arXiv subjects

M. Morassi

Publications and source records attributed to M. Morassi.

15 recordsLinked to original sources

arXiv2026

Crossing over universal scaling laws in two-dimensional driven dissipative condensates

In low dimensional systems, fluctuations are enhanced and prevent the spontaneous breaking of continuous symmetries. As a result, spatial and temporal correlation functions decay at large distances and long times. A well established example is given by two-dimensional bosonic condensates at equilibrium, which do not display long-range order of the coherence but algebraic decay belonging to the Berezinski-Kosterlitz-Thouless universality class. In contrast, the universal behaviors of non-equilibrium bosonic condensates are more diverse and many open questions remain. Here, we explore the spatio-temporal coherence properties of two-dimensional driven-dissipative polariton condensates in semiconductor optical microcavities. By tuning microscopic parameters, we observe a cross-over between two scaling laws that we attribute to the Edwards-Wilkinson (EW) and the Kardar-Parisi-Zhang (KPZ) universality classes. We demonstrate the collapse of the measured first-order correlations onto the EW and KPZ universal scaling functions and obtain critical exponents, well matching the values predicted theoretically. Our results highlight the intrinsic non-equilibrium nature of polariton condensates and establish them as a platform of choice for controlled exploration of the two-dimensional KPZ universality class.

cond-mat.quant-gas↗

arXiv2026

Dynamical decoupling of a quantum dot spin in a micropillar cavity for spin-multiphoton entanglement

Graph states of mutually entangled photons are key resources for quantum computation and communication and can be generated by leveraging the entanglement between a single resident spin and emitted photons from a charged semiconductor quantum dot (QD). This approach is intrinsically limited by the decoherence of the spin. We study how to mitigate this decoherence with dynamical decoupling of an electron spin in the weak transverse magnetic field regime using spin echo and Carr-Purcell-Meiboom-Gill (CPMG) techniques. Application of these techniques allows us to extend the coherence time of a spin by more than two orders of magnitude, extracting a $T_2^{CPMG}$ of $298\pm53$ ns. We further demonstrate that this technique is compatible with the generation of a spin-photon-photon entangled state at a high rate enabled by a micropillar cavity, with a 20% improvement in simulated state fidelity when using dynamical decoupling. These results pave the way for generating larger and more complex entangled states with QDs.

quant-ph↗

arXiv2026

Coupled Topological Interface States and Phonon Molecules in GaAs/AlAs Superlattices

Topological interface states in one-dimensional superlattices provide spatially localized phonon modes protected by the topology of the underlying band structure. In GaAs/AlAs distributed Bragg reflectors (DBRs), such states can be engineered through band inversion between superlattices with opposite Zak phases within the Su-Schrieffer-Heeger (SSH) framework. Here, we demonstrate topological phonon molecules and extended chains formed by coupled nanophononic interface states. By concatenating three superlattices with alternating topology, we realize two coupled interface states that hybridize into symmetric and antisymmetric modes, whose splitting can be tuned over tens of gigahertz by varying the reflectivity of the central DBR. Extending this concept, we engineer chains of up to N=6 coupled interface states that form narrow topological minibands while remaining strongly localized at the interfaces. We experimentally observe these coupled states in molecular-beam-epitaxy-grown GaAs/AlAs heterostructures using time-domain pump-probe transient reflectivity measurements, and reproduce their behavior using transfer-matrix calculations and a simple analytical model for the mode splitting. These results establish topological interface states as a robust platform for engineering coupled phononic systems and tunable nanophononic architectures in the GHz regime.

cond-mat.mes-hall↗

arXiv2025

Tailoring quantum walks in integrated photonic lattices

Unlike discrete photonic circuits, which manipulate photons step-by-step using a series of optical elements, arrays of coupled waveguides enable photons to interfere continuously across the entire structure. When composed of a nonlinear material, such arrays can also directly generate quantum states of light within the circuit. To clarify the similarities and distinctions between these two approaches of quantum walks, we conduct here a systematic comparison between linear waveguide arrays, injected with photons produced externally, and nonlinear arrays, where photon pairs are continuously generated via parametric down-conversion. We experimentally validate these predictions using III-V semiconductor nonlinear waveguide lattices with varied geometries, enabling us to tune the depth of the quantum walks over an order of magnitude and reveal the gradual emergence of non-classicality in the output state. Finally, we demonstrate an inverse-design approach to engineer \textit{aperiodic} waveguide arrays, whose optimized coupling profiles generate maximally entangled states such as the biphoton W-state. These results highlight the potential of continuously-coupled photonic systems to harness high-dimensional entanglement within compact architectures.

quant-ph↗

arXiv2025

Topological protection of photon-pair generation in nonlinear waveguide arrays

Harnessing topological effects offers a promising route to protect quantum states of light from imperfections, potentially enabling more robust platforms for quantum information processing. This capability is particularly relevant for active photonic circuits that generate quantum light directly on-chip. Here, we explore topological effects on photon-pair generation via spontaneous parametric down-conversion (SPDC) in nonlinear waveguide arrays, both theoretically and experimentally. A systematic comparison of homogeneous, trivial, and topological Su-Schrieffer-Heeger arrays reveals that only the topological configuration preserves a stable SPDC resonance spectrum under disorder in the tunnel couplings, with fluctuations in the resonance position reduced by more than one order of magnitude. An analytical model supports our experimental observations by linking this robustness to the band-structure properties of the interacting modes. These findings establish quadratic nonlinear waveguide arrays as a promising platform to explore the interplay of nonlinearity, topology, and disorder in quantum photonic circuits.

quant-ph↗

arXiv2025

The impact of hole $g$-factor anisotropy on spin-photon entanglement generation with InGaAs quantum dots

Self-assembled InGaAs/GaAs quantum dots (QDs) are of particular importance for the deterministic generation of spin-photon entanglement. One promising scheme relies on the Larmor precession of a spin in a transverse magnetic field, which is governed by the in-plane $g$-factors of the electron and valence band heavy-hole. We probe the origin of heavy-hole $g$-factor anisotropy with respect to the in-plane magnetic field direction and uncover how it impacts the entanglement generated between the spin and the photon polarization. First, using polarization-resolved photoluminescence measurements on a single QD, we determine that the impact of valence-band mixing dominates over effects due to a confinement-renormalized cubic Luttinger $q$ parameter. From this, we construct a comprehensive hole $g$-tensor model. We then use this model to simulate the concurrence and fidelity of spin-photon entanglement generation with anisotropic hole $g$-factors, which can be tuned via magnetic field angle and excitation polarization. The results demonstrate that post-growth control of the hole $g$-factor can be used to improve spin-photon cluster state generation.

cond-mat.mes-hall↗

arXiv2024

Deterministic and reconfigurable graph state generation with a single solid-state quantum emitter

Measurement-based quantum computing offers a promising route towards scalable, universal photonic quantum computation. This approach relies on the deterministic and efficient generation of photonic graph states in which many photons are mutually entangled with various topologies. Recently, deterministic sources of graph states have been demonstrated with quantum emitters in both the optical and microwave domains. In this work, we demonstrate deterministic and reconfigurable graph state generation with optical solid-state integrated quantum emitters. Specifically, we use a single semiconductor quantum dot in a cavity to generate caterpillar graph states, the most general type of graph state that can be produced with a single emitter. By using fast detuned optical pulses, we achieve full control over the spin state, enabling us to vary the entanglement topology at will. We perform quantum state tomography of two successive photons, measuring Bell state fidelities up to 0.80$\pm$0.04 and concurrences up to 0.69$\pm$0.09, while maintaining high photon indistinguishability. This simple optical scheme, compatible with commercially available quantum dot-based single photon sources, brings us a step closer to fault-tolerant quantum computing with spins and photons.

quant-ph↗

arXiv2024

Interference of ultrahigh frequency acoustic phonons from distant quasi-continuous sources

The generation of propagating acoustic waves is essential for telecommunication applications, quantum technologies, and sensing. Up to now, the electrical generation has been at the core of most implementations, but is technologically limited to a few gigahertz. Overcoming this frequency limit holds the prospect of faster modulators, quantum acoustics at higher working temperatures, nanoacoustic sensing from smaller volumes. Alternatively, the optical excitation of acoustic resonators has unlocked frequencies up to 1 THz, but in most cases, the acoustic energy cannot be efficiently extracted from the resonator into a propagating wave. Here, we demonstrate a quasi-continuous and coherent source of 20 GHz acoustic phonons, based on a ridge waveguide, structured in the vertical direction as a high-Q acousto-optic resonator. The high frequency phonons propagate up to 20 $\mu$m away from the source, with a decay rate of $\sim$1.14 dB/$\mu$m. We demonstrate the coherence between acoustic phonons generated from two distant sources through spatio-temporal interference. This concept could be scaled up to a larger number of sources, which enable a new generation of optically programmed, reconfigurable nanoacoustic devices and applications.

cond-mat.mes-hall↗

arXiv2024

Tunable Generation of Spatial Entanglement in Nonlinear Waveguide Arrays

Harnessing high-dimensional entangled states of light presents a frontier for advancing quantum information technologies, from fundamental tests of quantum mechanics to enhanced computation and communication protocols. In this context, the spatial degree of freedom stands out as particularly suited for on-chip integration. But while traditional demonstrations produce and manipulate path-entangled states sequentially with discrete optical elements, continuously-coupled nonlinear waveguide systems offer a promising alternative where photons can be generated and interfere along the entire propagation length, unveiling novel capabilities within a reduced footprint. Here we exploit this concept to implement a compact and reconfigurable source of path-entangled photon pairs based on parametric down-conversion in semiconductor nonlinear waveguides arrays. We use a double-pump configuration to engineer the output quantum state and implement various types of spatial correlations, exploiting a quantum interference effect between the biphoton state generated in each pumped waveguide. This demonstration, at room temperature and telecom wavelength, illustrates the potential of continuously-coupled systems as a promising alternative to discrete multi-component quantum circuits for leveraging the high-dimensional spatial degree of freedom of photons.

quant-ph↗

arXiv2023

Hybrid III-V/Silicon Quantum Photonic Device Generating Broadband Entangled Photon Pairs

The demand for integrated photonic chips combining the generation and manipulation of quantum states of light is steadily increasing, driven by the need for compact and scalable platforms for quantum information technologies. While photonic circuits with diverse functionalities are being developed in different single material platforms, it has become crucial to realize hybrid photonic circuits that harness the advantages of multiple materials while mitigating their respective weaknesses, resulting in enhanced capabilities. Here, we demonstrate a hybrid III-V/Silicon quantum photonic device combining the strong second-order nonlinearity and direct bandgap of the III-V semiconductor platform with the high maturity and CMOS compatibility of the silicon photonic platform. Our device embeds the spontaneous parametric down-conversion (SPDC) of photon pairs into an AlGaAs source and their vertical routing to an adhesively-bonded silicon-on-insulator circuitry, within an evanescent coupling scheme managing both polarization states. This enables the on-chip generation of broadband (> 40 nm) telecom photons by type 0 and type 2 SPDC from the hybrid device, at room temperature and with internal pair generation rates exceeding $10^5$ $s^{-1}$ for both types, while the pump beam is strongly rejected. Two-photon interference with 92% visibility (and up to 99% upon 5 nm spectral filtering) proves the high energy-time entanglement quality of the produced quantum state, thereby enabling a wide range of quantum information applications on-chip, within an hybrid architecture compliant with electrical pumping and merging the assets of two mature and highly complementary platforms in view of out-of-the-lab deployment of quantum technologies.

physics.optics↗

arXiv2023

Spin-momentum locking and ultrafast spin-charge conversion in ultrathin epitaxial Bi$_{1-x}$Sb$_x$ topological insulator

The helicity of 3D topological insulator surface states has drawn significant attention in spintronics owing to spin-momentum locking where the carriers' spin is oriented perpendicular to their momentum. This property can provide an efficient method to convert charge currents into spin currents, and vice-versa, through the Rashba-Edelstein effect. However, experimental signatures of these surface states to the spin-charge conversion are extremely difficult to disentangle from bulk state contributions. Here, we combine spin- and angle-resolved photo-emission spectroscopy, and time-resolved THz emission spectroscopy to categorically demonstrate that spin-charge conversion arises mainly from the surface state in Bi$_{1-x}$Sb$_x$ ultrathin films, down to few nanometers where confinement effects emerge. We correlate this large conversion efficiency, typically at the level of the bulk spin Hall effect from heavy metals, to the complex Fermi surface obtained from theoretical calculations of the inverse Rashba-Edelstein response. %We demonstrate this for film thickness down to a few nanometers, Both surface state robustness and sizeable conversion efficiency in epitaxial Bi$_{1-x}$Sb$_x$ thin films bring new perspectives for ultra-low power magnetic random-access memories and broadband THz generation.

cond-mat.mtrl-sci↗

arXiv2022

High-rate entanglement between a semiconductor spin and indistinguishable photons

Photonic graph states, quantum light states where multiple photons are mutually entangled, are key resources for optical quantum technologies. They are notably at the core of error-corrected measurement-based optical quantum computing and all-optical quantum networks. In the discrete variable framework, these applications require high efficiency generation of cluster-states whose nodes are indistinguishable photons. Such photonic cluster states can be generated with heralded single photon sources and probabilistic quantum gates, yet with challenging efficiency and scalability. Spin-photon entanglement has been proposed to deterministically generate linear cluster states. First demonstrations have been obtained with semiconductor spins achieving high photon indistinguishablity, and most recently with atomic systems at high collection efficiency and record length. Here we report on the efficient generation of three partite cluster states made of one semiconductor spin and two indistinguishable photons. We harness a semiconductor quantum dot inserted in an optical cavity for efficient photon collection and electrically controlled for high indistinguishability. We demonstrate two and three particle entanglement with fidelities of 80 % and 63 % respectively, with photon indistinguishability of 88%. The spin-photon and spin-photon-photon entanglement rates exceed by three and two orders of magnitude respectively the previous state of the art. Our system and experimental scheme, a monolithic solid-state device controlled with a resource efficient simple experimental configuration, are very promising for future scalable applications.

quant-ph↗

arXiv2022

Probing the dynamics and coherence of a semiconductor hole spin via acoustic phonon-assisted excitation

Spins in semiconductor quantum dots are promising local quantum memories to generate polarization-encoded photonic cluster states, as proposed in the pioneering Rudolph-Lindner scheme [1]. However, harnessing the polarization degree of freedom of the optical transitions is hindered by resonant excitation schemes that are widely used to obtain high photon indistinguishability. Here we show that acoustic phonon-assisted excitation, a scheme that preserves high indistinguishability, also allows to fully exploit the polarization selective optical transitions to initialise and measure single spin states. We access the coherence of hole spin systems in a low transverse magnetic field and directly monitor the spin Larmor precession both during the radiative emission process of an excited state or in the quantum dot ground state. We report a spin state detection fidelity of $94.7 \pm 0.2 \%$ granted by the optical selection rules and a $20\pm5$~ns hole spin coherence time, demonstrating the potential of this scheme and system to generate linear cluster states with a dozen of photons

cond-mat.mes-hall↗

arXiv2021

Optically defined cavities in driven-dissipative photonic lattices

The engineering of localised modes in photonic structures is one of the main targets of modern photonics. An efficient strategy to design these modes is to use the interplay of constructive and destructive interference in periodic photonic lattices. This mechanism is at the origin of defect modes in photonic bandgaps, bound states in the continuum and compact localised states in flat bands. Here we show that in lattices of lossy resonators, the addition of external optical drives with controlled phase enlarges the possibilities of manipulating interference effects and allows designing novel types of localised modes. Using a honeycomb lattice of coupled micropillars resonantly driven with several laser spots at energies within its photonic bands we demonstrate the localisation of light in at-will geometries down to a single site. These localised modes can be seen as fully reconfigurable optical cavities with the potentiality of enhancing nonlinear effects and of controlling light-matter interactions with single site resolution.

physics.optics↗

arXiv2021

Topological gap solitons in a 1D non-Hermitian lattice

Nonlinear topological photonics is an emerging field aiming at extending the fascinating properties of topological states to the realm where interactions between the system constituents cannot be neglected. Interactions can indeed trigger topological phase transitions, induce symmetry protection and robustness properties for the many-body system. Moreover when coupling to the environment via drive and dissipation is also considered, novel collective phenomena are expected to emerge. Here, we report the nonlinear response of a polariton lattice implementing a non-Hermitian version of the Su-Schrieffer-Heeger model. We trigger the formation of solitons in the topological gap of the band structure, and show that these solitons demonstrate robust nonlinear properties with respect to defects, because of the underlying sub-lattice symmetry. Leveraging on the system non-Hermiticity, we engineer the drive phase pattern and unveil bulk solitons that have no counterpart in conservative systems. They are localized on a single sub-lattice with a spatial profile alike a topological edge state. Our results demonstrate a tool to stabilize the nonlinear response of driven dissipative topological systems, which may constitute a powerful resource for nonlinear topological photonics.

cond-mat.mes-hall↗