Optics and photonics are key technologies in most strategic sectors, both for the advances of fundamental research and for the variety of application areas. In the past years, the possibility to functionalise and to engineer materials and components at nanometer scale opened up opportunities for new applications and new ways of radiation-matter interaction. The growing integration of chips on photonic devices will allow overcoming the intrinsic limitations in elaborating, transmitting and sharing information related to specific environments (e.g.: *internet of things*). In addition to these important potentialities, *quantum technologies* offer further opportunities to exploit the control of matter constituents and the quantum mechanics laws permitting to achieve performances incomparable to those based on classical systems. Optical technologies will contribute to tackle fundamental problems such as the Planet sustainable growth through the exemplary change in the systems for energy production and environmental parameters measurement.

The DSFTM has a relevant role in those sectors and, thanks to the transversal properties of those technologies, aims at facing some challenges in the following areas:

- Optical and photonic systems, multiparameter sensors networks and optoelectronic infrastructures (such as for
*Smart Cities*e*Smart Buildings*). - Photonic technologies for high efficiency energy production, conversion, storage and transportation.
- Photonic technologies for virtual reality, cognitive photonics and advanced man-machine interfaces.
- Photonic systems for advanced imaging, non-invasive diagnostics, living matter therapy and manipulation.
- Dissemination of ultra-precise time and frequency standards.
- Photonic technologies for the study of matter in extreme conditions (such as new materials synthesis, plasmas, nuclear fusion and charges acceleration).
- Advanced platforms based on photons and ultra-cold matter for communication networks, simulators and quantum computing.
- Quantum sensors for high precision time metrology, gravity, electromagnetic fields and physical properties of matter.

## Spotlights on research activity

## Topological order and thermal equilibrium in polariton condensates

The Berezinskii–Kosterlitz–Thouless phase transition from a disordered to a quasi-ordered state, mediated by the proliferation of topological defects in two dimensions, governs seemingly remote physical systems ranging from liquid helium, ultracold atoms and superconducting thin films to ensembles of spins. We have observed such a transition in a short–lived gas of exciton–polaritons, bosonic light–matter particles in semiconductor microcavities. The observed quasi–ordered phase, characteristic for an equilibrium two–dimensional bosonic gas, with a decay of coherence in both spatial and temporal domains with the same algebraic exponent, is reproduced with numerical solutions of stochastic dynamics, proving that the mechanism of pairing of the topological defects (vortices) is responsible for the transition to the algebraic order. This is made possible thanks to long polariton lifetimes in high–quality samples and in a reservoir–free region. Our results show that the joint measurement of coherence both in space and time is required to characterize driven–dissipative phase transitions and enable the investigation of topological ordering in open systems.

Contact person: Dario Ballarini, NANOTEC Lecce

## 0–π phase–controllable thermal Josephson junction

We have reported the first experimental realization of a thermal Josephson junction whose phase bias can be controlled from 0 to π. This is obtained thanks to a superconducting quantum interferometer that allows full control of the direction of the coherent energy transfer through the junction. This possibility, in conjunction with the completely superconducting nature of our system, provides temperature modulations with an unprecedented amplitude of about 100 mK and transfer coefficients exceeding 1 K per flux quantum at 25 mK. Then, this quantum structure represents a fundamental step towards the realization of caloritronic logic components such as thermal transistors, switches and memory devices. These elements, combined with heat interferometers and diodes, would complete the thermal conversion of the most important phase–coherent electronic devices and benefit cryogenic microcircuits requiring energy management, such as quantum computing architectures and radiation sensors.

Contact person: Antonio Fornieri, NANO Pisa

## Synthetic dimensions with magnetic fields and local interactions in photonic lattices

We have investigated how one can realize a photonic device that combines synthetic dimensions and synthetic magnetic fields with spatially local interactions. Using an array of ring cavities, the angular coordinate around each cavity spans the synthetic dimension. The synthetic magnetic field arises from the circumstance that intercavity photon hopping is associated with a change of angular momentum. Photon-photon interactions are local in the periodic angular coordinate around each cavity. We have also pointed out experimentally observable consequences of the synthetic magnetic field and of the local interactions.

Contact person: Iacopo Carusotto, INO Trento

## Toward cavity quantum electrodynamics with hybrid photon gap–plasmon states

Combining localized surface plasmons (LSPs) and diffractive surface waves (DSWs) in metallic nanoparticle gratings leads to the emergence of collective hybrid plasmonic–photonic modes known as surface lattice resonances (SLRs). These show reduced losses and therefore a higher Q factor with respect to pure LSPs, at the price of larger volumes V. By using aluminum nanoparticle square gratings with unit cells consisting of narrow–gap disk dimers (a geometry featuring a very small modal volume) we have demonstrated that an enhancement of the Q/V ratio with respect to the pure LSP and DSW is obtained for SLRs with a well–defined degree of plasmon hybridization. Simultaneously, we have reported a 5 times increase of the Q/V ratio for the gap–coupled LSP with respect to that of the single nanoparticle. The results of this work open the way toward more efficient applications for the exploitation of excitonic nonlinearities in hybrid plasmonic platforms.

Contact person: Milena De Giorgi, NANOTEC Lecce

## Repulsive Fermi polarons in a resonant mixture of ultracold 6Li atoms

We employ radio–frequency spectroscopy to investigate a polarized spin mixture of ultracold ^{6}Li atoms close to a broad Feshbach scattering resonance. Focusing on the regime of strong repulsive interactions, we observe well-defined coherent quasiparticles even for unitarity–limited interactions. We characterize the many-body system by extracting the key properties of repulsive Fermi polarons: the energy E+, the effective mass m*, the residue Z, and the decay rate Γ. Above a critical interaction, E+ is found to exceed the Fermi energy of the bath, while m* diverges and even turns negative, thereby indicating that the repulsive Fermi liquid state becomes energetically and thermodynamically unstable.

Contact person: Francesco Scazza, INO-CNR Sesto Fiorentino

## Triple–helical nanowires by tomographic rotatory growth for chiral photonics

We have reported three dimensional triple–helical nanowires, engineered by the innovative tomographic rotatory growth, on the basis of focused ion beam-induced deposition. These three dimensional nanostructures show up to 37% of circular dichroism in a broad range (500–1,000 nm), with a high signal-to-noise ratio (up to 24 dB). Optical activity of up to 8° only due to the circular birefringence is also shown, tracing the way towards chiral photonic devices that can be integrated in optical nanocircuits to modulate the visible light polarization.

Contact person: Vittorianna Tasco, NANOTEC–CNR Lecce

## Nanoscale phase engineering of thermal transport with a Josephson heat modulator

We have realized the first balanced Josephson heat modulator designed to offer full control at the nanoscale over the phase-coherent component of thermal currents. Our device provides magnetic–flux–dependent temperature modulations up to 40 mK in amplitude with a maximum of the flux–to–temperature transfer coefficient reaching 200 mK per flux quantum at a bath temperature of 25 mK. Foremost, it demonstrates the exact correspondence in the phase engineering of charge and heat currents, breaking ground for advanced caloritronic nanodevices such as thermal splitters, heat pumps and time–dependent electronic engines.

Contact person: Francesco Giazotto, NANO–CNR Pisa

## Josephson effect in fermionic superfluids across the BEC–BCS crossover

We have reported on the observation of the Josephson effect between two fermionic superfluids coupled through a thin tunneling barrier. We have shown that the relative population and phase are canonically conjugate dynamical variables throughout the crossover from the molecular Bose-Einstein condensate (BEC) to the Bardeen–Cooper–Schrieffer (BCS) superfluid regime. For larger initial excitations from equilibrium, the dynamics of the superfluids become dissipative, which we ascribe to the propagation of vortices through the superfluid bulk. Our results highlight the robust nature of resonant superfluids.

Contact person: Giacomo Roati, INO–CNR Sesto Fiorentino