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Full Programme

Date

10:00 - 10:30 Registration and coffee

10:30 - 12:30 Talks in LT 2.37

10:30 - 11:10 Non-local photon bunching in a four-mode interferometer

Holger F. Hoffmann (Hiroshima University, visiting Newcastle University)

Non-local multi-photon interference can be realized by applying a mode swap operation between two local multi-mode systems. The photon number correlations in the output can then be explained by photon bunching in non-local modes. This result establishes a fundamental relation between photon entanglement and the optical coherence between different optical paths.

11:10 – 11:30 Compressing Quantum Adaptive Agents with Tensor Networks

Rishi Sundar (University of Manchester)

Adaptive behaviour is central to how complex systems process information, from feedback controllers to learning and decision-making devices. An agent must continually combine new inputs with memory of the past to choose its next action, and the memory required to do so is a basic measure of its complexity. Recent work has shown that quantum models can simulate such agents with substantially less memory than the best classical representations. In passive, non-adaptive settings, these advantages can be analysed and optimised using one-dimensional tensor-network methods, which give efficient representations and controlled approximations. For genuinely adaptive agents, however, this machinery breaks down: dependence on unresolved future inputs generates a counterfactual branching structure, so the natural history is a tree rather than a line. Here we show how to restore a one-dimensional description. We introduce an isometric matrix-product-state representation for adaptive quantum agents, together with a certified compression scheme that compresses an agent for a chosen input distribution, controls the induced error, and repairs the truncated model into a fully operational approximate agent. We also show how the same framework extends to correlated inputs and connects naturally to higher-order quantum strategies through the quantum-comb formalism.

11:30 – 11:50 The quantum Mpemba effect in chaotic quantum systems

Tanmay Bhore (University of Leeds)

The Mpemba effect, where a system initially farther from equilibrium relaxes faster than one closer to equilibrium, has been extensively studied in classical systems and recently explored in quantum settings. While previous studies of the quantum Mpemba effect (QME) have largely focused on isolated systems with global symmetries, we argue that the QME is ubiquitous in generic, non-integrable many-body systems lacking such symmetries, including U(1) charge conservation, spatial symmetries, and even energy conservation. Using paradigmatic models such as the quantum Ising model with transverse and longitudinal fields, we show that the QME can be understood through the energy density of initial states and their inverse participation ratio in the energy eigenbasis. Our findings provide a unified framework for the QME, linking it with other anomalous phenoemna such as prethermalization and weak ergodicity breaking.

11:50 – 12:10 Thermal MDI-based Quantum Key Distribution

Alexandra Politi (University of York)

Exploring the performance of QKD protocols at frequencies beyond the optical regime is important for the possibility of integration with existing development for classical communications, like 5G and 6G. In this work, we extend the analysis of continuous-variable QKD to include trusted thermal noise within two distinct architectures: The asymmetric Measurement-Device-Independent (MDI) configuration, and modular star network for Conference-Key Agreement (CKA). We perform a rigorous security analysis that accommodates significantly higher levels of trusted noise than previous studies. We evaluate the performance of these schemes across the THz and microwave frequency regimes, considering both asymptotic and finite-size composable security frameworks. We then incorporate pulsed homodyne detectors, to mitigate the range-limiting effects of thermal noise, and to improve the schemes achievable transmission distances.

12:10 – 12:30 Continuous Variable Quantum Key Distribution channel emulator for the SPOQC mission

Emma Tien Hwai Medlock (University of York)

Continuous variable quantum key distribution (CV-QKD) uses amplitude and phase modulation of light to encode information and shot noise limited detectors for decoding, shows superior tolerance to noise and therefore a promising candidate for space-to-ground quantum communications, especially in daylight conditions. To enable space based quantum communications, the dynamic nature channel must be analysed and can be emulated in the lab for further protocol testing and optimisation. The dynamics of the channel will also cause issues for parameter estimation necessary for all CV-QKD protocols. The use of a satellite channel emulator enables lab based testing for various communication protocols. These protocols (quantum or classical) can be tested in the same system with minimal changes and at lower development costs and risk. With a satellite channel emulator the quantum payload's secure key generation rate under realistic conditions can be tested. Here, off the shelf optical components are used to emulate the channel.

12:30 - 1:30 Lunch in Room 1.34/1.49

1:30 - 3:30 Talks (including flash talks for poster session) in LT 2.37

1:30 – 1:50 Higher-Order Quantum Operations

Philip Taranto (University of Manchester)

 Many complex quantum phenomena require a description that goes beyond the standard paradigm of state preparations, evolutions, and measurements. A suitable framework for tackling such scenarios is that of higher-order quantum operations, i.e., quantum operations that transform quantum operations. These objects are fundamental to modern quantum theory and recent years have seen a proliferation of technical and conceptual results that fundamentally rely on them; they naturally emerge in quantum circuit architectures, correlated open dynamics, and investigations of quantum causality, to name but a few fields of application. In this talk, I will provide a pedagogical introduction to higher-order quantum operations, offering the tools to work with them in various contexts and firmly embed them within the current research landscape.

1:50 – 2:10 The quantum Zeno effect in the strong coupling, non-Markovian regime at finite temperatures

Sam Edmunds (Newcastle University)

We investigate the quantum Zeno effect (QZE) in open quantum systems using the reaction coordinate mapping to account for strong coupling and non-Markovian effects. Specifically, we consider a two-level system coupled to a finite-temperature bosonic bath with a Lorentzian spectral density. We show that frequent measurements project the system–environment state onto a Zeno subspace, inducing oscillations in the bath coherences. Using an effective non-unitary time-evolution operator, we demonstrate that this results in the survival probability (SP) depending not only on the measurement interval but also on the time-dependent state of the environment. Furthermore, we find that, in the long-time limit, the QZE is most readily accessed in the ultra-strong Markovian regime for finite-temperature baths.

2:10 – 2:30 A narrow linewidth cavity-enhanced SPDC source for long-distance QKD

Ormond Taylor (University of York)

Quantum Key Distribution (QKD) protocols that exploit single-photon interference, such as Twin-Field QKD, currently hold the record for secure communication transmission range. These protocols demand narrowband photon sources to ensure long coherence lengths and high-visibility first-order interference, a requirement that conventional photon sources do not easily meet. Sources based on Spontaneous Parametric Down-Conversion (SPDC) are attractive for their brightness and spectral flexibility, but typically emit over bandwidths of hundreds of GHz, resulting in coherence lengths far too short to practically achieve first-order interference. This work demonstrates a linear cavity-enhanced SPDC source at 1550 nm with a linewidth of 145 MHz, a three-order-of-magnitude reduction over single-pass SPDC. Signal and idler photons were detected using Superconducting Nanowire Single-Photon Detectors (SNSPDs) and temporal correlation measurements confirm the characteristic exponential decay expected from a cavity mode structure. This source represents a practical route toward the narrowband entangled-photon resources required for next-generation long-distance QKD.

2:30 – 2:50 Quantum many-body mixed phase space revealed by hybrid feedback control

Jie Ren (University of Leeds)

Understanding how complex systems transition between order and chaos is a central challenge of nonequilibrium physics. While weak perturbations of classical integrable systems give rise to a mixed phase space of coexisting regular and chaotic trajectories, analogous behavior in interacting quantum many-body systems has remained elusive. Here we develop and experimentally implement a hybrid quantum–classical feedback protocol that autonomously discovers and stabilizes long-lived regular trajectories in a superconducting quantum processor. Each iteration combines short-time quantum evolution with classical optimization that projects the dynamics back onto a low-entanglement variational manifold, effectively distilling coherence from chaotic evolution. The stabilized trajectories reveal signatures of a quantum many-body mixed phase space emerging from nonlinear variational dynamics, without a direct analog n few-body quantum systems described by a small number of effective degrees of freedom. Our results establish a versatile framework for the algorithmic identification and control of coherent dynamics, previously inaccessible to experiment.

2:50 – 3:10 How to map between any two pure states with a single unitary

Peter Bradshaw (Newcastle University)

It is well-known that any two pure quantum states (in the same Hilbert space) can be mapped to any other using unitary transformations. However, it was previously unknown how to determine the simplest transformation which achieves this for an arbitrary pair of pure states in arbitrary Hilbert space dimension. This talk shows how to utilise novel algebraic methods to construct such a unitary transformation in general; and proves that the desired mapping is always possible using a single exponential of unitary Lie algebra generators. This provides a useful tool for studying the relationships between systems of pure states in quantum information theory, as well as more elementary analyses of quantum circuits.

Flash talks for poster sessions:

These talks are 3 minutes each. If you like to participate in this session, please email your 2 or 3 slides to Almut [[email protected]] before Monday afternoon.

  • Topological edge states in a synthetic dimension with ultracold RbCs molecules

Francesca Blondell (Durham University)

Ultracold molecules offer an exciting platform for quantum simulation owing to their dipole moments, long-lived excited states and rich internal structures. Combining this with quantum gas microscopy techniques opens a wide array of possibilities for quantum simulation. Synthetic dimensions encode spatial information within the internal degrees of freedom of a system allowing for exploration of phenomena such as synthetic gauge fields, Anderson localisation, and Thouless pumping. We present the realisation of a synthetic dimension in ultracold RbCs molecules, in which we implement the Su-Schrieffer-Heeger (SSH) model in 4-,6-, and 8-site synthetic lattices. We realise the SSH model through coupling different rotational levels of the molecule using stroboscopic microwave pulses. Recent advances using magic wavelength trapping allow us to probe the synthetic dimension over tens of tunnelling times. We use this platform to probe the edge and bulk states of the system, accurately measure the energy splitting of edge states, and show topological protection of the edge states. The winding number of the system is extracted to probe the topological phase transition. The rich internal structure offered by molecules offer possibilities of investigating more complex synthetic dimensions in, such as investigating the effect of dipole-dipole interactions on synthetic lattice dynamics.

  • Warring contextualities - Provably classical vs Provably nonclassical

Enrico Bozzetto (Newcastle University)

In the literature, there are two differing definitions of contextuality: Kochen and Specker’s, and Spekkens’ (or “generalised”). However, researchers using one of these definitions rarely consider the other, meaning comparative analysis of these two notions is rare. In this paper, we advance the idea that Kochen-Specker contextuality provides a generalisation of the idea of system being fundamentally nonclassical, while Spekkens’ noncontextuality provides a generalisation of the idea of a system being classical. This allows us to reconcile the two approaches, as different stages in a hierarchy of classicality/nonclassicality.

  • Enhancing wave particle duality

Arwa Bukhari (University of Leeds)

To enhance the consistency between the quantum descriptions of waves and particles, we quantise mechanical point particles in this paper in the same physically motivated way as we previously quantised light in quantum electrodynamics (Bennett et al 2016 Eur. J. Phys. 37 014001). To identify the relevant Hilbert space, we notice that mechanical particles can occupy any position x while moving at any velocity v. Afterwards, we promote the classical states (x, v) to pairwise orthogonal quantum states and demand that these evolve according to Newton’s equations of motion. The resulting quantum theory is mass-independent, when Newton’s equations of motion are mass-independent, as one would expect. The basic formulation of quantum mechanics emerges from quantum mechanics in configuration space as a semi-classical approximation when a fixed mass is imposed and several other adjustments are made.

  • Theory of the correlated quantum Zeno effect in a monitored qubit dime

Gobinda Chakraborty (Lancaster University)

We theoretically investigate the stochastic dynamics of two qubits subject to one- and two-site correlated continuous weak measurements. When measurements dominate over the local unitary evolution, the system's dynamics is constrained, and part of the physical Hilbert space becomes inaccessible: a typical signature of the Quantum Zeno (QZ) effect. In this work, we show how the competition between these two measurement processes give rise to two distinct QZ regimes, we dubbed standard and correlated, characterised by a different topology of the allowed region of the physical Hilbert space being a simply and non-simply connected domain, respectively. We develop a theory based on a stochastic Gutzwiller ansatz for the wavefunction that is able to capture the structure of the phase diagram. Finally, we show how the two QZ regimes are intimately connected to the topology of the flow of the underlying non-Hermitian Hamiltonian governing the no-click evolution. 

  • Characterising carbon doping in hBN utilising near-field hyperbolic phonon-polariton propagation

Timothy Chester-Parsons (University of Sheffield)

Hexagonal boron nitride (hBN) has attracted significant attention as a van der Waals material due to its unusual optical properties, including a large negative dielectric response and strong optical anisotropy [1]. More recently, doped hBN has emerged as a promising platform for highly localised single-photon emitters (SPEs), with potential applications in integrated photonic circuits [2,3]. However, despite extensive optical studies, the atomic-scale bonding nature of these emitters remains poorly understood. In this work, we probe the nanoscale near-field optical response of pristine and carbon-doped hBN within the type-II Reststrahlen band. Phonon polaritons are launched from an s-SNOM tip, producing interference fringes arising from tip-launched and edge-reflected modes. Fourier analysis of these fringes, combined with an analytical model, enables extraction of the polariton dispersion. Using pristine hBN as a reference, we observe significant changes upon carbon doping. In particular, the high-frequency permittivity ε_∞ decreases, the longitudinal optical (LO) phonon frequency redshifts by ~ 10 cm⁻¹, and the transverse optical (TO) phonon frequency remains unchanged. These results indicate that carbon defects primarily modify long-range electrostatic lattice interactions, while leaving short-range bonding largely unaffected. This study provides new insight into the role of carbon defects in hBN and their influence on its nanoscale optical behaviour.

 [1] –  S. Dai et al. ,Tunable Phonon Polaritons in Atomically Thin van der Waals Crystals of Boron Nitride.Science343,1125-1129(2014).DOI:10.1126/science.1246833

[2] – L Spencer, J Horder et al, Monolithic Integration of Single Quantum Emitters in hBN Bullseye Cavities, ACS Photonics, 10, 12, 4417–4424 (2023), https://doi.org/10.1021/acsphotonics.3c01282

[3] – Mendelson, N., Chugh, D., Reimers, J.R. et al. Identifying carbon as the source of visible single-photon emission from hexagonal boron nitride. Nat. Mater. 20, 321–328 (2021). https://doi.org/10.1038/s41563-020-00850-y

  • Routing single photons with quantum emitters coupled to nanostructures

Mateusz Duda (University of Sheffield)

Quantum emitters coupled to nanophotonic structures are an excellent platform for controllable single-photon scattering. The tuneable light-matter interaction enables the construction of a single-photon switch – a device that can route a single photon from an input port to a selected output port. Such single-photon switching devices can be integrated into reconfigurable photonic circuits to actively control the photon propagation direction in a quantum network. Ideally, a single-photon switch should operate with high speed, efficiency, and fidelity, preserving the state of the input photon in the routing process. This talk/poster will provide an overview of single-photon switches based on quantum emitters coupled to nanophotonic structures such as waveguides and cavities, discussing different ways in which the properties of a quantum emitter can be tuned to route photons in a chosen direction.

  • Understanding magic in critical many-body systems

Andrew Hallam (University of Leeds)

“Magic,” or nonstabilizerness, is essential for universal quantum computation, but we still know surprisingly little about how it behaves in many-body quantum systems near criticality. In this talk, I show that the stabilizer Rényi entropy (SRE) of infinite matrix product states admits a natural spectral transfer-matrix description, and that its spectrum encodes universal subleading features. This leads to an SRE correlation length that is distinct from the conventional correlation length, diverges at continuous phase transitions, and controls the spatial response of the SRE to local perturbations. For the cluster-Ising model, I derive exact results for a low bond dimension MPS skeleton and numerically examine universal scaling along the Z_2 critical lines. These results establish nonstabilizerness as a sharp probe of criticality and perturbative response, and highlight a new connection between quantum computational resources and emergent many-body physics.

  • Photon emission without quantum jumps

Thomas Hartwell (University of Leeds)

When modelling photon emission, we often assume that the emitter experiences a random quantum jump. When a quantum jump occurs, the emitter transitions suddenly into a lower energy level, while spontaneously generating a single photon. However, this point of view is misleading when modelling quantum optical systems which rely on far-field interference effects for applications like distributed quantum computing and non-invasive photonic quantum sensing. Here we highlight that the dynamics of an emitter in the free radiation field can be described by simply solving a Schrödinger equation based on a locally acting Hamiltonian without invoking the notion of quantum jumps. Our approach is nevertheless consistent with quantum optical master equations. 

  • Deterministic entanglement of spectrally distinct quantum dots

Daniel Hodgson (University of Sheffield)

Quantum dots (QDs) are efficient semiconductor single-photon sources that can be readily integrated into photonic devices. Photon interactions with low-lying spin states enable QDs to be entangled together providing a step towards on-chip distributed quantum information processing. Existing protocols can generate a maximally entangled two-qubit state using two QDs with different emission spectra; however, the amount of entanglement that can be generated diminishes rapidly with the separation between the emission lines of the dots. Due to variations in the fabrication of QDs, creating the ability to entangle non-identical QDs is an important scientific challenge. Here I describe a protocol using an entangled photon pair for deterministically entangling two quantum dots with a separation between their emission lines limited only by the frequency of the input photons.

  • The role of polarization field terms in a model for a cavity quantum material

        Arwen Lloyd (University of Manchester)

Constructing models for cavity quantum materials requires a careful treatment of the light-matter coupling. In general, one must specify matrix elements constructed from the material wavefunctions, which are often unknown in a tight-binding framework. The Peierls substitution is often used to avoid introducing these additional parameters in the multi-center dipole (or Peierls) gauge, under the assumption that contributions from intraband and interband dipole moments can be neglected in the low-energy theory. We construct a toy model for a multi-band system with two sites, which we couple to a uniform field in the Coulomb, dipole, and Peierls gauges. We find that the Peierls substitution can be justified as a low-energy, effectively single-band description in one dimension, but it misses both self-polarization corrections and the direct coupling needed to describe interband transitions in the full Peierls gauge theory. Moreover, the Coulomb, dipole, and Peierls gauges define distinct partitions of the composite system into the light and matter subsystems. We illustrate the implications of this subsystem relativity for physical observables and on the performance of orbital truncations in each gauge.

  • Quantum Variational Methods for Supersymmetric Quantum Mechanics

Emanuele Mendicelli (University of Liverpool)

Quantum variational methods provide a powerful framework for investigating the ground states of fermion–boson systems, offering a pathway to detect supersymmetry breaking in nature. Classical simulations of supersymmetric models are affected by a severe sign problem, a limitation not present on quantum computing. In this talk, we examine a minimal single-site interacting fermion–boson system and assess the potential of hybrid quantum–classical variational algorithms. Using adaptive variational techniques, we construct optimal ansätze that scale efficiently, enabling robust identification of spontaneous supersymmetry breaking. We conclude by discussing the key challenges that must be addressed to extend these methods to larger and more complex systems.

  • Edge States Polaritons in hBN/WSe2 Double Grating Heterostructures

Oscar Palma (University of Sheffield)

We realise a topological photonic edge state at the boundary of two subwavelength gratings etched in an exfoliated hBN flake. The edge state is a photonic analogy of the celebrated Jackiw-Rebbi model describing a state that lies between two 1D media containing fermions with masses of opposite signs. The studied hBN gratings are designed to have photonic band-structures with different topology, achieved by varying the filling factor  leading to the band inversion, which can be probed in angle-resolved reflectance measurements. The experimental realisation of band inversion requires very accurate control of the refractive index contrast in the grating that we achieve by using “inverted” structures where an hBN grating is made first and then covered by an unetched hBN slab. Both the hBN flakes have precisely selected thicknesses creating an “inverted” grating with parameters unachievable by controlled depth etching of a single hBN flake. This approach also allows us to insert additional layers between the hBN grating and the top layer thus creating a photonic vdW heterostructure, in our case comprising a monolayer of semiconducting WSe 2 exhibiting excitons with a high oscillator strength. This allows us to observe the strong light matter interaction regime with formation of the edge state exciton-polaritons up to room temperature.

  • Ultracold Molecules and Rydberg Atoms as a Hybrid Quantum Platform

Daniel Ruttley (Durham University)

Ultracold polar molecules are an exciting future platform for quantum science and technology. Their rich internal structure enables dense quantum information storage, and their controllable interactions provide a mechanism for quantum information transfer. Here, I will discuss how we precisely control such molecules at Durham, with the aim of performing high-fidelity quantum gates. I will begin by describing our experiment, in which RbCs molecules are individually trapped in optical tweezers. By trapping molecules in magic-wavelength light, we engineer a decoherence-free environment that allows us to resolve hertz-scale dipolar interactions between pairs of molecules. I will show how we exploit these interactions to generate long-lived, high-fidelity entanglement between molecules. Finally, I will present work towards faster gates in which molecular interactions are mediated by highly dipolar Rydberg atoms. I will show that a molecule in a specific internal state can blockade Rydberg excitation of a neighbouring atom, and discuss how this mechanism enables entanglement between individual atoms and molecules. This unlocks the potential for scalable hybrid quantum systems in which Rydberg atoms serve as fast, controllable intermediaries for transferring quantum information between molecules.

  • Markovian approach to N-photon correlations beyond the quantum regression theorem

Mateusz Salamon (University of Manchester)

Multi-photon correlations from quantum emitters coupled to vibrational environments lie beyond the reach of standard tools such as the quantum regression theorem (QRT). In this talk I will introduce a Markovian framework for computing frequency-resolved N-photon correlation functions that overcomes this limitation. Applying our approach to a driven semiconductor quantum dot provides a tractable description of phonon effects on fluorescence beyond the single-photon spectrum. Our method accurately captures the emergence of the phonon sideband, missed by conventional QRT treatments, and reveals rich phonon-induced structure in the filtered two-photon spectrum. Strikingly, we find that photons emitted via the phonon sideband inherit second-order coherence properties of the Mollow triplet.

  • Maximum-Entropy Ensemble of Quantum Subsystems with a Thermal Average

Charlie Shakeshaft (University of Manchester)

The reduced density matrices of a bipartite Haar-random pure state are distributed according to the induced measure, over which statistical properties such as the eigenvalue density and the moments of entanglement entropy have been studied extensively. Here, we extend the treatment to incorporate equilibrium constraints, constructing a maximum entropy ensemble of quantum subsystems with a first moment fixed to the thermal state. We derive an expression for the partition function of the ensemble and study the resulting statistical mechanics, generalising previous results for the Gibbs weighted Hilbert-Schmidt measure to an arbitrary partitioning between the system and environment. To simulate the spectral and entanglement statistics, we provide both (i) a Metropolis-Hastings sampling algorithm for finite systems, and (ii) an efficient asymptotic algorithm, utilising the properties of the correlated Wishart ensemble of random matrices. Using this alongside tools from free probability theory, we examine thermally constrained modifications to the Marchenko-Pastur law for the eigenvalue density and the Page curve for entanglement entropy. Finally, we apply the formalism to the transverse-field Ising model, identifying signatures of criticality in the entanglement distribution of the random states.

3:30 - 4:00 Coffee break in GR 18

4:00 - 5:30 Poster session and networking in GR 18