Loredana Parlato

We investigate tunnel ferromagnetic Josephson junctions based on Superconductor-Insulator-thin superconductor-Ferromagnet-Superconductor multilayers. A comparative study of their electrodynamic properties is performed for junctions with niobium and aluminum (Al) electrodes, featuring different ferromagnetic interlayer materials and lateral dimensions ranging from the micrometric to the submicrometric scale. The parameters extracted from the fitting of the current-voltage characteristics using the tunnel junction microscopic model are found to be consistent with those independently estimated from switching current distribution measurements. Submicrometric Al-based devices exhibit electrodynamic properties comparable to those implemented in state-of-the-art transmon qubits and display clear signatures of quantum phase diffusion. The strong agreement between transport modelling and escape dynamics establishes a robust framework for describing hybrid ferromagnetic Josephson junctions consistent with their energy scales and supports their potential integration into superconducting quantum and classical digital circuits.

Quantum Key Distribution (QKD) is an optical communication technique that allows two authenticated parties to exchange an encryption key using protocols grounded in the laws of quantum mechanics, making the communication intrinsically secure. A fundamental requirement for key exchange through such protocols is the use of high-performance single-photon detectors and infrared single-photon sources with low multi-photon components. In addition to the rapid progress in single-photon sources (SPSs), achieving the ideal scenario where a source emits only single photons at telecom wavelength with high efficiency remains a theoretical concept. Multi-photon components emitted by non-ideal light sources such as lasers or Spontaneous Parametric Down Conversion (SPDC) sources can open the door to successful attacks from non-legitimate parties. In this work, we demonstrate, by using a Photon-Number-Resolving Detector (PNRD), based on parallel superconducting strips, the possibility of obtaining a single-photon source with extremely multi-photon components starting from a non-single-photon light source, namely an SPDC one. To estimate the reduction of the multi-photon component, we analyzed the second-order autocorrelation function, g(2)(0), of the light source using a PNRD modelled as 8 independent pixels in parallel, in a heralding protocol. We managed to obtain a reduction of the g(2)(0) using a CW SPDC with an emission rate of about 10 MHz using several sampling bin-widths, spanning from less than one nanoseconds to hundreds of nanoseconds. These results align with the goals of the Quantum Italy Deployment project, as sources with suppressed multi-photon components could play a key role in improving the performance of QKD systems.

We report on an innovative method for the characterization of the timing jitter on a picosecond timescale of single photon detectors and segmented photon number resolving detectors using the quantum properties of continuous spontaneous parametric down-converted light. We show that using both the strong correlation and the photon bunching of continuous spontaneous parametric down-converted light we can make more than enough independent data to determine the timing jitter of each individual channel using a weighted multivariable linear fit. The timing jitter of each channel is determined in this way is acceptable with a 0.05 acceptance criteria and it has a precision below 1 ps for each channel.

Single photon detectors based on superconducting nanostrips demonstrated high performance at 1550 nm, combining high efficiency and low noise. Arranged in arrays, these devices can implement photon-number resolution, expanding the range of applicability of superconducting detectors. A detector of this kind was used in our recent work to implement the annihilation operator and the characterization of quantum states in terms of their photon number distribution. The reconstructed distributions were consistent with the theoretical predictions, exhibiting fidelities higher than 99.9%. In this work, this procedure was repeated with each pixel singularly to define a detector fidelity matrix, evaluating the fidelity among all the possible pixel combinations. In this way, we introduce an additional tool that quantifies not only the quality of the reconstructed photon number distribution when compared to the theoretical expectation, but also the mutual fidelity when different pixels are chosen as independent, and then the fabrication uniformity.

We report on the fabrication and millikelvin char acterization of Nb/Al–AlOx/Nb Josephson junctions directly de posited on X-cut LiNbO3 substrates. Motivated by the strong electro-optic and piezoelectric properties of LiNbO3, such devices are of interest for future hybrid superconducting–electro-optic architectures. The junctions were fabricated using standard processes involving DC magnetron sputtering followed by controlled thermal oxidation of the aluminum layer to form an AlOx tunnel barrier with reproducible junction areas. Insulation of the junctions was achieved by anodization and SiOx deposition. Electrical measurements performed at 30 mK reveal high-quality Josephson tunnel junction behavior with a critical current density of J_c = 70 A/cm² and a characteristic voltage reaching values up to V_m = 286 mV. Differential conductance measurements exhibit pronounced quasiparticle peaks corresponding to a sum gap energy of 2Δ = (2.73 ± 0.05) meV, confirming the good quality of the Nb electrodes. Magnetic-field-dependent measure ments yield a Josephson penetration depth of λ_J = 45 μm, confirming operation in the small-junction regime, with deviations from the ideal Josephson interference pattern attributed to non-uniform current distribution across the junction area. Compared to our previous characterization at 4.2 K, this work demonstrates reliable Josephson junction operation on LiNbO3 at dilution-refrigerator temperatures relevant for quantum circuits, supporting the suitability of lithium niobate as a promising substrate for future hybrid superconducting–electro-optic device implementations.

Flux tuning of qubit frequencies in superconducting quantum processors is fundamental for implementing single and multi-qubit gates in quantum algorithms. Typical architectures involve the use of DC or fast RF lines. However, these lines introduce significant heat dissipation and undesirable decoherence mechanisms, leading to a severe bottleneck for scalability. Among different solutions to overcome this issue, we propose integrating tunnel Superconductor-Insulating-thin superconducting interlayer-Ferromagnet-Superconductor Josephson junctions (SIsFS JJs) into a novel transmon qubit design, the so-called ferrotransmon. SIsFS JJs provide memory properties due to the presence of ferromagnetic barriers and preserve at the same time the low-dissipative behavior of tunnel-insulating JJs, thus promoting an alternative tuning of the qubit frequency. In this work, we discuss the fundamental steps towards the implementation of this hybrid ferromagnetic transmon. We will give a special focus on the design, simulations, and preliminary experimental characterization of superconducting lines to provide in-plane magnetic fields, fundamental for an on-chip control of the qubit frequencies in the ferrotransmon.

Magnetic Josephson junctions (MJJs) are a special class of hybrid systems where antagonistic correlations coexist, thus providing a key for advances in weak superconductivity, superconducting spintronics, and quantum computation. So far, the memory properties of MJJs have been mostly investigated in view of digital electronics and for spintronic devices at liquid-helium temperature. At the operating temperature of quantum circuits, a magnetic order can rise in a superconductor (S) at the S/ferromagnet (F) interface, i.e., the inverse proximity effect (IPE), thus leading to a significant modification of the magnetic field patterns in MJJs. In this work, we have carried out a comparative investigation of the magnetic behavior of tunnel MJJs with a strong ferromagnetic layer inserted in the layout of both Nb and Al JJs, respectively. The comparative analysis validates the crucial role of the temperature, the fundamental scaling energies of S/F coupling systems, and the transparency of the S/F interface. This investigation points out that the IPE is a key aspect to consider when designing tunnel MJJs operating well below 4 K and thus in the perspective of hybrid superconducting quantum architectures.

Detectors capable of resolving the number of photons are essential in many applications, ranging from classic photonics to quantum optics and quantum communication. In particular, photon-number-resolving detectors based on arrays of superconducting nanostrips can offer a high detection efficiency, a low dark count rate and a recovery time of a few nanoseconds. In this work, we use a detector of this kind for the unbiased generation of random numbers by following two different methods based on the detection of photons. In the former, we exploit the property that the light is equally distributed on each strip of the entire detector, whereas in the latter we exploit the fact that, for a high average number of photons, the parity of the Poissonian distribution of the number of photons emitted by the laser tends to be zero. In addition, since these two methods are independent, it is possible to use them at the same time.

By measuring the current–voltage characteristics and the switching current distributions as a function of temperature, we have investigated the phase dynamics of Al tunnel ferromagnetic Josephson junctions (JJs), designed to fall in the typical range of parameters of state-of-the-art transmons, providing evidence of phase diffusion processes. The comparison with the experimental outcomes on non-magnetic JJs with nominally the same electrodynamical parameters demonstrates that the introduction of ferromagnetic barriers does not cause any sizeable detrimental effect and supports the notion of including tunnel ferromagnetic JJs in qubit architectures.

The forthcoming era of quantum computers can be a threat to the conventional cryptography and data security. Quantum Key Distribution (QKD) provides unconditional security under real-life conditions with several protocols over long distances in fibre and free space communication. Superconducting Nanowire Single Photon Detectors (SNSPDs) are becoming a dominant technology for QKD thanks to their unique characteristics, such as a near-unity efficiency in the infrared, low dark counts and picoseconds time resolution. Where the detector is typically a weakness of QKD, these SNSPDs characteristics make exploitation difficult. In this work, we characterized NbN SNSPDs at 2.2 K, using a CW laser source at 1550 nm, varying both bias currents and input photon rates to prove their high efficiency at low dark counts with a high counting rate, consistent with the requirements for QKD over long distances or with a high secure key rate.