We report on the development of a new family of magnetic field sensors with exceptionally low magnetic field noise, as low as 0.3 fT Hz −1/2 . Beside this, they exhibit high usable voltage swings of more than 150 μV pp and tolerable background fields during cool-down of up to 6.5 mT. In operation mode they recover completely from magnetization pulses of up to 76 mT, which makes them well suited for applications such as low-field magnetic resonance imaging.With respect to their easy and reliable use as well as their field resolution in the sub-fT Hz −1/2 range, the presented SQUID sensors are adequate for many applications, such as in geophysics or in biomagnetism.

We report on a technology for the fabrication of sub-micrometer sized cross-type Josephson tunnel junctions in niobium technology. We present the fabrication scheme and properties of cross-type junctions with linear dimensions from 10 down to 0.6 µm. Sidewall passivation of the junctions is achieved by anodization as well as by planarizing the junctions with SiO in a self-aligned deposition step. The measured ratio of the sub-gap resistance to the normal resistance is about 35. Because of their low sub-gap current and low parasitic capacitance such junctions are well suited for applications like high resolution SQUIDs.

We have constructed a microwave detector based on the voltage switching of an underdamped Josephson junction, that is positioned at a current antinode of a λ/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 yielded a sensitivity parameter of 0.5 in qualitative agreement with theoretical calculations.PACS numbers: 07.57. Kp, 74.78.Na, 85.25.Cp The light emission by single, microscopic quantum systems displays a number of non-classical features which have been exploited in fundamental investigations in quantum physics and which may result in applications in metrology, quantum communication and computing. Potential applications, however, would suffer from the rather weak coupling between atoms and single optical photons. This has stimulated efforts to study the same features with macroscopic artificial atoms. A particularly successful system relies on solid state superconducting circuits. Due to the Josephson non-linearity such circuits have an anharmonic excitation spectrum and may be restricted to an effective two-level systems which can interact resonantly with microwave fields. Besides the stronger coupling of superconducting circuits, an additional advantage is that they can be designed and fabricated on chip-scale, thereby allowing the integration in and scaling to larger systems with multiple components.Essential quantum optical effects with superconducting qubits, such as vacuum Rabi splitting [1], resonance fluorescence of a single artificial atom [2], and single atom lasing [3] have already been observed. Microwave fields can be amplified, detected and fully characterized in homodyne set-ups [4]. The effective coupling to transmission wave guides has made it possible to efficiently monitor the emitted radiation and verify the validity of the quantum trajectories of qubits conditioned on the detection record [5,6], as well as to apply feedback and stabilize coherent superposition states of the qubit [7].Quantum optics benefits from high efficiency single photon detectors. It relies on the energy of the individual photons being sufficient to exploit the photoelectric effect and liberate an electron which can be amplified and detected [8]. Transition edge sensors [9] and superconducting nanowire single photon detectors [10,11] also require a sufficiently large energy of the incident photon to heat and thus modify the current through the detector. The energy of microwave photons is too low to allow detection by these methods, and for this energy range both controllable single photon sources and efficient single photon detectors are still under development.When working in the single photon regime, it is an obvious choice to use the resonant coupling to qubit systems. Indeed...

We studied and optimised the properties of ultrathin superconducting niobium nitride films fabricated with a plasma-enhanced atomic layer deposition (PEALD) process. By adjusting process parameters, the chemical embedding of undesired oxygen into the films was minimised and a film structure consisting of mainly polycrystalline niobium nitride with a small fraction of amorphous niobium oxide and niobium oxo-nitrides were formed. For this composition a critical temperature of 13.8 K and critical current densities of 7 × 106 A cm–2 at 4.2 K were measured on 40 nm thick films. A fundamental correlation between these superconducting properties and the crystal lattice size of the cubic δ-niobium-nitride grains were found. Moreover, the film thickness variation between 40 and 2 nm exhibits a pronounced change of the electrical conductivity at room temperature and reveals a superconductor–insulator-transition in the vicinity of 3 nm film thickness at low temperatures. The thicker films with resistances up to 5 kΩ per square in the normal state turn to the superconducting one at low temperatures. The perfect thickness control and film homogeneity of the PEALD growth make such films extremely promising candidates for developing novel devices on the coherent quantum phase slip effect.

More than 50 years ago superconducting quantum interference devices (SQUIDs) were invented. Since then many applications opened up. Already in a 1980 workshop (Weinstock and Overton 1981 SQUID Applications to Geophysics (Society of Exploration Geophysicists)) the application of SQUIDs in geosciences was reviewed. The fabrication and cooling technologies, electronics and other SQUID system components underwent significant improvement within the past years. Thus, SQUIDs are today better suited, more sensitive and effective as well as robust and reliable in operation for geophysical measurements. Many successful application examples, demonstrations and discoveries of mineral resources have been made using them in laboratory devices for investigation of magnetic properties, magnetic exploration, transient electromagnetics and for superconducting gravimeters as well as gravity gradiometers. Therefore, this article intends to review the past, present, and some future aspects of SQUIDs in geo-scientific applications such as e.g. mineral exploration. Since this field is still very active and quite a number of developments are ongoing, this review cannot be comprehensive.