arXiv subjects

Marco Liscidini

Publications and source records attributed to Marco Liscidini.

At least 19 recordsLinked to original sources

arXiv2026

Coexistence of polariton bound states in the continuum and 2D radiative excitons

Bound states in the continuum (BICs) enable optical modes with ideally infinite radiative lifetimes despite lying within the radiation continuum. Radiation-matter interaction in periodically patterned planar waveguides embedding two-dimensional (2D) optically active excitations can be described quantum mechanically by diagonalizing a non-Hermitian operator, known as the Hopfield matrix, which can be generalized to incorporate independent photonic and excitonic losses into the polaritonic states. However, since 2D excitons undergo intrinsic wavevector-dependent radiative decay within the light cone, whether the Hopfield formalism can consistently account for this process while preserving polariton BICs has remained an open question. Here we show that a non-Hermitian Hopfield formalism incorporating excitonic radiative losses correctly captures the existence of genuine $k=0$ polariton BICs with diverging radiative lifetimes. The theory provides a unified microscopic framework for radiative excitons and polariton BICs, and an efficient predictive tool for designing photonic-crystal platforms coupled to quantum wells, transition-metal dichalcogenides, and other 2D excitonic materials.

cond-mat.mes-hall↗

arXiv2026

Quantum work extraction from partial information and with finite resources

Information can be converted into work, but in quantum mechanics information about a state is not freely available: it must be inferred statistically from measurements on a finite number of copies. We study work extraction in this finite-resource setting by introducing a partial-information and finite-resources (PIFR) quantum Maxwell's demon. Given $N$ identical copies of a state with known Hamiltonian, the demon measures $M$ copies to estimate the state and the corresponding ergotropic unitary, which is then applied to the remaining $N-M$ copies. This protocol induces a trade-off between information acquisition, reconstruction accuracy, and thermodynamic yield, making the total extracted work normalized to the ideal ergotropic benchmark the relevant figure of merit. As our central result, we derive a universal closed-form trade-off bound that places this ergotropic efficiency between a Carnot-type ceiling $1-M/N$ and a floor controlled by a reconstruction precision rooted in finite-sample quantum estimation theory; optimizing the copy allocation yields $M^*\propto N^{2/3}$ and an $N^{-1/3}$ approach to the ideal limit, set by a conservative, worst-case reconstruction precision. By considering standard quantum state tomography, we numerically verify the presence of an optimal resource distribution, which also depends on the purity of the state under consideration. Our results identify finite-copy work extraction as a genuinely task-dependent inference problem, in which estimation strategies should be judged by thermodynamic performance rather than reconstruction fidelity alone.

quant-ph↗

arXiv2026

Harnessing bound states in the continuum for quantum and nonlinear photonics in silicon nitride microresonators

In this work we theoretically and experimentally investigate bound states in the continuum (BICs) in a reconfigurable integrated photonic structure, focusing on spontaneous four-wave mixing. We consider a configuration in which only one mode, the idler, is tuned to the BIC condition, and show that suppressing the radiative loss of this single mode profoundly reshapes the overall nonlinear dynamics. In particularly, we realize electrically tunable Friedrich-Wintgen BICs with Q > 10^6 in an interferometrically coupled silicon-nitride microresonator. Although inaccessible through linear excitation, the BIC is populated by four-wave mixing and read out through its radiative photon partner. In the low-gain regime, we investigate the spectral, correlation, and coherence properties of the generated photon pairs, revealing distinctive signatures in the signal-idler correlations and an extracted signal whose coherence time approaches the intrinsic-loss-limited lifetime of the dark idler. In the high-gain regime, making one mode participating in optical parametric amplification a BIC enhances the parametric gain by 12 dB and lowers the oscillation threshold by 1.6 dB. These results establish BICs as a tool for mode-selective lifetime engineering, enabling control over both biphoton wavepackets and the threshold dynamics of integrated parametric devices.

physics.optics↗

arXiv2026

A Universal All-Fiber Quantum Buffer for the Telecom Band

The realization of a scalable quantum internet relies on the ability to temporally align asynchronous photonic signals through on-demand buffering. While matter-based quantum memories achieve long storage times, their extremely narrow bandwidths and cryogenic requirements pose significant barriers to integration with existing telecommunications infrastructure. Conversely, current all-optical memories operate at room temperature but are hampered by high input/output losses and a lack of universality across different photonic degrees of freedom. Here, we demonstrate a universal, fully fiber-integrated quantum buffer operating over the full telecom C-band that overcomes these fundamental trade-offs. By implementing an actively switched dual-Sagnac cavity driven by cross-phase modulation, we achieve an ultra-low input/output loss of 0.46 dB and a storage time exceeding 18 $\mu$s. The device exhibits an operational bandwidth exceeding 12.5 THz ($\sim$100 nm), covering the full telecom C-band. We show the simultaneous buffering of over 200 temporal modes with the ability to address them either collectively or one by one. We demonstrate high-fidelity storage for all three degrees of freedom compatible with optical fiber propagation, namely time-bin, frequency-bin, and polarization qubits, along with faithful preservation of entanglement, confirming the platform's true universality. These results provide a robust, room-temperature solution for the high-rate synchronization of multidimensional quantum states, clearing a major hurdle for the deployment of global photonic quantum networks.

quant-ph↗

arXiv2026

Quantum inference on a classically trained quantum extreme learning machine

Quantum extreme learning machines (QELMs) are unconventional computing architectures that bear remarkable promise in both classical and quantum machine-learning tasks, such as the estimation of quantum state properties. However, the probabilistic nature of quantum measurements demands extensive repetitions for training to precisely estimate expectation values, imposing stringent trade-offs among experimental resources, acquisition time, and signal-to-noise ratio, particularly for large datasets. Here, we introduce a paradigm shift by training the QELM exclusively with intense classical fields, namely twin beams generated by stimulated emission, while performing inference directly on previously unseen genuine quantum input states to predict their quantum properties. This strategy dramatically reduces acquisition times while substantially enhancing the signal-to-noise ratio. Using frequency-bin encoded biphoton states, implemented here for the first time in a quantum machine learning architecture, we demonstrate entanglement witnessing of two-qubit states with (93 +- 4)% accuracy, multi-dimensional entanglement detection, and learning of the Hamiltonian governing photon-pair generation with a fidelity of (96 +- 4)%. Our results open a new pathway toward faster and more robust training of photonic quantum extreme learning machines for quantum feature extraction.

quant-ph↗

arXiv2026

Compact linearly uncoupled resonators for efficient spontaneous parametric downconversion via angular phase matching

We report an integrated platform for efficient second-order nonlinear interactions based on linearly uncoupled resonators and angular phase matching. The proposed architecture confines phase control to a limited section of the device, maximizing field enhancement and effective nonlinear interaction length while simultaneously reducing the overall footprint. As an example we show the results for an AlGaAs-on-insulator structure demonstrating a photon-pair generation rate of 3.16 GHz/mW in the continuous-wave regime and 5.89 MHz under pulsed pumping. The generated biphoton state exhibits a Schmidt number K=1.02, indicating nearly uncorrelated photon pairs. The compact and reconfigurable nature of this approach, together with its independence from material-specific poling techniques, makes it applicable to a broad class of integrated $\chi_2$ nonlinear platforms.

physics.optics↗

arXiv2026

Coherent control of photon pairs via quantum interference between second- and third-order quantum nonlinear processes

Genuine quantum interference between independent nonlinear processes of different order provides a route to coherent control that cannot be reduced to a classical field interference. Here we present an all-optical analogue of coherent carrier injection by exploiting interference between second- and third-order quantum nonlinear processes in an integrated photonic platform. Photon pairs generated via spontaneous parametric down-conversion and spontaneous four-wave mixing coherently contribute to the same final two-photon state, resulting in a phase-dependent modulation of both the generation rate and the spectral structure of the emitted biphoton state. We illustrate the features of such interference and how it can be used to shape biphoton wavefunctions and their quantum correlations. These results identify interference between nonlinear processes of different order as a distinct form of coherent quantum control within quantum nonlinear optics.

physics.optics↗

arXiv2025

Tunable integrated ring resonators by femtosecond laser micromachining

Femtosecond Laser Micromachining (FLM) is a powerful technology for the fabrication of photonic devices. In this context, the integration of resonant elements within the platform represents a key advancement, enhancing both its versatility and its compatibility with a wide range of optical and fluidic components specifically enabled by this technique. Here, we report the realization of a tunable racetrack resonator fabricated by FLM and operating at telecom wavelengths. Leveraging low-loss waveguides, we obtained a Q factor of the resonator as high as 8 x 10^5 at critical coupling. Moreover, by integrating two thermo-optic phase shifters, we achieved both resonance tuning and dynamic control of the Q factor. This capability makes the device highly versatile for applications requiring dynamic spectral control, such as tunable filters, gyroscopes, and sensors.

physics.optics↗

arXiv2025

Photon triplets from integrated microrings: A path towards deterministic non-Gaussianity on a chip

We propose cascaded spontaneous four-wave mixing (SFWM) in microring resonators as a scalable and efficient approach for directly generating non-Gaussian states of light. Focusing on the well-understood "low-gain" regime, we demonstrate that triplet generation through cascaded SFWM can be achieved with high efficiency and favorable spectral characteristics using realistic microring sources in AlGaAs. The ability to achieve the generation of light in a single set of supermodes -- and the predicted accessibility of the "high-gain" regime at realistic pump powers -- makes this source a promising candidate as a direct and deterministic source of non-Gaussian light for photonic quantum information processing.

quant-ph↗

arXiv2025

Time-resolved characterization of pulsed squeezed light from a strongly driven silicon nitride microresonator

Silicon nitride microresonators driven by strong pump pulses can generate squeezed light in a dominant spectral-temporal mode, a central resource for continuous-variable quantum computation. In the high parametric gain regime, several effects, including self- and cross-phase modulation as well as time-ordering corrections, become significant and can degrade source performance. In this work, we comprehensively investigate the generation of squeezed light from a silicon nitride resonator under pulsed pumping, spanning from low to high parametric gain up to 16 photons/pulse. We experimentally study how the average photon number and the first- and second- order correlations of the squeezed marginal modes evolve with increasing pulse energy, across various frequency detunings and pulse durations. Furthermore, we analyze the errors introduced by multi-pair emissions in estimating the joint temporal intensity via time-resolved coincidence measurements. We propose and demonstrate an error-correction strategy based on the marginal distributions of time-resolved multi-photon events. Our results provide a practical strategy for optimizing the gain and the temporal mode structure of pulsed squeezed light sources in microresonators, elucidating the physical mechanisms and limitations that govern source performance in the high gain regime.

quant-ph↗

arXiv2025

An asymptotic field approach for the control of dipole emission in integrated structures

We present a general framework to model spontaneous emission in integrated photonic structures by exploiting quantization of the electromagnetic field in terms of asymptotic in/out modes. This approach allows for an efficient and physically meaningful calculation of the emission rate into each radiative channel of an arbitrary structure, without relying on approximations such as Lorentzian lineshapes or point-like system-bath coupling. We show that with this approach one can recover well-known results for dipole emission in waveguides or ring resonators, and that such results can be easily extended to include the effect of backscattering. Finally, as an application, we design a tunable integrated single-photon source that enables full control over both the emission rate and output mode. This flexibility makes our method particularly well-suited for the design and analysis of integrated single-photon sources in various material platforms.

quant-ph↗

arXiv2025

Generation of hyperentangled photon pairs in the time and frequency domain on a silicon photonic chip

Multi-dimensional entangled photon states represent an important resource in quantum communication networks. Specifically, hyperentangled states presenting simultaneous entanglement in several degrees of freedom (DoF), stand out for their noise resilience and information capacity. In this work, we demonstrate the generation of hyperentangled photon pairs in the time and frequency-bin domain by spontaneous four-wave mixing from the coherent driving of two integrated Silicon microresonators. We demonstrate entanglement in each DoF by proving the violation of the Clauser Horne Shimony Holt (CHSH) inequality by more than 27 standard deviations (STDs) in each reduced space. Genuine hyperentanglement is then assessed from the negativity of an hyperentanglement witness, which is verified by more than 60 STDs. These results mark, to the best of our knowledge, the first demonstration of time-frequency bin hyperentanglement in an integrated silicon photonic device.

quant-ph↗

arXiv2025

Experimental verification of Threshold Quantum State Tomography on a fully-reconfigurable photonic integrated circuit

Reconstructing the state of a complex quantum system represents a pivotal task for all quantum information applications, both for characterization purposes and for verification of quantum protocols. Recent technological developments have shown the capability of building quantum systems with progressively larger number of qubits in different platforms. The standard approach based on quantum state tomography, while providing a method to completely characterize an unknown quantum state, requires a number of measurements that scales exponentially with the number of qubits. Other methods have been subsequently proposed and tested to reduce the number of measurements, or to focus on specific properties of the output state rather than on its complete reconstruction. Here, we show experimentally the application of an approach, called threshold quantum state tomography, in an advanced hybrid photonic platform with states up to n=4 qubits. This method does not require a priori knowledge on the state, and selects only the informative projectors starting from the measurement of the density matrix diagonal. We show the effectiveness of this approach in a photonic platform, showing that a consistent reduction in the number of measurement is obtained while reconstructing relevant states for quantum protocols, with only very limited loss of information. The advantage of this protocol opens perspective of its application in larger, more complex, systems.

quant-ph↗

arXiv2025

Modelling and optimization of pulsed squeezed state generation in a ring-resonator system

We present a semi-analytic formalism for calculating the squeezing and antisqueezing spectrum in a channel waveguide side-coupled to a lossy ring resonator. Our approach first uses the semi-analytic evolution of the density matrix inside the ring up to the time where the squeezing is maximized. Then noting that a conservative approximate result for the squeezing can be obtained by ignoring the effect of the pump at later times, we calculate the free evolution of the field operators in the channel waveguide at all later times. We then calculate the quadrature squeezing spectrum in the waveguide, assuming that the measurement starts at the time when the squeezing in the ring is maximized. Using these results, we determine the optimum values for the pump pulse duration and amplitude and the ring-channel coupling for the pump and signal. We find that squeezing above 10 dB can be easily achieved in the channel for antisqueezing levels of less than 22 dB.

quant-ph↗

arXiv2025

A Permutation-equivariant Deep Learning Model for Quantum State Characterization

The characterization of quantum states is a fundamental step of any application of quantum technologies. Nowadays there exist several approaches addressing this problem, also based on machine and deep learning techniques. However, all these approaches usually require a number of measurement that scales exponentially with the number of parties composing the system. Threshold quantum state tomography (tQST) addresses this problem and, in some cases of interest, can significantly reduce the number of measurements. In this paper, we study how to combine a permutation-equivariant deep learning model with the tQST protocol. We test the model on quantum state tomography and purity estimation. Finally, we validate the robustness of the model to noise. We show results up to 4 qubits.

quant-ph↗

arXiv2025

Enhanced Compressive Threshold Quantum State Tomography for Qudit Systems

We propose an efficient quantum state tomography method inspired by compressed sensing and threshold quantum state tomography that can drastically reduce the number of measurement settings to reconstruct the density matrix of an $N$-qudit system. We validate our algorithm with simulations on IBMQ and demonstrate the efficient and accurate reconstruction of $N\leq7$ qubit systems, reproducing GHZ, $W$, and random states with $O(1)$, $O(N^2)$, and $O(N)$ settings.

quant-ph↗

arXiv2025

Quantum interference with time-frequency modes and multiple-photons generated by a silicon nitride microresonator

We demonstrate bipartite gaussian boson sampling with squeezed light in 6 mixed time-frequency modes. Non-degenerate two-mode squeezing is generated in two time-bins from a silicon nitride microresonator with simultaneous high spectral purity (>0.86(3)) and indistinguishability (0.985(2)). An unbalanced interferometer embedding electro-optic modulators, which is stabilized by exploiting the continuous energy-time entanglement of the generated photon pairs, controls time and frequency-bin modes. We measure 144 collision-free events with 4 photons at the output, achieving a fidelity >0.98 with the theoretical probability distribution. We use this result to identify the similarity between families of isomorphic graphs with 6 vertices, and present an approach for the realization of universal operations on time-frequency modes.

quant-ph↗

arXiv2024

A Versatile Chip-Scale Platform for High-Rate Entanglement Generation using an AlGaAs Microresonator Array

Integrated photonic microresonators have become an essential resource for generating photonic qubits for quantum information processing, entanglement distribution and networking, and quantum communications. The pair generation rate is enhanced by reducing the microresonator radius, but this comes at the cost of increasing the frequency mode spacing and reducing the quantum information spectral density. Here, we circumvent this rate-density trade-off in an AlGaAs-on-insulator photonic device by multiplexing an array of 20 small-radius microresonators each producing a 650-GHz-spaced comb of time-energy entangled-photon pairs. The resonators can be independently tuned via integrated thermo-optic heaters, enabling control of the mode spacing from degeneracy up to a full free spectral range. We demonstrate simultaneous pumping of five resonators with up to $50$ GHz relative comb offsets, where each resonator produces pairs exhibiting time-energy entanglement visibilities up to 95$\%$, coincidence-to-accidental ratios exceeding 5,000, and an on-chip pair rate up to 2.6 GHz/mW$^2$ per comb line -- more than 40 times improvement over prior work. As a demonstration, we generate frequency-bin qubits in a maximally entangled two-qubit Bell state with fidelity exceeding 87$\%$ (90$\%$ with background correction) and detected frequency-bin entanglement rates up to 7 kHz ($\sim 70$ MHz on-chip pair rate) using $\sim 250$ $\mu$W pump power. Multiplexing small-radius microresonators combines the key capabilities required for programmable and dense photonic qubit encoding while retaining high pair-generation rates, heralded single-photon purity, and entanglement fidelity.

quant-ph↗