The extremely high precision attainable today in Metrology laboratories wouldn't be achievable without devices based on quantum physics.
We describe here two such devices based on Superconductivity, aimed at the extension of the Josephson voltage standard to a standard for AC voltage measurements and at the development of sensors for Photon counting.

Nowadays arrays of superconducting Josephson junctions are routinely used in many National Metrology Laboratories to reproduce the unit of voltage. From a physical viewpoint, the array behaviour is described by the Josephson equation V = f n h/(2e), expressing the linear dependence of array DC voltage V on the frequency f of a microwave signal radiated on the array. What is most important for Metrology is the value of the proportionality factor, nh/(2e), which can be calculated in terms of an integer number n and two fundamental constants: the electron charge e and the Planck constant h. Fundamental constants can be determined with high accuracy and, being "fundamental", their value is independent of measurement conditions. This is essential for metrological applications: the fundamental constant proportionality between voltage and frequency is an "intrinsic guarantee" of the accuracy of the generated voltage, once the frequency is known. The attentive reader might now argue that, up to now, we are just shifting the problem from voltage to frequency measurements: luckily enough for metrologists, frequency measurements are, by far, the most accurate we can do today and frequency standards are so precise that up to 15 digits are needed to write down their value. With Josephson arrays then we can exploit the accuracy of frequency standards in voltage measurements, making it possible to measure DC voltages, with relative uncertainties as low as 10^-11.
Researchers are now putting much effort into trying to develop standards for AC (sinusoidal) electrical signals. For this purpose the Josephson arrays must be "programmable", i.e. capable of rapidly modulating their voltage by controlling another electrical parameter of the device, typically its current. This is not achievable with the "hysteretic" arrays of Superconductor-Insulator-Superconductor (SIS) junctions used in DC metrology, and non-hysteretic arrays with a single-valued current to voltage (I-V) characteristic have to be used (see fig. 1).
Relevant parameters are the values of critical current Ic and characteristic voltage, Vc, which define the noise immunity of the device, the operating frequency and the number of junctions needed to achieve a voltage suited to metrological applications.
Nb/Al-AlOx/Nb junctions (so called SNIS: Superconductor, Normal, Insulator, Superconductor), developed at INRIM in recent years [1], are based on the simple fabrication technology derived from the well known tri-layer process of SIS junctions, yet show very peculiar properties among which the possibility of maximising both Ic and Vc and operating above liquid helium temperature (4.2 K). Quantized voltage steps at temperatures near the niobium transition from the superconductive to the normal state (9 K) have been achieved with these junctions [2]. INRIM is now working to demonstrate their use in arrays like the one shown in fig. 1.

Fig. 1 Geometrical pattern of an array with 1600 Nb/Al-AlOx/Nb overdamped SNIS junctions. Right, the I-V characteristic of the array is shown.

The advancement of nanotechnology enables scientists to investigate the influence of reduced dimensionality on phase transition. Some examples are represented by the niobium nitride (NbN) superconducting nanowires that have acquired a lot of attention recently due to their potential application in telecommunications as fast superconducting single photon detectors. These detectors exploit the large resistance change at the transition from the normal to the superconducting state. The operation principle is based on the formation of a resistive region in place of photon absorption (so called "hot-spot") with the subsequent redistribution of the bias current around the hot-spot. When the current density in the hot-spot exceeds the critical current density the entire cross-section of the stripe becomes resistive which manifests itself as a voltage pulse on the terminals. According to theoretical predictions faster detectors working at a higher temperature could be realised by replacing NbN with MgB₂. As the absorption of a photon results in a hot-spot with ~ 20 nm diameter in a 10 nm thick stripe, the detector has to be made up of an ultra-thin strip with sub-micrometric width. MgB₂ films are fabricated by the so-called all-in situ method, consisting of the co-evaporation of Boron and Magnesium, followed by annealing in-situ [3].

Fig. 2 (a)Scanning Electron Microscope image of MgB₂ meander lines 270 nm wide. (b)Critical current density J_c vs temperature T of a meander line in (a).

Electron beam lithography was used to obtain the detector's geometry [4]. Typical nanomeander geometries are shown in figure 2(a). The high critical current density, 8·10⁶ A/cm² at 9.5 K in fig. 2 (b), measured in the best devices, shows that long nanostructures of MgB₂ can be realized maintaining good superconducting properties. First characterizations of MgB₂ meanders have shown that single photon detectors are feasible, starting from a homogeneous film thinner than 30 nm on sapphire substrate.

[1] V. Lacquaniti, C. Cagliero, S. Maggi, R. Steni: "Over damped Nb/Al-ALOx/Nb Josephson junctions", Appl. Phys. Lett., Vol. 86, pp. 042501-042503, 2005.
[2] V. Lacquaniti, N. De Leo, M. Fretto, S. Maggi, A. Sosso: "Nb/Al-AlOx/Nb overdamped Josephson junctions above 4.2 K for voltage metrology", Appl. Phys. Lett., Vol. 91, pp. 252505, 2007.
[3] E. Monticone, C. Gandini, C. Portesi, M. Rajteri, et al.: "MgB₂ thin films on silicon nitride substrates prepared by an in situ method", Supercond. Sci. Technol., Vol. 17, pp. 649-652, 2004.
[4] C. Portesi, S. Borini, E. Monticone, G. Amato: "Fabrication of superconducting MgB₂ nanostructures by an electron beam lithography-based technique", J. Appl. Phys., Vol. 99, pp. 066115-066117, 2006.