arXiv subjects

Martina Morassi

Publications and source records attributed to Martina Morassi.

At least 19 recordsLinked to original sources

arXiv2026

Boosted Optomechanics with a Fluid of Nonlinear Polaritons

Merging optomechanics and polaritonics opens stimulating perspectives like the giant enhancement of optomechanical interaction and the enrichment of optomechanics with effective nonlinear photons. The experimental implementation of these concepts has however remained elusive. Here we report on the resonant optical control of polaritonic optomechanical resonators constituted of semiconductor disks embedding quantum wells. Whispering gallery photons and quantum well excitons strongly couple, leading to the emergence of polaritons that couple to the mechanical vibrations of the disk. We perform resonant optomechanical frequency response experiments on these resonators, modeled introducing a minimal set of constitutive equations, from which we extract the polariton-modified optomechanical coupling $g_0$ and the polariton nonlinearities. We observe a boost of $g_0$ by more than a decade compared to bare photons, reaching to a record $g_0$ for whispering gallery resonators of $22$ MHz, and analyze experimentally and theoretically its evolution as function of the polariton's composition. We also measure a clear hierarchy of three polaritonic nonlinearities, again analyzed as function of polariton composition, establishing a bridge between past unconciliated reports in polaritonics. Grounded on experimental and theoretical foundations, resonant polaritonic optomechanics is set ready for an optomechanical exploration of quantum fluids of polaritons.

quant-ph↗

arXiv2026

Industry-ready spin-photon interfaces for hybrid photonic quantum computing

Hybrid photonic quantum computers, combining stationary matter qubits and flying photonic qubits, offer an intrinsically networked and resource-efficient route to large-scale, error-corrected quantum computation. Their core components are cavity-coupled matter qubits that act as light--matter interfaces, enabling: high-efficiency on-demand single-photon generation, stable near-unity photon indistinguishability and spin--multi-photon entanglement. Semiconductor quantum dots in microcavities are a leading platform for realizing such devices. Yet reaching the performance, reproducibility and spin-coherence thresholds for large-scale error correction remains a major challenge requiring industrial fabrication and control. Here we report thousands of monolithic semiconductor quantum-dot devices fabricated using a III--V pilot production-line process compatible with large-scale deployment. Systematic control of source parameters yields state-of-the-art efficiency and supports a path to optical losses below fault-tolerance thresholds. Using field-quadrature state reconstruction as a stringent joint test of efficiency and indistinguishability, we observe near-unity photon quantum purity stable over tens of minutes and a record single-photon Wigner-function negativity. We further demonstrate seven-partite spin--multi-photon entanglement and spin coherence extendable to microsecond timescales in the low-magnetic-field regime. Finally, photons from distant sources are as indistinguishable as photons emitted successively by a single source. These results establish foundry-compatible III--V quantum dots as a scalable platform for hybrid photonic quantum computing.

quant-ph↗

arXiv2026

Near-identical photons from distant quantum dot-cavity devices

Scalable optical quantum technologies require interference between large numbers of indistinguishable single-photons emitted by independent sources. Semiconductor quantum dots are known to be excellent on-demand sources of single-photons. They show record efficiency when inserted into optical cavities to control their spontaneous emission and generate trains of near identical photons over microsecond timescales. However, generating perfectly identical photons from distant cavity-based sources has remained a long-standing challenge. It requires precise matching of the emission wavelengths and emission dynamics, while simultaneously minimizing spectral noise across all time scales for distant emitters in uncorrelated environments. Here, we report on the nanofabrication of a large number of quantum dot-cavity sources with ultra-low spectral noise and wavelength dispersion. The high source efficiency and the use of two tuning mechanisms enable precise optimization of the spectral overlap between distant sources. With this approach, we demonstrate a two-photon indistinguishability of $88\pm1$ % between photons emitted from two distant sources. Remarkably, this value reaches the upper bound set by the intrinsic indistinguishability of photons emitted successively by each source. These results represent a key milestone for scaling photon-based quantum technologies.

quant-ph↗

arXiv2026

Tensor-driven geometric phase in nonlinear AlGaAs metasurfaces

Dielectric metasurfaces provide a unique platform for efficient harmonic generation and optical wavefront manipulation at the nanoscale. While several approaches are available for performing wavefront shaping, the one exploiting geometric phase streamlines significantly the design and fabrication process. It has been recently shown that, in III-V semiconductor alloys, the rotation of the crystal axes affects the phase and amplitude of second-harmonic generation (SHG) induced by circularly polarized light [1]. Based on this notion, we fabricated and characterized two aluminum gallium arsenide metasurfaces displaying the versatility of the geometric phase design approach through nonlinear beam steering and structured-light generation on the harmonic field.

physics.optics↗

arXiv2026

Emission of time-ordered photon pairs from a coherently-driven Kerr microcavity

Weakly-interacting many-body systems possess remarkable quantum properties that are essential components of quantum technologies, and constitute a topic of fundamental interest. Here we show that in a solid-state nonlinear microcavity embedding discrete modes of exciton-dressed photons, we can isolate a single eigenmode of quantum fluctuations from the much brighter coherent fraction of the field. In this regime, we perform frequency- and time-resolved correlations measurements between photons on the red and blue side of the fluctuations spectrum. When the average number of fluctuation quanta is smaller than one, we observe the formation of large pairwise time-ordered correlations: red photon first and blue photon second. We show that this peculiar time-ordering correlation emerges spontaneously from the interplay between frequency-resolved detection, and the non-trivial internal quantum structure of the elementary fluctuations.

cond-mat.quant-gas↗

arXiv2025

Biphoton state generation and engineering with bright hybrid III-V/Silicon photonic devices

Hybrid photonic circuits, harnessing the complementary strengths of multiple materials, represent a key resource to enable compact, scalable platforms for quantum technologies. In particular, the availability of bright sources of tunable biphoton states is eagerly awaited to meet the variety of applications currently under development. In this work we demonstrate a heterogeneously integrated device that merges biphoton generation and on-chip quantum state engineering, combining an AlGaAs photon-pair source with a CMOS-compatible silicon-on-insulator (SOI) circuit. Photon pairs are generated in the C telecom band via spontaneous parametric down-conversion and transferred to the SOI chip through a multimode evanescent coupling scheme. This design achieves a pair generation rate above 10$^{6}$ s$^{-1}$mW$^{-1}$ and a coincidence-to-accidental ratio up to 600. Crucially, the coupling design induces strong and predictable transformations of the biphoton joint spectral amplitude, enabling complex quantum state engineering entirely on-chip in a compact device compliant with electrical pumping.

quant-ph↗

arXiv2025

Measuring non-Abelian quantum geometry and topology in a multi-gap photonic lattice

Recent discoveries in semi-metallic multi-gap systems featuring band singularities have galvanized enormous interest in particular due to the emergence of non-Abelian braiding properties of band nodes. This previously uncharted set of topological phases necessitates novel approaches to probe them in laboratories, a pursuit that intricately relates to evaluating non-Abelian generalizations of the Abelian quantum geometric tensor (QGT) that characterizes geometric responses. Here, we pioneer the direct measurement of the non-Abelian QGT. We achieve this by implementing a novel orbital-resolved polarimetry technique to probe the full Bloch Hamiltonian of a six-band two-dimensional (2D) synthetic lattice, which grants direct experimental access to non-Abelian quaternion charges, the Euler curvature, and the non-Abelian quantum metric associated with all bands. Quantum geometry has been highlighted to play a key role on macroscopic phenomena ranging from superconductivity in flat-bands, to optical responses, transport, metrology, and quantum Hall physics. Therefore, our work unlocks the experimental probing of a wide phenomenology of multi-gap systems, at the confluence of topology, geometry and non-Abelian physics.

cond-mat.mes-hall↗

arXiv2025

Benchmarking quantum key distribution by mixing single photons and laser light

Quantum key distribution is a key application of quantum mechanics, shaping the future of privacy and secure communications. Many protocols require single photons, often approximated by strongly attenuated laser pulses. Here, we harness the emission of a quantum dot embedded in a micropillar and explore a hybrid approach where the information is encoded on a mixture of single photons and laser pulses. We derive a phenomenological analysis of the configuration where both sources of light are mixed incoherently to perform the BB84 protocol, showing nearly perfect matching between theory and experiment. This provides a flexible technology compensating limited collected brightnesses of single-photon sources as well as a thorough investigation of single-photon statistics advantage scenarios over Poisson-distributed statistics. Explicitly, our model highlights an efficiency threshold for unconditional advantage of single photons over laser along with insights on the interplay between single-photon purity and collected brightness in the performances of BB84.

quant-ph↗

arXiv2025

Dispersion engineered AlGaAs-on-insulator nanophotonics by distributed feedback

Technological advances in the fabrication of nanophotonic circuits have driven the scientific community to increasingly focus on the precise tailoring of their key optical properties, over a broadband spectral domain. In this context, the modulation of the local refractive index can be exploited to customize an effective reflectivity by the use of distributed Bragg mirrors, enabling the on-chip integration of Fabry-P\'erot resonators. The resulting cavity length is strongly wavelength-dependent, offering practical solutions to the growing demand of dispersion engineering. Owing to their typically high core-to-cladding refractive index contrast and exceptional nonlinear properties, III-V semiconductor-based platforms represent promising candidates for the fabrication of Bragg reflectors. In this work, we propose an AlGaAs-on-insulator linear resonator based on distributed Bragg mirrors. We discuss the first experimental demonstration of a systematic, shape-constrained inverse design technique which tailors a prescribed dispersion profile, showing a strong agreement between simulations and measurements. In perspective, the proposed approach offers an efficient and general response to the challenge of dispersion engineering in integrated optical circuits.

physics.optics↗

arXiv2025

All-optical nonlinear phase modulation in open semiconductor microcavities

We report a significant advancement in ultra low power light-by-light phase modulation using open semiconductor microcavities in the strong light-matter coupling regime. We achieve cross-phase modulation of up to 247$\pm$17 mrad per particle between laser beams attenuated to single-photon average intensities. This breakthrough extends the potential for quantum information processing and nonlinear quantum optics in strongly coupled light-matter systems, setting a new benchmark in the field without relying on atom-like emitters. Our findings suggest promising new avenues for scalable quantum optical technologies.

physics.optics↗

arXiv2025

A sublattice Stokes polarimeter for bipartite photonic lattices

The concept of pseudo-spin provides a general framework for describing physical systems featuring two-component spinors, including light polarization, sublattice degrees of freedom in bipartite lattices, and valley polarization in 2D materials. In all cases, the pseudo-spin can be mapped to a Stokes vector on the Poincar\'e sphere. Stokes polarimeters for measuring the polarization of light are a powerful tool with a wide range of applications both in classical and quantum science. Generalizing Stokes polarimetry to other spinor degrees of freedom is thus a challenge of prime importance. Here, we introduce and demonstrate a Stokes polarimeter for the sublattice polarization in a bipartite photonic lattice. Our method relies on k-space photoluminescence intensity measurements under controlled phase shifts and attenuations applied independently to each sublattice. We implement our method using honeycomb arrays of coupled microcavities realizing photonic analogs of graphene and hexagonal boron nitride. Using our sublattice polarimeter, we reconstruct the Bloch modes in amplitude and phase across the Brillouin zone, achieving sub-linewidth precision in the determination of their eigenenergies, including near band touching points. This enables full access to the system Bloch Hamiltonian and quantum geometric tensor. Our approach can readily be extended to more complex systems with additional internal degrees of freedom, enabling experimental investigations of trigonal warping, Chern insulating phases, and Euler-class topology in multigap systems.

cond-mat.mes-hall↗

arXiv2025

Monitoring the generation of photonic linear cluster states with partial measurements

Quantum states of light with many entangled photons are key resources for photonic quantum computing and quantum communication. In this work, we exploit a highly resource-efficient generation scheme based on a linear optical circuit embedding a fibered delay loop acting as a quantum memory. The single photons are generated with a bright single-photon source based on a semiconductor quantum dot, allowing to perform the entangling scheme up to 6 photons. We demonstrate $2$, $3$, $4$ and $6$-photon entanglement generation at respective rates of $6$kHz, $120$Hz, $2.2$Hz, and $2$mHz, corresponding to an average scaling ratio of $46$. We introduce a method for real-time control of entanglement generation based on partially post-selected measurements. The visibility of such measurements carries faithful information to monitor the entanglement process, an important feature for the practical implementation of photonic measurement-based quantum computation.

quant-ph↗

arXiv2025

Optical and magnetic response by design in GaAs quantum dots

Quantum networking technologies use spin qubits and their interface to single photons as core components of a network node. This necessitates the ability to co-design the magnetic- and optical-dipole response of a quantum system. These properties are notoriously difficult to design in many solid-state systems, where spin-orbit coupling and the crystalline environment for each qubit create inhomogeneity of electronic g-factors and optically active states. Here, we show that GaAs quantum dots (QDs) obtained via the quasi-strain-free local droplet etching epitaxy growth method provide spin and optical properties predictable from assuming the highest possible QD symmetry. Our measurements of electron and hole g-tensors and of transition dipole moment orientations for charged excitons agree with our predictions from a multiband k.p simulation constrained only by a single atomic-force-microscopy reconstruction of QD morphology. This agreement is verified across multiple wavelength-specific growth runs at different facilities within the range of 730 nm to 790 nm for the exciton emission. Remarkably, our measurements and simulations track the in-plane electron g-factors through a zero-crossing from -0.1 to 0.3 and linear optical dipole moment orientations fully determined by an external magnetic field. The robustness of our results demonstrates the capability to design - prior to growth - the properties of a spin qubit and its tunable optical interface best adapted to a target magnetic and photonic environment with direct application for high-quality spin-photon entanglement.

quant-ph↗

arXiv2025

Excitonic oscillator-strength saturation dominates polariton-polariton interactions

Exciton-polaritons in semiconductor microcavities exhibit large two-body interactions that, thanks to ever refined nanotechnology techniques, are getting closer and closer to the quantum regime where single-photon nonlinearities start being relevant. To foster additional progress in this direction, in this work we experimentally investigate the microscopic mechanism driving polariton-polariton interactions. We measure the dispersion relation of the collective excitations that are thermally generated on top of a coherent fluid of interacting lower-polaritons. By comparing the measurements with the Bogoliubov theory over both the lower and upper polariton branches simultaneously, we find that polariton-polariton interactions stem dominantly from a mechanism of saturation of the exciton oscillator strength.

cond-mat.mes-hall↗

arXiv2024

Efficient fiber-pigtailed source of indistinguishable single photons

Semiconductor quantum dots in microcavities are an excellent platform for the efficient generation of indistinguishable single photons. However, their use in a wide range of quantum technologies requires their controlled fabrication and integration in compact closed-cycle cryocoolers, with a key challenge being the efficient and stable extraction of the single photons into a single-mode fiber. Here we report on a novel method for fiber-pigtailing of deterministically fabricated single-photon sources. Our technique allows for nanometer-scale alignment accuracy between the source and a fiber, alignment that persists all the way from room temperature to 2.4 K. We demonstrate high performance of the device under near-resonant optical excitation with g$^{(2)}$(0) = 1.3 %, a photon indistinguishability of 97.5 % and a fibered brightness of 20.8 %. We show that the indistinguishability and single-photon rate are stable for over ten hours of continuous operation in a single cooldown. We further confirm that the device performance is not degraded by nine successive cooldown-warmup cycles.

quant-ph↗

arXiv2024

Experimental quantum triangle network nonlocality with an AlGaAs multiplexed entangled photon source

The exploration of the concept of nonlocality beyond standard Bell scenarios in quantum network architectures unveils fundamentally new forms of correlations that hold a strong potential for future applications of quantum communication networks. To materialize this potential, it is necessary to adapt theoretical advances to realistic configurations. Here we consider a quantum triangle network, for which is was shown in theory that, remarkably, quantum nonlocality without inputs can be demonstrated for sources with an arbitrarily small level of independence. We realize experimentally such correlated sources by carefully engineering the output state of a single AlGaAs multiplexed entangled-photon source, exploiting energy-matched channels cut in its broad spectrum. This simulated triangle network is then used to violate experimentally for the first time a Bell-like inequality that we derive to capture the effect of noise in the correlations present in our system. We also rigorously validate our findings by analysing the mutual information between the generated states. Our results allow us to deepen our understanding of network nonlocality while also pushing its practical relevance for quantum communication networks.

quant-ph↗

arXiv2024

Energy-efficient picosecond spin-orbit torque magnetization switching in ferro- and ferrimagnetic films

Electrical current pulses can be used to manipulate magnetization efficiently via spin-orbit torques (SOTs). Pulse durations as short as a few picoseconds have been used to switch the magnetization of ferromagnetic films, reaching the THz regime. However, little is known about the reversal mechanisms and energy requirements in the ultrafast switching regime. In this work, we quantify the energy cost for magnetization reversal over 7 orders of magnitude in pulse duration, in both ferromagnetic and ferrimagnetic samples, bridging quasi-static spintronics and femtomagnetism. To this end, we develop a method to stretch picosecond pulses generated by a photoconductive switch by an order of magnitude. Thereby, we can create current pulses from picoseconds to durations approaching pulse width available with commercial instruments. We show that the energy cost for SOT switching decreases by more than an order of magnitude in all samples when the pulse duration enters the picosecond range. We project an energy cost of 9 fJ for a 100 x 100 nm 2 ferrimagnetic device. Micromagnetic and macrospin simulations unveil a transition from a non-coherent to a coherent magnetization reversal with a strong modification of the magnetization dynamical trajectories as pulse duration is reduced. Our results cement the potential for high-speed magnetic spin-orbit torque memories and highlights alternative magnetization reversal pathways at fast time scales.

cond-mat.mtrl-sci↗

arXiv2023

Elliptical micropillars for efficient generation and detection of coherent acoustic phonons

Coherent acoustic phonon generation and detection assisted by optical resonances are at the core of efficient optophononic transduction processes. However, when dealing with a single optical resonance, the optimum generation and detection conditions take place at different laser wavelengths, i.e. different detunings from the cavity mode. In this work, we theoretically propose and experimentally demonstrate the use of elliptical micropillars to reach these conditions simultaneously at a single wavelength. Elliptical micropillar optophononic resonators present two optical modes with orthogonal polarizations at different wavelengths. By employing a cross-polarized scheme pump-probe experiment, we exploit the mode splitting and couple the pump beam to one mode while the probe is detuned from the other one. In this way, at a particular micropillar ellipticity, both phonon generation and detection processes are enhanced. We report an enhancement of a factor of ~3.1 when comparing the signals from elliptical and circular micropillars. Our findings constitute a step forward in tailoring the light-matter interaction for more efficient ultrahigh-frequency optophononic devices.

physics.optics↗