R. Leoni

Waveguide superconducting single-photon detectors for integrated quantum photonic circuits

The monolithic integration of single-photon sources, passive optical circuits, and single-photon detectors enables complex and scalable quantum photonic integrated circuits, for application in linear-optics quantum computing and quantum communications. Here, we demonstrate a key component of such a circuit, a waveguide single-photon detector. Our detectors, based on superconducting nanowires on GaAs ridge waveguides, provide high efficiency (∼20%) at telecom wavelengths, high timing accuracy (∼60 ps), and response time in the ns range and are fully compatible with the integration of single-photon sources, passive networks, and modulators.

Feasibility of the high Tc superconducting bolometer

A design analysis is given for a bolometric infrared detector that uses the resistive transition of a high‐temperature superconductor as the temperature sensing element, and liquid nitrogen (LN) as the coolant. It is shown that for highly oriented c‐axis films, the measured low‐frequency noise causes little or no degradation of the performance. With the incoming radiation chopped at 10 Hz, noise equivalent powers (NEP) in the range (1–20)×10−12 W Hz−1/2 should be achievable. These values compare favorably with the NEP of other detectors operating at or above LN temperatures for wavelengths greater than 20 μm.

Trapping of micrometre and sub-micrometre particles by high-frequency electric fields and hydrodynamic forces

We demonstrate that micrometre and sub-micrometre particles can be trapped, aggregated and concentrated in planar quadrupole electrode configurations by positive and negative dielectrophoresis. For particles less than in diameter, concentration is driven by thermal gradients, hydrodynamic effects and sedimentation forces. Liquid streaming is induced by the AC field itself via local heating and results, under special conditions, in vortices which improve the trapping efficiency. Microstructures were fabricated by electron-beam lithography and modified by UV laser ablation. They had typical gap dimensions between 500 nm and several micrometres. The theoretical and experimental results illustrate the basic principles of particle behaviour in ultra-miniaturized field traps filled with aqueous solutions. The smallest single particle that we could stably trap was a Latex bead of 650 nm. The smallest particles which were concentrated in the central part of the field trap were 14 nm in diameter. At high frequencies (in the megahertz range), field strengths up to 56 MV can be applied in the narrow gaps of 500 nm. Further perspectives for microparticle and macromolecular trapping are discussed.

A cascade switching superconducting single photon detector

We have realized superconducting single photon detectors with reduced inductance and increased signal pulse amplitude. The detectors are based on a parallel connection of ultrathin NbN nanowires with a common bias inductance. When properly biased, an absorbed photon induces a cascade switch of all the parallel wires generating a signal pulse amplitude of 2mV. The parallel wire configuration lowers the detector inductance and reduces the response time well below 1ns.

Biomagnetic measurements of spontaneous brain activity in epileptic patients

In the last few years there has been an increasing interest in the magnetic activity due to bioelectrical currents flowing in the brain. In this paper preliminary results are reported concerning spontaneous magnetic brain activity in 36 patients affected by different kinds of brain disease; in most of these cases the symptoms were induced by localized pathology (atrophies, scars, tumors). Measurements were carried out with the simultaneous recording of the EEG. At present one of the most interesting features of magnetic detection seems to be its high localizing ability in cases of cortical foci, and sometimes its ability to show activities not evident in the EEG. These features seem to be very encouraging for the search for technical improvements, with the aim of making the magnetic technique a candidate for current diagnostic purposes.

Nanowire superconducting single-photon detectors on GaAs for integrated quantum photonic applications

We demonstrate efficient nanowire superconducting single photon detectors (SSPDs) based on NbN thin films grown on GaAs. NbN films ranging from 3 to 5 nm in thickness have been deposited by dc magnetron sputtering on GaAs substrates at 350 °C. These films show superconducting properties comparable to similar films grown on sapphire and MgO. In order to demonstrate the potential for monolithic integration, SSPDs were fabricated and measured on GaAs/AlAs Bragg mirrors, showing a clear cavity enhancement, with a peak quantum efficiency of 18.3% at λ=1300 nm and T=4.2 K.

Low frequency resistance fluctuations in films of high temperature superconductors

Low-frequency voltage fluctuations in thin films of YBa/sub 2/Cu/sub 3/O/sub 7-x/ at and above the superconducting transition temperature have a spectral density proportional to the ratio of the average voltage across the film to the frequency. The ratio of the spectral density to the average voltage decreases markedly as the microstructure of the films is improved. In contrast to classic superconductors, the noise at the resistive transition does not arise from equilibrium temperature fluctuations. >

Evidence of carrier number fluctuation as origin of 1/f noise in polycrystalline silicon thin film transistors

A systematic study of the noise performances of polycrystalline silicon thin film transistors is presented. The drain current spectral density of these devices shows an evident 1/ f behavior and scales, when operating in the linear regime, with the square of the mean value of the drain current. The origin of the noise can be ascribed to carrier number fluctuations related to the dynamic trapping and detrapping of the oxide traps.

Ultrastrong coupling in the near field of complementary split-ring resonators

Ultrastrong coupling of split ring resonators to the cyclotron transition in two-dimensional electron gases is studied in the terahertz regime, clarifying the importance of the resonator geometry. The use of the complementary type of resonator allows removal of the signal from the uncoupled areas. The experimental results are of spectacular quality and quantity. A record high light-matter coupling ratio (normalized vacuum Rabi frequency) of 0.87 is achieved.

Characterization of parallel superconducting nanowire single photon detectors

Superconducting nanowire single photon detectors (SNSPDs) have been realized using an innovative parallel wire configuration. This configuration allows, at the same time, a large detection area and a fast response, with the additional advantage of large signal amplitudes. The detectors have been thoroughly characterized in terms of signal properties (amplitude, risetime and falltime), detector operation (latching and not latching) and quantum efficiency (at 850 nm). It has been shown that the parallel SNSPD is able to provide significantly higher maximum count rates for large area SNSPDs than meandered SNSPDs. Through a proper parallel wire configuration the increase in maximum count rate can be obtained without latching problems.