Although they enabled several advances in the field of optomechanics, optomechanical disk resonators have not yet been qualified for operation in the quantum regime of motion. We present the experimental demonstration of an optomechanical disk resonator prepared in the quantum ground state. With a gigahertz frequency, the mechanical breathing mode of the investigated semiconductor disk reaches a level of excitation below a single phonon when cooled in a dilution refrigerator. We quantify the phonon occupancy by performing sideband thermometry: a conical optical fiber is evanescently coupled to the disk optical whispering-gallery mode, and Stokes and anti-Stokes photons scattered by phonon emission and absorption are counted on a single-photon detector. We measure a suppression of the absorption process corresponding to a mean phonon occupancy of 0.66±0.20. Beyond ground-state cooling, we experimentally investigate the mechanisms ruling laser-induced heating, which ultimately limits the lowest measurable phonon occupancy, and witness both intracavity and extracavity heating.

The evaluation of cognitive functions interactions has become increasingly implemented in the cognition exploration. In the present study, we propose to examine the organization of the cognitive network in healthy participants through the analysis of behavioral performances in several cognitive domains. Specifically, we aim to explore cognitive interactions profiles, in terms of cognitive network, and as a function of participants’ handedness. To this end, we proposed several behavioral tasks evaluating language, memory, executive functions, and social cognition performances in 175 young healthy right-handed and left-handed participants and we analyzed cognitive scores, from a network perspective, using graph theory. Our results highlight the existence of intricate interactions between cognitive functions both within and beyond the same cognitive domain. Language functions are interrelated with executive functions and memory in healthy cognitive functioning and assume a central role in the cognitive network. Interestingly, for similar high performance, our findings unveiled differential organizations within the cognitive network between right-handed and left-handed participants, with variations observed both at a global and nodal level. This original integrative network approach to the study of cognition provides new insights into cognitive interactions and modulations. It allows a more global understanding and consideration of cognitive functioning, from which complex behaviors emerge.

Entangled photon states are a fundamental resource for optical quantum technologies and investigating the fundamental predictions of quantum mechanics. Up to now such states are mainly generated in macroscopic nonlinear optical systems with elaborately tailored optical properties. In this theoretical work, we extend the understanding on the generation of entangled photonic states toward the nanoscale regime by investigating the fundamental properties of photon-pair generation in sub-wavelength nonlinear nanoresonators. Taking materials with Zinc-Blende structure as an example, we reveal that such systems can naturally generate various polarization-entangled Bell states over a very broad range of wavelengths and emission directions, with little to no engineering needed. Interestingly, we uncover different regimes of operation, where polarization-entangled photons can be generated with dependence on or complete independence from the pumping wavelength and polarization, and the modal content of the nanoresonator. Our work also shows the potential of nonlinear nanoresonators as miniaturized sources of biphoton states with highly complex and tunable properties.

Frequency upconversion of near-infrared photons to the visible range is strategical for information technology, as it can provide an alternative for the read out of telecom signals using efficient silicon-based detectors. Light upconversion is a nonlinear process mediated by matter that consists in the interaction of either energy-degenerate photons, such as in second-harmonic and third-harmonic generation (THG), or photons with different energies, such as in sum frequency generation (SFG). We recently investigated frequency upconversion in both plasmonic and dielectric nanoantennas [1], [2]. Thanks to the adopted dual-beam pump scheme, where an ultrashort pulse ( $\omega$ ) at telecom wavelength ( $\lambda=1551\ \text{nm}$ ) impinges on the sample along with its frequency-doubled replica ( $2 \omega$ ), THG and SFG are degenerate in energy. This, along with coherence, enables the interference between the processes. Yet, we found that in individual nanoantennas symmetry plays a major role in enhancing/suppressing the interference between SFG and THG. By tuning the relative phase between the two impinging pulses, we performed all-optical switching of upconverted light with efficiency > 50% in asymmetric plasmonic antennas [2].

Entangled photon states are a fundamental resource for optical quantum technologies and investigating the fundamental predictions of quantum mechanics. Up to now such states are mainly generated in macroscopic nonlinear optical systems with elaborately tailored optical properties. In this theoretical work, we extend the understanding on the generation of entangled photonic states towards the nanoscale regime, by investigating the fundamental properties of photon-pair-generation in sub-wavelength nonlinear nanoresonators. Taking materials with Zinc-Blende structure as example, we reveal that such systems can naturally generate various polarization-entangled Bell states over a very broad range of wavelengths and emission directions, with little to no engineering needed. Interestingly, we uncover different regimes of operation, where polarization-entangled photons can be generated with dependence on or complete independence from the pumping wavelength and polarization, and the modal content of the nanoresonator. Our work also shows the potential of nonlinear nanoresonators as miniaturized sources of biphoton states with highly complex and tunable properties.

We propose a dual-beam pumping scheme whereby a pulse of telecom frequency ω (1550 nm wavelength) is mixed with its frequency-doubled replica at 2ω. This brings about sum-frequency generation at ω+2ω = 3ω on top of third harmonic at ω+ω+ω = 3ω. We exploited the diffraction by an AlGaAs metasurface for realizing the symmetry breaking which is key to avoid averaging out the interference between the two coherent frequency-tripling pathways. The upconverted light can thereby be routed into different diffraction orders with high selectivity, based on the pumps’ relative phase, providing a platform for GHz-rate amplitude modulation and information encoding.

We address the surface vs bulk origin of the second-order optical nonlinearity in AlGaAs nanocylinders through polarization-resolved measurements. By comparing numerical simulations accounting just for bulk second-order nonlinearity with experimental results, we show that the surface contribution to second-harmonic generation (SHG) cannot be neglected and depends on the resonant conditions of the nanocylinder. Additionally, our analysis suggests that bulk and surface SHG are competing effects, and that their interference might influence the overall efficiency.

Crystals and fibers doped with rare-earth (RE) ions provide the basis for most of today's solid-state optical systems, from lasers and telecom devices to emerging potential quantum applications such as quantum memories and optical to microwave conversion. The two platforms, doped crystals and doped fibers, seem mutually exclusive, each having its own strengths and limitations, the former providing high homogeneity and coherence and the latter offering the advantages of robust optical waveguides. Here we present a hybrid platform that does not rely on doping but rather on coating the waveguide-a tapered silica optical fiber-with a monolayer of complexes, each containing a single RE ion. The complexes offer an identical, tailored environment to each ion, thus minimizing inhomogeneity and allowing tuning of their properties to the desired application. Specifically, we use highly luminescent Yb3+[Zn(II)MC (QXA)] complexes, which isolate the RE ion from the environment and suppress nonradiative decay channels. We demonstrate that the beneficial optical transitions of the Yb3+ are retained after deposition on the tapered fiber and observe an excited-state lifetime of over 0.9 ms, on par with state-of-the-art Yb-doped inorganic crystals.

We experimentally demonstrate the possibility to modulate the second harmonic (SH) power emitted by nonlinear AlGaAs metasurfaces embedded in a liquid crystal (LC) matrix. This result is obtained by changing the relative in-plane orientation between the LC director and the linear polarization of the light at the excitation wavelength. According to numerical simulations, second-harmonic is efficiently radiated by the metasurfaces thanks to the sizeable second-order susceptibility of the material and the resonant excitation of either electric or magnetic dipole field distributions inside each meta-atom at the illuminating fundamental wavelength. This resonant behavior strongly depends on the geometric parameters, the crystallographic orientation, and the anisotropy of the metasurface, which can be optimized to modulate the emitted SH power by about one order of magnitude. The devised hybrid platforms are therefore appealing in view of enabling the electrical control of flat nonlinear optical devices.

Photon-pair sources based on spontaneous parametric down-conversion (SPDC) are the backbone of a vast number of experimental implementations in quantum optics. Sub-wavelength-sized dielectric nanoresonators have recently emerged as a new platform for the implementation of SPDC sources [1] , that promise much potential in generating photons with engineered properties, as these nanoresonators offer a strong control over many parameters of the scattered light (directionality, polarization, …) However, the many degrees of freedom in these highly multi-modal and non-Hermitian systems makes design and formal description of pair-generation challenging.

Crystals and fibers doped with Rare Earth (RE) ions provide the basis to most of today's solid-state optical systems, from lasers and telecom devices to emerging potential quantum applications such as quantum memories and optical to microwave conversion. The two platforms, doped crystals and doped fibers, seem mutually exclusive, each having its own strengths and limitations- the former providing high homogeneity and coherence, and the latter offering the advantages of robust optical waveguides. Here we present a hybrid platform that does not rely on doping but rather on coating the waveguide - a tapered silica optical fiber - with a monolayer of complexes, each containing a single RE ion. The complexes offer an identical, tailored environment to each ion, thus minimizing inhomogeneity and allowing tuning of their properties to the desired application. Specifically, we use highly luminescent Yb$^{+3}$[Zn(II)$_{MC}$(QXA)] complexes, which isolate the RE ion from the environment and suppress non-radiative decay channels. We demonstrate that the beneficial optical transitions of the Yb$^{+3}$ are retained after deposition on the tapered fiber, and observe an excited-state lifetime of over 0.9 ms, on par with state-of-the-art Yb doped inorganic crystals.

We model SPDC in dielectric nanoresonators based on quasinormal modes (QNMs). Using QNMs, the process reduces to a few interacting modes, providing intuition and enabling the design of nanoscale SPDC sources with complex functionalities.

Nanophotonics systems have recently been studied under the perspective of non-Hermitian physics. Given their potential for wavefront control, nonlinear optics and quantum optics, it is crucial to develop predictive tools to assist their design. We present here a simple model relying on the coupling to an effective bath consisting of a continuum of modes to describe systems of coupled resonators, and test it on dielectric nanocylinder chains accessible to experiments. The effective coupling constants, which depend non-trivially on the distance between resonators, are extracted from numerical simulations in the case of just two coupled elements. The model predicts successfully the dispersive and reactive nature of modes for configurations with multiple resonators, as validated by numerical solutions. It can be applied to larger systems, which are hardly solvable with finite-element approaches.

We report on the integration of AlGaAs whispering-gallery mode microdisks into a monolithic photonic chip as a platform for frequency conversion and non-classical state generation based on parametric nonlinear optical processes. Quasi phase-matching, resonant field enhancement and confinement ensure efficient nonlinear interactions: second-harmonic generation has been achieved with a conversion efficiency of 5%W−1, and spontaneous parametric down-conversion with a photon pair generation rate of 1.2 kHz/μW.

Dielectric optical metasurfaces provide a powerful and versatile way to explore exotic physics on engineered systems as well as conceive novel miniaturized optical devices for wavefront shaping, harmonic generation and quantum photonics [1] . Since their sub-wavelength constitutive elements are non Hermitian due to low quality factors, such assemblies of optical nanoantennas can implement a wide range of functionalities. Non-Hermitian components were formerly studied in plasmonics [2] , and more recently their dielectric counterparts have also drawn a growing interest [3] , [4] . In this work, we theoretically study one-dimensional chains of N dissipative dielectric nanoresonators by building an analytical non-Hermitian Hamiltonian model. The latter has been fed with complex coupling parameters obtained from a numerical simulation in the case of N=2 , and then validated with longer chains ( N>2 ) by comparison with brute-force calculations. It thereby becomes a predictive tool, essential to overcome numerical limitations to study and design large systems.

We present a scheme for deterministic ion-photon qubit exchange, namely a SWAP gate, based on realistic cavity-QED systems with 171Yb+, 40Ca+ and 138Ba+ ions. The gate can also serve as a single-photon quantum memory, in which an outgoing photon heralds the successful arrival of the incoming photonic qubit. Although strong coupling, namely having the single-photon Rabi frequency be the fastest rate in the system, is often assumed essential, this gate (similarly to the Duan-Kimble C-phase gate) requires only Purcell enhancement, i.e. high single-atom cooperativity. Accordingly, it does not require small mode volume cavities, which are challenging to incorporate with ions due to the difficulty of trapping them close to dielectric surfaces. Instead, larger cavities, potentially more compatible with the trap apparatus, are sufficient, as long as their numerical aperture is high enough to maintain small mode area at the ion's position. We define the optimal parameters for the gate's operation and simulate the expected fidelities and efficiencies, demonstrating that efficient photon-ion qubit exchange, a valuable building block for scalable quantum computation, is practically attainable with current experimental capabilities.

We report on a monolithic photonic chip including a microdisk for chi(2) optical interactions which is evanescently coupled with two suspended waveguides used for pump injection and nonlinear signal collection: one for critical coupling at λω ≈ 1600 nm and the other for critical coupling at λ2ω ≈ 800 nm. Our sources are CW tunable lasers: an external-cavity laser diode at ω and a Ti:Sapphire laser at 2ω, both connected to a microlensed fiber. Thanks to critical coupling at input/output wavelengths, we achieved 11% W−1 SHG efficiency. The same chip enabled us to demonstrate spontaneous parametric down-conversion (SPDC).

A novel theoretical non-Hermitian formalism for analyzing nonlinear nano-optics is proposed. Its main strength lies in the unique way it analytically incorporates the quasinormal modes of the nanostructures, which have to be resonantly excited to achieve significant nonlinear efficiency. Owing to the analyticity, the formalism is computationally more effective than the usual multipolar Mie-scattering expansions for nonlinear nano-optics. It is also more general and applies to nanostructures laying on substrates or embedded in thin films. It additionally provides guidelines for the multiparameter design and optimization of nonlinear photonic nanostructures. In particular, it reveals an important phase-matching condition at the subwavelength scale between the linear and nonlinear harmonics, which was not clarified in earlier theoretical works based on Hermitian theory for waveguides and high-Q cavities. Its closed form is a major asset that enables us to propose a systematic approach to design phased-matched nanostructures offering drastic second harmonic generation enhancements with engineered.(2) and pump beams.

We demonstrate photon-pair generation via spontaneous parametric down-conversion (SPDC) from two types of metasurfaces composed by AlGaAs nanocylinders: 1) monolithically fabricated on a selectively oxidized layer of AlAs epitaxially grown on a GaAs wafer; 2) fabricated by reporting the AlGaAs nanostructures on a transparent wafer via wafer bonding. In these samples, we observed SPDC both in back- and forward-scattering configurations, under excitation with a CW pump around 775 nm and single-photon detection on the signal and idler channels. The Bragg modulation of Mie-resonances enables paraxial SPDC, which demonstrates the potential of all-dielectric metasurfaces for quantum applications like on-axis quantum imaging.

In this Letter, we report on the fabrication and characterization of a monolithic III–V semiconductor photonic chip, designed to perform nonlinear parametric optical processes for frequency conversion and non-classical state generation. This chip co-integrates an AlGaAs microdisk that is evanescently coupled to two distinct suspended waveguides designed for light injection and collection around 1600 nm and 800 nm, respectively. Quasi-phase matching provided by the resonator geometry and material symmetry, resonant field enhancement, and confinement ensure efficient nonlinear interactions. We demonstrate second-harmonic generation efficiency of 5 % W − 1 and a biphoton generation rate of 1.2 kHz/µW through spontaneous down-conversion.