We report on the development of magnetic background field-tolerant superconducting quantum interference filter (SQIF) based on low-capacitance sub-micrometer sized cross-type Josephson junctions either as current sensing amplifiers—even on chip—for advanced superconducting quantum interference device (SQUID) readout circuits or as magnetic field sensor in flux transformer configuration especially for geophysical measurement systems. Their very small parasitic magnetic sensitive areas enable them to operate in the Earth magnetic field and allow for magnetically unshielded cool-down. The careful consideration of magnetic sensitive areas inside each SQUID in the SQIF in all three dimensions result in developed SQIFs with 28 SQUIDs in series exhibiting large voltage swing and transfer function of more than 2 mV and 500VA−1 , respectively. We report on the electrical parameters and field stability as well as on the noise performance of the devices under investigation. SQIFs and devices with additional on-chip flux-transformers show input referred current noise levels of 5.9 and 1.1pAHz−1/2 , respectively. We furthermore demonstrate their potential as SQIF-based magnetometers using them in addition with thin-film pickup loops resulting in a white magnetic field noise of 1.6fTHz−1/2 . The implemented SQIFs are thus highly compatible with state-of-the-art single SQUID-devices offering beneficial features such as a unique working point, making them excellent suited for implementation of e.g. geophysical instruments.

In this paper, we present new integrated on-chip planar-type superconducting quantum interference device (SQUID) first order gradiometers, which are manufactured by a new mix and match fabrication technology combining sub-μm sized Josephson junctions (JJs) with cm-scale pickup loops. The fabrication technology is described and the design is schematically shown. These sensors have high voltage swing and low flux noise and provide thus the best so far reported gradient resolution of about 13 fT / m · Hz . These gradiometers were developed and implemented in instruments for applications in mineral exploration. They allow a better system dynamic range (ratio of maximal to minimal detectable signals) and a more effective use of the chip area. The gradiometer performance in terms of gradient resolution is compared against state of the art planar-type SQUID based gradiometers. Two models predicting a gradient noise limit as a function of the chip area and baseline of the gradiometer are discussed. The small geometric dimensions of the gradiometers are very important for future miniaturized instruments in mineral exploration. The comparison of our results with those models highlights the importance of a smaller JJs size for the reduction of the intrinsic gradiometer noise and geometrical dimensions.

We report on a novel concept and prototype development of a coreless SQUID-based charged-particle beam monitor as a non-destructive diagnostic tool for accelerator facilities. Omitting the typically used pickup coil with a high magnetic permeability core leads to a significant improvement in low-frequency noise performance. Moreover, a revised shielding geometry allows for very compact and rather lightweight device designs. Based on highly sensitive SQUIDs featuring sub-micron cross-type Josephson tunnel junctions, our prototype device exhibits a current sensitivity of about 6 pA Hz−1/2 in the white noise region. Together with a measured shielding factor of about 135 dB this opens up the way for its widespread use in modern accelerator facilities.

The effect of flux trapping on the flux-voltage characteristics of multi-loop SQUID magnetometers was investigated by means of repeated cool-down cycles in a stepwise increased magnetic background field. For a SQUID with N parallel loops, N different flux offsets, each separated by Φ 0 / N , were observed even in zero magnetic field. These flux offsets further split into a so called fine structure, which can be explained by minor asymmetries in the SQUID design. The observed results are discussed with particular regard to their impact on the previously presented absolute SQUID cascade vector magnetometer.

We construct a microwave detector based on the voltage switching of an underdamped Josephson junction that is positioned at a current antinode of lambda/4 coplanar waveguide resonator. By measuring the switching current and the transmission through a waveguide capacitively coupled to the resonator at different drive frequencies and temperatures, we are able to fully characterize the system and assess its detection efficiency and sensitivity. Testing the detector by applying a classical microwave field with the strength of a single photon yields a sensitivity parameter of 0.5, in qualitative agreement with theoretical calculations.

Technological processes for the fabrication of low- and high-Tc Josephson junctions, aimed for certain applications, are described. On the one hand, the integration of low-Tc superconductor digital electronics with superconducting sensor arrays enables input signal processing with quantum limited resolution at millikelvin temperatures. We describe this mixed signal superconductor technology for analogue sensor readout and signal multiplexing for operating temperatures down to 300 mK. On the other hand, by making use of modern high-Tc Josephson junction technology, sensitive magnetometers, which require a modest cooling power, can be developed. Examples of the application of the mentioned processes are shown.

We report on the development of a new family of superconducting quantum interference device (SQUID) current sensors based on sub-micron cross-type Josephson tunnel junctions. Their low total junction capacitance permits high usable voltage swings of more than 100 μV and exceptional low noise of the SQUIDs at 4.2 K. Integrated rf-filters as well as high tolerable background fields during cool-down of up to 9.6 mT enable their highly reliable and easy use. With input coil inductances ranging from 10 nH to 2.8 μH and current sensitivities and coupled energy resolution down to 65 fA Hz–1/2 and below 10 h, respectively, they are a versatile tool for numerous applications.

Silicon nitride membrane based cryogenic bolometers exhibit high sensitivity and enable ultra-sensitive detector applications. Multi-pixel instruments were already introduced as devices for submillimeter-wave imaging. Nevertheless, the numbers of pixels are limited by the readout process which is typically a time-division multiplexing or code-division multiplexing technique. To overcome this challenge, a replacement of the transition-edge sensor as thermometer by a lumped-element resonance circuit seems to be a promising solution. Therefore, one can benefit from the intrinsic capability of frequency-division multiplexing that allows the readout of large detector arrays simultaneously and in real-time. The number of pixels is then limited by the available readout bandwidth and the quality factors of each individual resonance circuit. We successfully demonstrated, based on our feasibility study, the principal operation of such a device, what we call kinetic-inductance bolometer (KIBO). But the overall performance of the achievable noise-equivalent power (NEP) was limited by implementation and operation temperature of KIBO. Therefore, improved KIBO designs were developed and fabricated with niobium thin-film technology. In this paper, we describe the improvement procedure and estimate the expected NEP value.

We present two possible methods for the fabrication of sub-micron sized Josephson junctions, namely the shadow-evaporation technique and the cross-type technology. Their importance for the field of modern super-conducting technology is discussed. As examples we present measurement results of a two-qubit sample and a prototype of a microwave detector fabricated each by one of the described methods. We review potential applications of superconducting quantum circuits based on the developed methods.

Fabrication of devices composed of Josephson junctions in trilayer technology is usually compromised by the topography of the subjacent layers. If the structures of subsequent layers traverse steps, several adverse effects arise. Lithography may be impaired because of issues with resist thickness uniformity and focus depth, and the superconducting ampacity of the Nb wiring is reduced. Various methods have been tried to planarize perturbing structures. In this contribution, we address a few of the challenges of chemical mechanical polishing. The proposed improvements are based on the well-established IPHT cross-type junction technology.

We report on the development of nearly quantum limited SQUIDs with miniature pickup loop dimensions. The implemented high quality and low capacitance cross-type Nb/AlOx/Nb Josephson junctions offer large ICRN-products and therefore enable an exceptional low noise level of the SQUIDs. Devices with loop dimensions of 1 μm exhibit white flux noise levels as low as 45 nΦ0 Hz−1/2 corresponding to an energy resolution ε of about 1 h at 4.2 K, with h being Planck's constant. Moreover, the large usable voltage swings of the devices of about 300 μV allow highly sensitive and easy single-stage operation while exploring nearly the intrinsic noise of the SQUIDs, beneficial e.g. for sensor arrays in SQUID microscopy.

Josephson junctions generate, when subjected to microwave irradiation, voltages with a very high precision and are used in metrology applications. So-called PJVS (programmable Josephson voltage-standards) are capable of generating both AC and DC voltages of up to 10 V. Our work addresses a full fabrication scenario for 10 V PJVS arrays driven at 70 GHz to be used in low microwave-power conditions as in, but not limited to GUNN diodes or cryocooler applications. Nb x Si1−x in its function as a barrier material was characterised with AFM, RBS and reflectometry in order to establish a reliable technological foundation. A 10 V PJVS array driven with microwave power below 50 mW is further presented, which was achieved by optimising the fabrication technology regarding the degree of homogeneity of the Josephson junctions composition and thickness. Control over these parameters is crucial in choosing a stable and well-suited characteristic voltage (I c R n product) and critical current density j c. With this, a low-power operation of a PJVS array is possible without the need for liquid helium cooling, which is currently limiting the availability of PJVS based metrology.

We report on the development of an ultralow-noise thin-film-based superconducting quantum interference device (SQUID) magnetometer. A niobium thin-film pickup coil is connected to the input coil of a SQUID current sensor. The low capacitance of the used submicrometer cross-type Josephson junctions enables superior noise performance of the device. Application scenarios, e. g., in geophysics and ultralow-field magnetic resonance imaging, are discussed.

We report on the development of a three-axis absolute vector magnetometer suited for mobile operation in the Earth's magnetic field. It is based on low critical temperature dc superconducting quantum interference devices (LTS dc SQUIDs) with sub-micrometer sized cross-type Josephson junctions and exhibits a white noise level of about 10 fT/Hz(1/2). The width of superconducting strip lines is restricted to less than 6 μm in order to avoid flux trapping during cool-down in magnetically unshielded environment. The long-term stability of the flux-to-voltage transfer coefficients of the SQUID electronics is investigated in detail and a method is presented to significantly increase their reproducibility. We further demonstrate the long-term operation of the setup in a magnetic field varying by about 200 μT amplitude without the need for recalibration.

The paper describes a dry-cooled AC quantum voltmeter system operated up to kilohertz frequencies and 7 V rms. A 10 V programmable Josephson voltage standard (PJVS) array was installed on a pulse tube cooler (PTC) driven with a 4 kW air-cooled compressor. The operating margins at 70 GHz frequencies were investigated in detail and found to exceed 1 mA Shapiro step width. A key factor for the successful chip operation was the low on-chip power consumption of 65mW in total. A thermal interface between PJVS chip and PTC cold stage was used to avoid a significant chip overheating. By installing the cryocooled PJVS array into an AC quantum voltmeter setup, several calibration measurements of dc standards and calibrator ac voltages up to 2 kHz frequencies were carried out to demonstrate the full functionality. The results are discussed and compared to systems with standard liquid helium cooling. For dc voltages, a direct comparison measurement between the dry-cooled AC quantum voltmeter and a liquid-helium based 10 V PJVS shows an agreement better than 1 part in 10(10).

The article contains sections titled:IntroductionJunction CharacterizationNb–Al/AlOx–NbJunction TechnologyCircuits, Applications, and Resulting Requirements forJosephson Junctions

Cryogenic bolometers based on thin silicon nitride membranes show a very high sensitivity, which makes them ideal for ultrasensitive detector applications, particularly in the field of submillimeter-wave imaging. For that, transition-edge sensors (TESs) have been established as a viable approach toward developing multipixel sensor arrays. However, current multiplexer techniques as time- or code-division multiplexing do not scale well to multiplexing levels of several hundred detectors per channel. To achieve such a scalable readout solution, frequency-division multiplexing (FDM) would be an applicable way. For that, the temperature-sensing element of a bolometer has to be resonant or coupled to a resonance circuit, which changes its microwave behavior with the temperature of the absorber.