Quantum Sensing is a rapidly expanding research field that finds one of its applications in Fundamental Physics, as the search for Dark Matter. Recent developments in the fabricationof superconducting qubits are contributing to driving progress in Quantum Sensing. Such devices have already been successfully applied in detecting few-GHz single photons via Quantum Non-Demolition measurement (QND). This technique allows us to detect the presence of the same photon multiple times without absorbing it, with remarkable sensitivity improvements and dark count rate suppression in experiments based on high-precision microwave photon detection, such as Axions and Dark Photons search experiments. In this context, the INFN Qub-IT project goal is to realize an itinerant single-photon counter based on superconducting qubits that will exploit QND. The simulation step is fundamental for optimizing the design before manufacturing and finally characterizing the fabricated chip in a cryogenic environment. In this study we present Qub-IT’s status towards the characterization of its first superconducting transmon qubit devices, illustrating their design and simulation.
The quasicharge superconducting qubit realizes the dual of the transmon and shows strong robustness to flux and charge fluctuations thanks to a very large inductance closed on a Josephsonjunction. At the same time, a weak anharmonicity of the spectrum is inherited from the parent transmon, that introduces leakage errors and is prone to frequency crowding in multi-qubit setups. We propose a novel design that employs a quartic superinductor and confers a good degree of anharmonicity to the spectrum. The quartic regime is achieved through a properly designed chain of Josephson junction loops that avoids strong quantum fluctuations without introducing a severe dependence on the external flux.
Superconducting interferometers are quantum devices able to transduce a magnetic flux into an electrical output with excellent sensitivity, integrability and power consumption. Yet,their voltage response is intrinsically non-linear, a limitation which is conventionally circumvented through the introduction of compensation inductances or by the construction of complex device arrays. Here we propose an intrinsically-linear flux-to-voltage mesoscopic transducer, called bi-SQUIPT, based on the superconducting quantum interference proximity transistor as fundamental building block. The bi-SQUIPT provides a voltage-noise spectral density as low as ∼10−16 V/Hz1/2 and, more interestingly, under a proper operation parameter selection, exhibits a spur-free dynamic range as large as ∼60 dB, a value on par with that obtained with state-of-the-art SQUID-based linear flux-to-voltage superconducting transducers. Furthermore, thanks to its peculiar measurement configuration, the bi-SQUIPT is tolerant to imperfections and non-idealities in general. For the above reasons, we believe that the bi-SQUIPT could provide a relevant step-beyond in the field of low-dissipation and low-noise current amplification with a special emphasis on applications in cryogenic quantum electronics.
Setting up strong Josephson coupling in van der Waals materials in close proximity to superconductors offers several opportunities both to inspect fundamental physics and to developnovel cryogenic quantum technologies. Here we show evidence of Josephson coupling in a planar few-layer black Phosphorus junction. The planar geometry allows us to probe the junction behavior by means of external gates, at different carrier concentrations. Clear signatures of Josephson coupling are demonstrated by measuring supercurrent flow through the junction at milli Kelvin temperatures. Manifestation of Fraunhofer pattern with a transverse magnetic field is also reported, confirming the Josephson coupling. These findings represent the first evidence of proximity Josephson coupling in a planar junction based on a van der Waals material beyond graphene and open the way to new studies, exploiting the peculiar properties of exfoliated black phosphorus thin flakes.
We propose a solid state refrigeration technique based on repeated adiabatic magnetization/demagnetization cycles of a superconductor which acts as the working substance. The gradualcooling down of a substrate (normal metal) in contact with the working substance is demonstrated for different initial temperatures of the substrate. Excess heat is given to a hot large-gap superconductor. The on-chip refrigerator works in a cyclic manner because of an effective thermal switching mechanism: Heat transport between N/N versus N/S junctions is asymmetric because of the appearance of the energy gap. This switch permits selective cooling of the metal. We find that this refrigeration technique can cool down a 0.3cm3 block of Cu by almost two orders of magnitude starting from 200mK, and down to about 1mK starting from the base temperature of a dilution fridge (10mK). The corresponding cooling power for a 1cm×1cm interface are 25 nW and 0.06 nW respectively, which scales with the area of the interface.
Heat management and refrigeration are key concepts for nanoscale devices operating at cryogenic temperatures. The design of an on-chip mesoscopic refrigerator that works thanks to theinput heat is presented, thus realizing a solid state implementation of the concept of cooling by heating. The system consists of a circuit featuring a thermoelectric element based on a ferromagnetic insulator-superconductor tunnel junction (N-FI-S) and a series of two normal metal-superconductor tunnel junctions (SINIS). The N-FI-S element converts the incoming heat in a thermovoltage, which is applied to the SINIS, thereby yielding cooling. The cooler’s performance is investigated as a function of the input heat current for different bath temperatures. We show that this system can efficiently employ the performance of SINIS refrigeration, with a substantial cooling of the normal metal island. Its scalability and simplicity in the design makes it a promising building block for low-temperature on-chip energy management applications.
The emerging field of coherent caloritronics (from the Latin word „calor“, i.e., heat) is based on the possibility to manipulate the phase-coherent heat currents flowingin mesoscopic superconducting structures. The goal is to design and implement quantum technologies able to master energy transfer with the same degree of accuracy reached for charge transport in contemporary electronic devices. This can be obtained by exploiting the macroscopic quantum coherence intrinsic to superconducting condensates, which manifests itself through the Josephson and the proximity effect. Here, we review recent experimental results obtained in the realization of heat interferometers and thermal rectifiers, and discuss a few proposals for exotic non-linear phase-coherent caloritronic devices, such as thermal transistors, solid-state memories, coherent heat splitters, microwave refrigerators, thermal engines and heat valves. Besides being extremely attractive from the fundamental physics point of view, these systems are expected to have a vast impact on all cryogenic microcircuits requiring energy management, and (possibly) lay the first stone for the foundation of electronic thermal logic.
We present a microwave quantum refrigeration principle based on the Josephson effect. When a superconducting quantum interference device (SQUID) is pierced by a time-dependent magneticflux, it induces changes in the macroscopic quantum phase and an effective finite bias voltage appears across the SQUID. This voltage can be used to actively cool well below the lattice temperature one of the superconducting electrodes forming the interferometer. The achievable cooling performance combined with the simplicity and scalability intrinsic to the structure pave the way to a number of applications in quantum technology.