Shigeyuki Miyajima

Single-Flux-Quantum Circuit Based Readout System for Detector Arrays by Using Time to Digital Conversion

We propose a single-flux-quantum (SFQ) based readout circuit for a transition edge sensor (TES) array for X-ray radiation detection. Utilization of SFQ circuits for this purpose enables large-scale integration of TESs due to very high speed processing ability of the received signals and using an already established integrated circuit design environment. We utilize the cooling time dependence of the TES on the incident X-ray energy. Time to digital conversion is made by using an SFQ based quasi-one-junction SQUID (QOS), which works as a 1-bit comparator with an adjustable current threshold level, and an SFQ based high speed counter. The readout system is composed of two separate chips that are connected to each other with flexible superconducting wiring. The QOS-multiplexer chip is directly connected to the TES array and it performs the digital conversion of the TES output. The demultiplexer-counter chip, which will be placed at elevated temperatures, receives the multiplexed SFQ pulses and determines the duration of the TES output which is above the predetermined threshold level. Final goal of this work is to readout more than 10 k TES pixels and in this paper, feasibility of the system and current status of the development process is demonstrated.

Current-Biased Kinetic Inductance Detector Using

We propose a current-biased kinetic inductance detector (CB-KID). This detector senses a change in kinetic inductance under a dc bias current, in contrast to a current-biased transition edge detector, reported previously, which probes the transient change in the bias current at the transition edge. Our CB-KID consists of a 200-nm-thick thin-film meander line with 3- wire. It is operated at 4 K. A scanning laser spot can be achieved by an XYZ piezo-driven stage and an optical fiber with an aspheric focused lens. We succeeded in observing a signal image in the contour at 4 K. The magnitude of the signal should be proportional to the number of quasi-particles excited by the laser. Because our CB-KID has a typical signal of 3-ns pulse width and can operate at 4 K, it will be possible to assemble a large-scale array of neutron detectors with single flux quantum readout circuits.

\hbox{MgB}_{2}

The decision of a Josephson comparator is influenced by thermal noise which is limiting the practical sensitivity in many applications of superconductor electronics. We analyzed the Gaussian state distribution theoretically and identified the most effective way to improve the practical sensitivity of a Josephson comparator. We suggest a modified damping concept and demonstrate a gray zone width as low as 840 nA experimentally. This value is about ten times smaller than for conventional Josephson comparators and very close to the theoretical limit at liquid helium temperature.

Nanowires for Detecting Neutrons

A current-biased kinetic inductance detector (CB-KID) is a new type of superconducting detector and senses a change in kinetic inductance in the superconducting nanowire biased by weak dc current Ib. Kinetic inductance depends on the density of Cooper pairs. Therefore, when Cooper pairs are broken by local energy dissipation, a change in kinetic inductance ΔLk can be obtained by monitoring a voltage V across the CB-KID sensor. The CB-KID has a wide operating temperature regime in the superconducting state whereas a current-biased transition edge detector senses a change in resistance at the superconductive transition edge and operates only at near vicinity of Tc. The confirmation for the validity of the CB-KID idea was preceded by using an MgB2 CB-KID meanderline detector. We extended this CB-KID method to a conventional Nb nanowire. We consider that our Nb-based CB-KID has versatile potentials in the various future applications. It is for the first time that the Nb-based CBKID operates at 4 K and has a capability of submicrometer position resolution.

Josephson comparator with modified dynamic behavior for improved sensitivity

A superconducting nanowire detector with a 10B conversion layer is useful for sensing a single neutron at a high operation rate. We use 7Li ion and 4He ion emitted as two independent heat sources in opposite direction, which are associated with nuclear reaction 10B(n,4He)7Li to identify the XV position of neutron arrival. We probe a change rate in the kinetic inductance Lk originating from kinetic inertia of the Cooper pairs. Our detector is different from a well-known microwave kinetic inductance detector and, hence, is named as a current-biased kinetic inductance detector. We use two sets of meanderlines made of Nb nanowires with superconducting readout taps to monitor local signals. In between the X meander and the V meander, we sandwiched a 10B layer acting as a neutron conversion layer to two charged particles. We succeeded in observing signal induced by nuclear reaction. We plan to build a megapixel neutron imager by coupling 10-bit linear position-sensitive arrays with single-flux quantum circuits to read (X,V) position of neutron arrival in the future.

20 ps Pulsed Laser Irradiation to Current-Biased Kinetic Inductance Detector Made of Nb Nanowires

We demonstrate neutron detection using a solid state superconducting current biased kinetic inductance detector (CB-KID), which consists of a superconducting Nb meander line of 1 μm width and 40 nm thickness. 10B-enriched neutron absorber layer of 150 nm thickness is placed on top of the CB-KID. Our neutron detectors are able to operate in a wide superconducting region in the bias current–temperature diagram. This is in sharp contrast with our preceding current-biased transition edge detector, which can operate only in a narrow range just below the superconducting critical temperature. The full width at half maximum of the signals remains of the order of a few tens of ns, which confirms the high speed operation of our detectors.

Toward Neutron Radiography Using Two Arrays of Nb-Based Current-Biased Kinetic Inductance Detectors With 10 B Converter Sandwiched In-Between

We evaluated the relationship between the gray zone width and the operating margin for comparators composed of quasi-one-junction superconducting quantum interference devices (QOSs) with shunt resistors, which are often used as high-speed readout circuits in multiple superconductor detector systems. The gray zone width is a good measure of current sensitivity of a single-bit comparator. We numerically analyzed the gray zone width of a QOS comparator and determined the circuit parameters. The gray zone width obtained from the experiments concurred with the results of the numerical analysis and was 2–3 µA at 4.2 K in a QOS comparator composed of three Nb/AlOx/Nb junctions with critical currents of less than 90 µA. The experimentally obtained operating margin for the bias current provided to the comparator was ±15% at the bias current of around 140 µA. These results show that QOS comparators are promising for readout circuits operating up to tens of GHz and imply that gray zone width is the thermal noise in the resistors at 4.2 K.

Neutron detection using a current biased kinetic inductance detector

In this paper, we present a novel design for a signal processor based on rapid-single-flux-quantum (RSFQ) circuits for a compact neutron diffraction system with an array of 10B-enriched MgB2 nanowire neutron detectors. By introducing an RSFQ signal processor, the system is able to accommodate a large number of detectors and determine the kinetic energy of each incident neutron. The processor consists of quasi-one-junction SQUIDs (QOSs), time-to-digital converters, and a multiplexer. A QOS is connected to each detector that operates at 27 K. The QOS picks up a voltage impulse with a width smaller than 2 ns generated across a detector and creates an SFQ pulse for RSFQ signal processing. The diffraction patterns are determined by the position and the kinetic energy of the incident neutrons. The time-to-digital converter is used to measure these energies by the time-of-flight method. Time resolution down to tens of picoseconds is possible, even with an array of detectors, and it is easy to multiplex signals in the time domain. We successfully demonstrated the RSFQ signal processor in liquid helium, and also tested a prototype, consisting of the processor connected with four MgB2 nanowire detectors in a Gifford-McMahon cryocooler, by irradiating a focused pulsed laser instead of neutrons, and obtained preliminary experimental results.

Experimental Demonstration and Numerical Analysis of Microampere Gray Zone Width with Enhanced Operating Margin in Shunted Quasi-One Junction Superconducting Quantum Interference Device Comparators

We propose a novel system of a 3-D scanning superconducting quantum interference device (SQUID) microscope with high sensitivity for magnetic fields and high spatial resolution. Our 3-D scanning SQUID microscope consists of an XYZ piezodriven scanner for controlling the position of the pickup coils, three SQUID sensors, and the three-channel SQUID electronics at room temperature. Three SQUID sensors are fabricated in the same chip, and each SQUID has a pickup coil for XYZ directions, respectively. The SQUIDs used in our system are reproducibly fabricated using a Nb multilayer process of a fabrication center named CRAVITY. Pickup coils were built either in multiply winding numbers for enhancing sensitivity or in smaller inner diameters for improving spatial resolution. Three pickup coils with readout circuits are configured in orthogonal with each other and are acting as a vector sensor of the local magnetic field. At this moment, we succeeded to design some SQUIDs with 1-D and 3-D pickup coils. The critical current density Jc of a Josephson junction (JJ) and the minimum critical current I√ of JJ are 320 A/cm2 and 12.5 μA, respectively. The size of a chip is 2.9 mm × 2.9 mm, and the inner diameter of the pickup coil can be reduced down to 4 μm. The experimental I-V characteristic of fabricated SQUIDs was suitable for use in a scanning SQUID system. We have also fabricated a SQUID stage for installation in a cryostat for the experiment of 1-D SQUID sensors.

High-Throughput RSFQ Signal Processor for a Neutron Diffraction System With Multiple

We propose and demonstrate a low-power and low-current cryogenic readout interface for a superconducting nanowire single-photon detector (SSPD) using adiabatic quantum-flux-parametron (AQFP) logic. The AQFP readout interface samples and digitizes the current signal from an SSPD, generating binary output data in accordance with the detection behavior of the SSPD. We demonstrate the correct operation of an SSPD with the interface, where the AQFP readout interface and the SSPD are placed in separate 0.1-W Gifford–McMahon (GM) cryocoolers and are interconnected via coaxial cables. It was found that the temperature of the sample stage did not change even after the AQFP readout interface was turned on, which revealed that the AQFP readout interface uses sufficiently low power and current for a compact cryocooler.