Detection of single infrared photons in superconducting microstrips of 4 nm thick disordered Nb_0.15Re_0.85 has been investigated. Microstrips with a critical temperature of 5.15 K and widths from 1.0 to 2.5 μm have been fabricated by optical lithography. We demonstrate single photon detection sensitivity at 1.5 μm wavelength at a temperature of 1.79 K. By investigating the detection process at this temperature, we find that the current bias threshold is at 21% of the depairing current. This threshold is similar to what should be observed in typical amorphous superconductors, which confirms that ultrathin disordered Nb_0.15Re_0.85 is an interesting material for superconducting microstrip single photon detectors that operate above 1 K.

Josephson junctions (JJs) containing ferromagnetic (F) materials are being considered for applications as cryogenic random access memories (RAM). In this work, we report on the fabrication and characterization of tunnel JJs, based on Nb technology with a strong ferromagnetic interlayer Ni₈₀Fe₂₀ alloy (Permalloy), which is suitable for the realization of devices with reduced area and guarantees relative low saturation and coercive fields in the use of JJs as RAM elements. We have successfully realized Josephson memory elements that work well down to 7 μm² preserving high values of the characteristic voltage. We have also investigated the role of the F layer thickness, and by measuring the critical current dependence on the external applied magnetic field, we have optimized our devices as memory elements using thin ferromagnetic layers with thickness down to 3 nm. We have experimentally proved their functioning as memory elements by applying magnetic field pulses in opposite directions that can change the F layer magnetization.

Thermal fluctuations in hybrid superconductor/ferromagnetic $\mathrm{NbN}/\mathrm{NiCu}$ bilayers, as well as in pure superconducting NbN, two-dimensional (2D), nanostripes, have been investigated in order to understand the origin of dark counts in superconducting nanostripes when operated as single-photon detectors in the temperature range from $4.2\mathrm{to}8\mathrm{K}$. In 2D superconductors, the dynamics of vortex motion play a significant role in the formation of a transient normal state, leading to dark-count events in current-biased nanostripes. By introducing a weak ferromagnetic overlayer on top of pure NbN, we managed to control the vortex dynamics, which subsequently enabled us to differentiate between several proposed theoretical models. In particular, a $6\text{−}\text{nm}\text{−}\text{thick}$ NiCu film grown on top of $8\text{−}\text{nm}\text{−}\text{thick}$ NbN nanostripes led to an enhanced critical current density in the resulting nanostructure, as well as significantly lowered fluctuation rates, as compared to pure NbN structures, leading to reduced dark counts. The enhancement of pinning in $\mathrm{NbN}/\mathrm{NiCu}$ bilayers provided evidence that thermal excitations of single vortices (vortex hopping) near the edge of a 2D nanostripe were the dominant mechanism of the observed dark-count transients. On the other hand, in pure NbN the leading source of thermal fluctuations was the current-assisted thermal unbinding of vortex-antivortex pairs.

Time-resolved optical pump-probe and photoimpedance studies of proximized ferromagnet/superconductor nanobilayers are presented. The weak ferromagnetic nature of an ultrathin ${\mathrm{Ni}}_{0.48}{\mathrm{Cu}}_{0.52}$ film makes it possible to observe the dynamics of the nonequilibrium superconductivity in $\mathrm{Ni}\mathrm{Cu}∕\mathrm{Nb}$ hybrids through time-resolved measurements of a near-surface optical reflectivity change, which is generated by femtosecond optical pump pulses and discussed within a nonequilibrium two-temperature electron-heating model. We observed that the NiCu overlay significantly reduced the slow bolometric contribution present in the photoresponse of a pure Nb film, resulting in a strong enhancement of the nonequilibrium kinetic-inductive component of the transient photoimpedance, measured as an $\sim 700\text{−}\mathrm{ps}$-wide voltage waveform generated across an optically excited current-biased $\mathrm{Ni}\mathrm{Cu}∕\mathrm{Nb}$ bilayer microbridge. The sensitive picosecond photoresponse makes our $\mathrm{Ni}\mathrm{Cu}∕\mathrm{Nb}$ heterostructures suitable for “engineered” ultrafast superconducting photodetectors since the photoimpedance signals observed in plain Nb bridges were at least $10\mathrm{ns}$ long and were due to the light-induced simple-heating effect.

Aluminum Josephson junctions are the building blocks for the realization of superconducting quantum bits. Attention has been also paid to hybrid ferromagnetic Josephson junctions, which allow switching between different magnetic states, making them interesting for applications such as cryogenic memories, single-photon detectors, and spintronics. In this paper, we report on the fabrication and characterization of high-quality ferromagnetic Josephson junctions based on aluminum technology. We employed an innovative fabrication process inspired by niobium-based technology, allowing us to obtain very high-quality hybrid aluminum Josephson junctions; thus, supporting the use of ferromagnetic Josephson junctions in advanced quantum circuits. The fabrication process is described in detail and the main DC transport properties at low temperatures (current–voltage characteristic, critical current as a function of the temperature, and the external magnetic field) are reported. Here, we illustrate in detail the fabrication process, as well as the main DC transport properties at low temperatures (current–voltage characteristic, critical current as a function of the temperature, and the external magnetic field).

The competition between superconducting and ferromagnetic orderings in Josephson devices has promoted fundamental and applicative studies of high impact for superconducting digital technology, cryogenic memories, and spintronics, where the possibility of switching between different magnetic states is a crucial advantage. Here, we report on fabrication and characterization of very high quality tunnel ferromagnetic Josephson junctions (JJs) with aluminum electrodes, demonstrating hysteretic behavior of the magnetic field pattern preserving typical transport properties of Al junctions, underdamped behavior, and very low quasiparticle dissipation. The proposed technology paves the way for the possible implementation of Al tunnel-ferromagnetic JJs in superconducting quantum circuits, toward alternative approaches based on digital control of the Josephson device.