrating the reduced current signals. In the Ir-TES detector, a position dependent response of pulse height distribution was observed, however, 15.4 eV (FWHM) energy resolution and 60μsec response time are obtained for collimated 3 keV x-rays.

White noise of dissipationless microwave radio frequency superconducting quantum interference device (RF-SQUID) multiplexers has been experimentally studied to evaluate their readout performance for transition edge sensor (TES) photon counters ranging from near infrared to gamma ray. The characterization has been carried out at 4 K, first to avoid the low-frequency fluctuations present at around 0.1 K, and second, for a feasibility study of readout operation at 4 K for extended applications. To increase the 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 ≤ Q ≤ 2 × 104 and the square root of spectral density of current noise referred to the SQUID input √SI = 31 pA/√Hz. The former and the latter are factor-of-five and seven improvements from our previous results on Nb-based resonators, respectively. Two-directional...

We are developing a tungsten transition edge sensor (W-TES) for x-ray imaging spectroscopy applications. The conventional readout scheme for TES array has a great difficulty in reading out multi-channel x-ray signals. Therefore we have considered a new readout scheme for a W-TES array. This scheme is based on a cooling time measurement in a saturation mode. In this scheme, we only need time information, therefore we can expect to simplify a readout system. Although the currently achieved energy resolution of our W-TES (0.5 mm×0.5 mm) using the cooling time measurement is 55 eV (FWHM) at 5.9 keV, much higher energy resolution is expected by making smaller devices.

We are developing an Ir-TES x-ray microcalorimeter with high energy-resolution. First we fabricated an Ir film of 0.5 mm×0.5 mm. The measured energy resolution was 194 eV (FWHM) for 5.9 keV x-rays and this value is extremely worse than the theoretical resolution. We considered the limitation in the energy resolution arises from non-uniformity in the heat diffusion process. To improve the heat diffusion property, we have developed a new small pixel geometry that has a shorter length between electrodes and some thin slits to separate the heat diffusion area of the Ir-TES. This device has shown an improved energy resolution of 47 eV (FWHM) at 5.9 keV. Furthermore the risetime and falltime map has shown the possibility of identifying the pixel position which would be useful for an imaging TES array.