arXiv subjects

Stefano Paesani

Publications and source records attributed to Stefano Paesani.

At least 19 recordsLinked to original sources

arXiv2026

Purification of photonic graph states

Graph states constitute the main building block for quantum computing with photons. Quantum emitters with a hosted spin can deterministically generate photonic graph states, strongly lowering the overhead of multiplexing highly probabilistic linear-optics graph state generation. However, they generally suffer from various noise sources, resulting in reduced fidelities of the produced states. To mitigate this issue, we develop purification schemes for entangled photonic states. We first develop purification schemes to purify arbitrary photonic GHZ and other CSS states, which we generalize to all photonic graph states and stabilizer states. The proposed purification schemes have a high success probability of up to $1/2$ and require only linear optics and photon detectors. We optimize cascaded purification schemes for various graph states, taking into account phenomenological Pauli errors or physical noise in time-bin-encoded graph state generation with quantum emitters.

quant-ph↗

arXiv2026

A Platform-aware Compilation Framework for Fault-tolerant Quantum Computation

The compilation of an algorithm can vary significantly with the choice of physical hardware platform and error correction model. Yet, current compilation frameworks typically commit to a single architecture-hardware configuration, making it difficult to assess resource estimates across platforms. We present a platform-aware compilation framework that re-compiles a quantum circuit into a hardware-compatible instruction set as well as fault-tolerant operations and provides end-to-end resource estimates in terms of physical-qubit count, time-to-solution, and classical processing time. We benchmark the framework by obtaining end-to-end resource estimates for different compilers, each tailored to the functionalities of specific hardware modalities: connectivity, clock speed, and noise model. As part of this framework, we introduce a transversal active volume (t-AV) compilation architecture designed for the efficient execution of fault-tolerant operations in platforms supporting long-range logical connectivity. We benchmark the framework for Hamiltonian simulation of the 2D Fermi Hubbard model as well as for eigenenergy estimation of a small molecule (trimethylenemethane) as a candidate for early fault-tolerant demonstration of quantum chemistry. For the latter, we show that end-to-end quantum simulations can be achieved with $\sim10^4$ physical qubits and runtimes ranging from $10^2$ ms (photonics, superconducting) to $10^5$ ms (neutral atoms).

quant-ph↗

arXiv2026

Fine structure of the M-center in Si

Color centers in silicon offer great possibilities for scalable quantum technologies. The M-center, proposed to originate from a carbon-hydrogen complex, offers telecommunications-band emission and a paramagnetic ground state similar to the T-center. Here, we report on photoluminescence lines in the vicinity of the M-center and investigate their properties, including their dependence on temperature, implantation fluence, implantation isotope, and annealing temperature. Three emission lines are observed that are blue-shifted by 1.1, 2.8 and 3.6 meV relative to the 761 meV zero phonon line of the M center, where the 2.8 meV line exhibits negative thermal quenching and is therefore proposed to originate from a second excited state of the M-center. The remaining blue-shifted emission lines, together with two additional red-shifted (4.7 and 6.4 meV) emission lines display normal thermal quenching, and are unaffected by an isotope shift of the implanted carbon atom ($^{12}$C versus $^{13}$C implantation) and implantation fluence. Thus, they likely arise either from other defects with similar emission energies or from a perturbed configuration of the M-center.

cond-mat.mtrl-sci↗

arXiv2026

Epitaxial single T centres in silicon-on-insulator

Spin-photon interfaces based on silicon quantum emitters offer a scalable platform for quantum computing and networking. However, achieving coherent photon emission remains a primary challenge due to stringent material quality requirements. To overcome this, we utilise high-purity molecular-beam epitaxy (MBE) to epitaxially incorporate single T centres in silicon-on-insulator (SOI) wafers. We demonstrate single T-centre emission coupled to a nanophotonic waveguide and observe significant suppression of homogeneous broadening, yielding optical linewidths as narrow as 30 MHz using natural silicon for crystal growth. These results establish epitaxial T centres as a robust foundation for coherent spin-photon interfaces in silicon quantum photonics.

quant-ph↗

arXiv2026

Fusion-based implementation of qLDPC codes with quantum emitters

Quantum low-density parity check (qLDPC) codes offer higher encoding rate than topological codes, e.g. surface codes, making them favourable for practical, fault-tolerant quantum computing with low overhead. These codes are particularly well-suited for fusion-based photonic implementations as this platform readily supports non-local connections. We propose an architecture specifically tailored to quantum emitters which can implement any Calderbank-Shor-Steane (CSS) qLDPC code. In this architecture, the photonic resource states are deterministically produced via quantum emitters and a conditional repeat-until-success strategy is incorporated to achieve high photon loss tolerance. We simulate small exemplary Bivariate Bicycle qLDPC codes and analyse the performance of our constructions under relevant physical noise mechanisms, including erasures due to fusion failure or photon loss, as well as Pauli errors. We obtain performances comparable with topological architectures though with significantly higher encoding rates.

quant-ph↗

arXiv2026

Observation of associative-memory retrieval and spin-glass phases on a photonic quantum simulator

Models of interacting complex systems provide the fundamental statistical physics reference frame for the study and the understanding of associative memories, machine learning, and the dynamics of neural networks. On the other hand, simulating complex multi-synaptic interactions on a classical hardware is computationally demanding due to the super-linear scaling of the system complexity. Photonic quantum technologies provide a promising solution to these limitations by leveraging on their inherent speed and parallel processing ability in order to simulate complex networks. Recently, a connection between multiphoton processes and generalized $p$-body Hopfield models has been theoretically established. Here, we design and demonstrate an experimental platform that exploits single photons distributed across a set of optical modes, in which controlled arrays of binary phase shifters act as Ising-like neurons. We focus specifically on a fully connected Hopfield Hamiltonian with four-body local interaction terms, realized via two-photon processes. Through quantum simulations on programmable photonic processors, the study identifies three distinct regimes: a memory retrieval phase, a spin-glass memory "black-out" phase, and a paramagnetic phase. Experimental results confirm successful memory retrieval at low storage capacities and temperatures, where the system consistently relaxes to fixed points with high memory overlap, effectively reconstructing the stored patterns. Future research will extend the platform design to investigate networks with local or dilute interactions, while advances in the realization of scalable photonic circuits will enable architectures that encompass very large numbers of interacting spins.

quant-ph↗

arXiv2026

Practical blueprint for low-depth photonic quantum computing with quantum dots

Fusion-based quantum computing is an attractive model for fault-tolerant computation based on photonics requiring only finite-sized entangled resource states followed by linear-optics operations and photon measurements. Large-scale implementations have so far been limited due to the access only to probabilistic photon sources, vulnerability to photon loss, and the need for massive multiplexing. Deterministic photon sources offer an alternative and resource-efficient route. By synergistically integrating deterministic photon emission, adaptive repeat-until-success fusions, and an optimised architectural design, we propose a complete blueprint for a photonic quantum computer using quantum dots and linear optics. It features time-bin qubit encoding, reconfigurable entangled-photon sources, and a fusion-based architecture with low optical connectivity, significantly reducing the required optical depth per photon and resource overheads. We present in detail the hardware required for resource-state generation and fusion networking, experimental pulse sequences, and exact resource estimates for preparing a logical qubit. We estimate that one logical clock cycle of error correction can be executed within microseconds, which scales linearly with the code distance. We also simulate error thresholds for fault-tolerance by accounting for a full catalogue of intrinsic error sources found in real-world quantum dot devices. Our work establishes a practical blueprint for a low-optical-depth, emitter-based fault-tolerant photonic quantum computer.

quant-ph↗

arXiv2026

Photon Sorting with a Quantum Emitter

High-quality photonic Bell state measurements (BSMs) enable scalable universal quantum computing and long distance quantum communication. However, when implemented with linear optics, BSMs are fundamentally probabilistic, introducing substantial hardware overheads and limiting noise tolerance in photonic quantum computing architectures. Nonlinear interactions at the single-photon level can overcome these limitations by enabling near-deterministic photon-photon gates. Here, we demonstrate a passive photon-sorting circuit based on the induced nonlinearity arising from photon scattering in a solid-state quantum emitter. The scattering is implemented in a directional waveguide-emitter coupling interface and embedded on-chip into a linear optical circuit, through which we demonstrate sorting of one- and two-photon components with a success probability of 62%. We find that the current system can enable BSMs with a 57% post-selected success probability without ancillary photons, exceeding the linear-optical limit of 50%, and can be readily improved to >65% with design optimisations.

quant-ph↗

arXiv2026

High-Dimensional Quantum Photonics: Roadmap

The field of high-dimensional quantum photonics involves the use of multimode photonic degrees-of-freedom such as the spatial, temporal, or spectral structure of light to encode multi-level quantum states. Recent years have seen rapid progress in the development of methods to generate, manipulate, and distribute such quantum states of light and their use in a range of quantum technology applications that offer practical advantages over conventional qubit-based approaches. High-dimensional quantum states of light encoded in photonic time-bins, frequency-bins, transverse-spatial modes, waveguide paths, and temporal modes have enabled noise-robust fundamental tests of quantum mechanics, error-resilient and high-capacity quantum communication protocols, andas well as efficient approaches for quantum information processing, to name just a few examples. However, research in this field has progressed fairly independently, with little exchange across different photonic degrees-of-freedom or between experiment and theory and no comprehensive comparison between degrees-of-freedom. This roadmap aims to bridge this gap by surveying progress in each area and identifying shared challenges and opportunities that cut across two or more photonic degrees-of-freedoms. We review early work and state-of-the-art experimental techniques under development for high-dimensional quantum states encoded in single and entangled photons, as well as theoretical tools for their measurement and certification. We outline the main outstanding challenges for theory and each experimental degree-of-freedom, identifying promising future directions of research that may enable these to be overcome. We end by discussing interconnections and shared challenges centered around their distribution, measurement, and manipulation, with a view towards their integration into next-generation quantum technology platforms and applications.

quant-ph↗

arXiv2026

Colour Centre Formation in Silicon-On-Insulator for On-Chip Photonic Integration

Colour centres in silicon have great potential as single photon sources for quantum technologies. Some of them - like the T centre - also possess optically-active spins that enable spin-photon interfaces for generating entangled photons and multi-spin registers. This paper explores the generation of several types of colour centres in silicon for mass-manufacturable silicon-on-insulator quantum devices. We investigate how different processes in the device development affect the presence of the quantum emitters, including thermal annealing and fabrication steps for optical nanostructures. The study reveals coupled formation dynamics between different colour centres, identifies optimal parameters for annealing processes, and reports on the sensitivity to annealing duration and nanofabrication procedures for photonic integrated circuits. Furthermore, we discern stable optical signals from colour centres in silicon which have not been identified before.

quant-ph↗

arXiv2025

High-dimensional Path-Encoded Entanglement Distribution Between Photonic Chips Enabled by Multimode Phase Stabilisation

The reliable distribution of high-dimensional entangled quantum states, an important resource in quantum technologies, through optical fibre networks is challenging due to the need to maintain coherence across multiple modes. Here we demonstrate the distribution of four-dimensional path-encoded entangled quantum states between photonic chips, enabled by a novel multimode phase stabilisation algorithm. The algorithm utilises the reconfigurability of the integrated photonic circuits to complete one iteration of phase stabilisation in just two measurement rounds for an arbitrary number of modes, and requires no additional hardware to the quantum measurements it enables. As a result, we are able to perform complete quantum state tomography across two chips using the minimum number of local projective measurements to verify the fidelity of the distributed entangled state to be 86% (compared to 8.1% without the phase stabilisation) with an entanglement entropy of 0.995+/-0.002.

quant-ph↗

arXiv2025

Multiphoton quantum simulation of the generalized Hopfield memory model

In the present work, we introduce, develop, and investigate a connection between multiphoton quantum interference, a core element of emerging photonic quantum technologies, and Hopfieldlike Hamiltonians of classical neural networks, the paradigmatic models for associative memory and machine learning in systems of artificial intelligence. Specifically, we show that combining a system composed of Nph indistinguishable photons in superposition over M field modes, a controlled array of M binary phase-shifters, and a linear-optical interferometer, yields output photon statistics described by means of a p-body Hopfield Hamiltonian of M Ising-like neurons +-1, with p = 2Nph. We investigate in detail the generalized 4-body Hopfield model obtained through this procedure and show that it realizes a transition from a memory retrieval to a memory black-out regime, i.e. a spin-glass phase, as the amount of stored memory increases. The mapping enables novel routes to the realization and investigation of disordered and complex classical systems via efficient photonic quantum simulators, as well as the description of aspects of structured photonic systems in terms of classical spin Hamiltonians.

quant-ph↗

arXiv2025

Hybrid Path-Transverse Electric Mode Qudit Encoding on an Integrated Photonic Chip

Hybrid encodings, where multiple degrees of freedom are used to encode quantum information, can increase the size of the Hilbert space with minimal increase to hardware requirements. We show a reprogrammable integrated photonic device, with multimodal components designed to allow for control over the transverse electric modes. We use this device to generate qudit states entangled in the path and transverse electric mode degrees of freedom. We generate and verify a hyperentangled state with a fidelity of $\mathcal{F}_{\text{HE}} = 67.3 \pm 0.2\%$ and a GHZ$_{4}$-style state with a fidelity of $\mathcal{F}_{\text{GHZ}_{4}} = 85.2 \pm 0.4 \%$. We use our hyperentangled state in a single-copy entanglement distillation protocol, resulting in an average $9.1 \%$ increase in the fidelity of the distilled Bell state for up to a $50\%$ probability of bit flip error. By utilising degrees of freedom which are readily compatible with integrated photonics, our work highlights how this hybrid encoding demonstrates a first step in using the transverse electric mode to reduce the footprint of integrated quantum photonic experiments.

quant-ph↗

arXiv2025

Purifying photon indistinguishability through quantum interference

Indistinguishability between photons is a key requirement for scalable photonic quantum technologies. We experimentally demonstrate that partly distinguishable single photons can be purified to reach near-unity indistinguishability by the process of quantum interference with ancillary photons followed by heralded detection of a subset of them. We report on the indistinguishability of the purified photons by interfering two purified photons and show improvements in the photon indistinguishability of $2.774(3)$\% in the low-noise regime, and as high as $10.2(5)$ \% in the high-noise regime.

quant-ph↗

arXiv2025

Transforming graph states via Bell state measurements

Graph states are key resources for measurement-based quantum computing, which is particularly promising for photonic systems. Fusions are probabilistic Bell state measurements, measuring pairs of parity operators of two qubits. Fusions can be used to connect/entangle different graph states, making them a powerful resource for measurement-based and related fusion-based quantum computing. There are several different graph structures and types of Bell state measurements, yet the associated graph transformations have only been analyzed for specific cases. Here, we provide a full set of graph transformation rules and give an intuitive visualization based on Venn diagrams of local neighborhoods of graph nodes. We derive these graph transformations for all types of rotated type-II fusion, showing that there are five different fusion success cases. Finally, we give application examples of the derived graph transformation rules and show that they can be used to construct graph codes or simulate fusion networks.

quant-ph↗

arXiv2025

Tailoring fusion-based photonic quantum computing schemes to quantum emitters

Fusion-based quantum computation is a promising quantum computing model where small-sized photonic resource states are simultaneously entangled and measured by fusion gates. Such operations can be readily implemented with scalable photonic hardware: resource states can be deterministically generated by quantum emitters and fusions require only shallow linear-optical circuits. Here, we propose fusion-based architectures tailored to the capabilities and noise models in quantum emitters. We show that high tolerance to dominant physical error mechanisms can be achieved, with fault-tolerance thresholds of 8% for photon loss, 4% for photon distinguishability between emitters, and spin noise thresholds well above memory-induced errors for typical spin-photon interfaces. Our construction and analysis provide guidelines for the development of photonic quantum hardware targeting fault-tolerant applications with quantum emitters.

quant-ph↗

arXiv2025

Generating graph states with a single quantum emitter and the minimum number of fusions

Graph states are the key resources for measurement- and fusion-based quantum computing with photons, yet their creation is experimentally challenging. We optimize a hybrid graph-state generation scheme using a single quantum emitter and linear optics Bell-state measurements called fusions. We first generate a restricted class of states from a single quantum emitter and then apply fusions to create a target graph state, where we use a dynamic programming approach to find the construction that requires the lowest possible number of fusions. Our analysis yields a lookup table for constructing $\sim 2.8\times 10^7$ non-isomorphic graph states with the minimum number of fusions. The lookup table covers all graph states with up to eight qubits and several other ones with up to 14 qubits. We present construction protocols of selected graph states and provide the lookup table. For large graph states that are not in the lookup table, we derive bounds for the required number of fusions using graph-theoretic properties. Finally, we use the lookup table to search for the best graph codes for loss-tolerant encodings, given a fixed number of fusions for their construction.

quant-ph↗

arXiv2024

Breadth-first graph traversal union-find decoder

Fast decoding algorithms are decisive for real-time quantum error correction and for analyzing properties of error correction codes. Here, we develop variants of the union-find decoder that simplify its implementation and provide potential decoding speed advantages. Furthermore, we show how these methods can be adapted to decode non-topological quantum low-density-parity-check (qLDPC) codes. All the developed decoders can directly include both qubit erasures and Pauli errors in the decoding step, thus addressing the dominant noise mechanisms for photonic quantum computing. We investigate the strengths and weaknesses of the different decoder variants, benchmark their speed and threshold error rates on several codes, and provide the used source code.

quant-ph↗