In this article, we present advancements of our sub-micrometer cross-type Nb/AlOx/Nb Josephson junction technology by implementing an additional highly conductive resistivelayer, e.g., for thermalization structures. This allows for shiftingthe typically observed thermal decoupling of shunt resistors ofhighly sensitive dc superconducting quantum interference devices(SQUIDs) and SQUID arrays down to temperatures in the mil-likelvin range. We show results on the technological integration,device design of series SQUID arrays, and their characterizationat 4.2 K down to 65 mK. Measurements were performed in anadiabatic demagnetization refrigerator. As an application scenario,we have successfully implemented them as readout circuitry foroptical transition edge sensors operated at 150 mK requiring bothlow-noise and large bandwidth of the readout implemented in aflux-locked loop configuration

We present an approach allowing an optically pumped magnetometer (OPM) to be operated within the Earth’s magnetic field as a vector magnetometer, the sensitive axis of which can be freely defined. This approach enables the measurement of any vector component of the Earth’s magnetic field with the same sensitivity. The OPM is realized by a microfabricated cesium-vapor cell with nitrogen buffer gas, which is immersed in a constant homogeneous bias field of approximately 730μT. Since this bias field is about one order of magnitude stronger than the Earth’s magnetic field, it defines the sensitive axis of the OPM. The bias field is generated by solid-state magnets and has been designed to exhibit a very low relative inhomogeneity (<±10−4 in relative units) within the vapor-cell dimensions as well as a point of vanishing temperature dependence at around 40∘C. The OPM utilizes the light-narrowing effect, which enables effective suppression of spin-exchange relaxation even in such a large magnetic field amplitude. Based on this implementation, we demonstrate a white-noise floor of below 60fT/Hz in the frequency interval between 100Hz and 600 Hz and a 3-dB sensor bandwidth of >5kHz. Our approach enables unshielded ultrasensitive vectorial measurement capabilities that are relevant in many important applications. Published by the American Physical Society 2025

The thermal sensitivity of fibre Bragg gratings (FBGs) is extensively employed in diverse industrial and scientific applications. Notably, it lies at the core of flexible, low-cost and highly precise sensors, featuring stability in harsh environments and distributed sensing capability. This study assesses the thermal properties of FBGs in fluoride fibres within a temperature range of 4—373 K. Despite having higher thermal expansion coefficients, these fibres do not exhibit significantly higher sensitivity at room or higher temperatures. Still, the pronounced dependence of their thermal sensitivity on the Bragg wavelength in the light of the extended transmission range up to 4—5.5 µm shows the potential for sensing applications. Most importantly, employing FBGs inscribed in fluoride fibres enables the further expansion of fibre-based sensing systems to cryogenic environments, as they exhibit a sensitivity of 1—1.35 pm/K in a temperature range of 4—50 K. Moreover, exposure to low temperatures provides valuable information on glass stability and physical parameters, which is beneficial for further development of photonic systems based on fluoride fibres.

Axions detection requires the ultimate sensitivity down to the single-photon limit. In the microwave region, this corresponds to energies in the yJ range. This extreme sensitivity has to be combined with an extremely low dark-count rate since the probability of axions conversion into microwave photons is supposed to be very low. To face this complicated task, we followed two promising approaches that both rely on the use of superconducting devices based on the Josephson effect. The first one is to use a single Josephson junction (JJ) as a switching detector (i.e., exploiting the superconducting to normal state transition in the presence of microwave photons). We designed a device composed of a coplanar waveguide terminated on a current-biased JJ. We tested its efficiency to pulsed (pulse duration 10 ns) microwave signals since this configuration is closer to an actual axions search experiment. We show how our device is able to reach detection capability of the order of ten photons with the frequency of 8 GHz. The second approach is based on an intrinsically quantum device formed by two resonators coupled only via a superconducting qubit network. This approach relies on quantum nondemolition measurements of the resonator photons. We show that by injecting radiofrequency power into the resonator, the frequency position of the resonant drop in the transmission coefficient (S21) can be modulated up to 4 MHz. We anticipate that, once optimized, both the devices have the potential to reach single-photon sensitivity.

In mobile applications such as geomagnetic surveying, two major effects hamper the use of optically pumped magnetometers: dead zones, sensor orientations where the sensors signal amplitude drops; and heading errors, a dependence of the measured magnetic field value on the sensor orientation. We present a concept for an omnidirectional magnetometer to overcome both of these effects. The sensor uses two cesium vapor cells, interrogated by circularly-polarized amplitude-modulated laser light split into two beams propagating perpendicular to each other. This configuration is experimentally investigated using a setup wherein the laser beam and magnetic field direction can be freely adjusted relative to each other within a magnetically shielded environment. We demonstrate that a dead-zone-free magnetometer can be realized with nearly isotropic magnetic-field sensitivity. While in the current configuration we observe heading errors emerging from light shifts and shifts due to the nonlinear Zeeman effect, we introduce a straightforward approach to suppress these systematic effects in an advanced sensor realization.

Waveguides with superconducting Josephson junction-based metamaterial are widely used as parametric amplifiers. However, the precise estimation of power entering the device is crucial for the estimation of gain and noise temperature. This is nontrivial when the measurement tract is not symmetrical. We present a basic framework for the analysis of properties of such nonlinear systems and calibration of the input power. Utilizing measurements with varied temperature and power of the input signal, we estimate additional attenuation of the input line. We demonstrate a precise calibration procedure of a Josephson junction metamaterial.

We experimentally investigate the influence of the orientation of optically pumped magnetometers in Earth’s magnetic field. We focus our analysis to an operational mode that promises femtotesla field resolutions at such field strengths. For this so-called light-shift dispersed $M_{z}$ (LSD-Mz) regime, we focus on the key parameters defining its performance. That are the reconstructed Larmor frequency, the transfer function between output signal and magnetic field amplitude as well as the shot noise limited field resolution. We demonstrate that due to the use of two well balanced laser beams for optical pumping with different helicities the heading error as well as the field sensitivity of a detector both are only weakly influenced by the heading in a large orientation angle range.

Purpose The purpose of this paper is to present a simulation study using a model of a new optically pumped magnetometer sensor for application in the field of magnetoencephalography. The effects of sensor distance and orientation on the measurement information and the sensitivity to neuronal sources are investigated. Further, this paper uses a combinatorial optimization approach for sensor placement to measure spontaneous activity in the region of the occipital cortex. Design/methodology/approach This paper studies the effects of sensor distance and orientation on sensitivity to cortical sources and measurement information. A three-compartment model of the head, using the boundary element method, is applied. For sensor setup optimization, a combinatorial optimization scheme is developed. Findings The sensor distance to sources considerably affects the sensitivity and the retrieved information. A specific arrangement of four sensors for measuring spontaneous activity over the occipital part of the head is optimized by effectively avoiding position conflicts. Research limitations/implications Individual head models, as well as more detailed noise and signal models, will increase the significance for specific-use cases in future studies. Originality/value Effects of sensor distance and orientation are specifically evaluated for a new optically pumped magnetometer. A discrete optimization scheme for sensor optimization is introduced. The presented methodology is applicable for other sensor characterization and optimization problems. The findings contribute significantly to the development of new sensors.

Superconducting qubits were initially developed with the goal of realizing a superposition of macroscopically distinct quantum states by exploiting superconducting circuits. This basic idea resulted from the quantum mechanical description of the Josephson junction, the key element for producing superconducting qubits. Because the phase across a Josephson junction and its charge are canonical conjugates, there are two alternative realizations of superconducting qubits. The first one is based on the charge degree of freedom, termed charge qubit. The second utilizes the phase (or flux) degree of freedom and correspondingly are called phase (flux) qubits. Nowadays, the most robust superconducting qubit is the transmon. In practical applications, quantum state initialization and manipulations are heavily restricted by the quantum coherence of the qubit itself and of the qubit‐based systems. The main source of decoherence is interactions with the environment. Their relatively large values result from the macroscopic size of the quantum bits. Still, their circuit architecture enables the implementation of different types of coupling schemes between superconducting qubits and qubit‐resonator systems. The handling of superconducting quantum structures requires special experimental methods, including qubit fabrication, cooling to milliKelvin temperatures, experimental characterization, and readout. Concerning applications, superconducting qubits are promising candidates for both quantum simulators and universal quantum computing. This article covers a description of basic types of superconducting qubits and gives a general description of their use that includes dissipation and decoherence, coupling schemes, experimental realization, and basic measurement techniques. Finally, their use as building blocks for the realization of quantum computation is discussed.

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

The key issue for the implementation of a metamaterial is to demonstrate the existence of collective modes corresponding to coherent oscillations of the meta-atoms. Atoms of natural materials interact with electromagnetic fields as quantum two-level systems. Artificial quantum two-level systems can be made, for example, using superconducting nonlinear resonators cooled down to their ground state. Here we perform an experiment in which 20 of these quantum meta-atoms, so-called flux qubits, are embedded into a microwave resonator. We observe the dispersive shift of the resonator frequency imposed by the qubit metamaterial and the collective resonant coupling of eight qubits. The realized prototype represents a mesoscopic limit of naturally occurring spin ensembles and as such we demonstrate the AC-Zeeman shift of a resonant qubit ensemble. The studied system constitutes the implementation of a basic quantum metamaterial in the sense that many artificial atoms are coupled collectively to the quantized mode of a photon field.

A Lagrangian formalism is used to derive the Hamiltonian for a $\lambda$/4 resonator shunted by a current-biased Josephson junction. The eigenstates and the quantum dynamics of the system are analyzed numerically, and we show that the system can function as an efficient detector of weak incident microwave fields.