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Jake Iles-Smith

Publications and source records attributed to Jake Iles-Smith.

At least 19 recordsLinked to original sources

Efficient optimisation of multi-parameter quantum control protocols for strongly-coupled systems

Achieving high-fidelity control in the presence of strong non-Markovian noise is critical for the optimization of emergent solid-state quantum devices. We present a highly efficient optimization framework that combines automatic differentiation with the non-Markovian uniTEMPO algorithm, enabling direct gradient-based optimization of complex objective functions. We apply this method to semiconductor quantum dots, optimizing multi-pulse excitation schemes: specifically Swing-UP of a Quantum EmmiteR (SUPER) and Floquet-engineered Two-Photon Excitation (FTPE) for single- and bi-exciton generation. Our approach yields high preparation fidelities within experimentally accessible parameter regimes. By integrating adiabatic rapid passage (ARP), we systematically enhance both SUPER and FTPE, demonstrating that these optimized protocols consistently outperform standard resonant pi-pulses and two-photon excitation. Notably, this performance gap widens at elevated temperatures, establishing the superior thermal robustness of our optimized multi-pulse strategies for real-world quantum hardware.

quant-ph

Beyond the Quantum Regression Theorem in Variational Polaron Master Equations: from Ohmic to Super-Ohmic Environments

While the quantum regression theorem (QRT) is the standard tool for computing multi-time correlation functions in open quantum systems, it relies on system-bath separability and an environment that remains in equilibrium, assumptions that are violated once dynamical correlations develop. Using the projection operator formalism, we derive an extension to the QRT that explicitly incorporates these correlation-induced corrections, enabling us to capture multi-time correlation functions in non-Markovian regimes. We apply this framework to the variational polaron master equation for the spin-boson model in ohmic and super-ohmic regimes, where the polaron transformation mixes system-bath degrees of freedom to produce a non-thermal effective environment. Benchmarking against numerically exact tensor-network simulations, we demonstrate quantitative agreement for single- and two-time observables, including linear-response spectra, even at strong coupling. Our approach broadens the reach of analytic master equations to multi-time observables in strong-coupling and non-Markovian regimes, where environmental memory effects and system-bath correlations are crucial.

quant-ph

Resonant Excitation Induced Vibronic Mollow Triplets

The Mollow triplet is the definitive spectral signature of an optically dressed quantum emitter. We predict that for emitters coupled to localized phonons, this signature is not confined to the zero-phonon line. Under a strong resonant drive, we show that Mollow triplets are strikingly replicated on the associated phonon sidebands -a surprising result, given that phonon sidebands are typically viewed as incoherent, inelastic scattering pathways. These vibronic Mollow triplets are a direct fingerprint of dynamically generated dressed states that hybridize the emitter's electronic, photonic, and vibrational degrees of freedom. We develop a scalable analytical formalism to model this effect in complex, multi-mode molecular systems, such as dibenzoterrylene. Our work provides the precise driving conditions for observing these novel spectral features, establishing a new signature of coherence in vibronically coupled systems.

quant-ph

Numerically exact open quantum system work statistics with process tensors

Accurately quantifying the thermodynamic work costs of quantum operations is essential for the continued development and optimisation of emerging quantum technologies. This present a significant challenge in regimes of rapid control within complex, non-equilibrium environments - conditions under which many contemporary quantum devices operate and conventional approximations break down. Here, we introduce a process tensor framework that enables the computation of the full numerically exact quantum work statistics of driven open quantum systems. We demonstrate the utility of our approach by applying it to a Landauer erasure protocol operating beyond the weak-coupling, Markovian, and slow-driving limits. The resulting work probability distributions reveal distinct quantum signatures that are missed by low-order moments yet significantly impact the erasure fidelity of the protocol. Our framework delivers non-perturbative accuracy and detail in characterising energy-exchange fluctuations in driven open quantum systems, establishing a powerful and versatile tool for exploring thermodynamics and control in the operating regimes of both near-term and future quantum devices.

quant-ph

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

Multi-photon correlations from quantum emitters coupled to vibrational environments lie beyond the reach of standard tools such as the quantum regression theorem (QRT). Here, we 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.

quant-ph

Non-Markovian Quantum Heat Statistics with the Reaction Coordinate Mapping

The definition of heat in quantum mechanics is ambiguous. Complications arise in particular when the coupling between a quantum system and a thermal environment is non-negligible, as the boundary between the two becomes blurred, making the distinction between system and environment difficult to draw. The reaction coordinate mapping can be used in such regimes to redraw the boundary between the system and environment. In this paper we combine the reaction coordinate technique with a two-point measurement protocol to compare two different definitions of heat: energetic changes with respect to the full environment Hamiltonian (prior to the mapping), and energetic changes with respect to the residual environment Hamiltonian (after the mapping). We find that the latter definition displays behaviour more expected of a heat bath in the highly non-Markovian regime considered.

quant-ph

Capturing non-Markovian polaron dressing with the master equation formalism

Understanding the dynamics of open quantum systems in strong coupling and non-Markovian regimes remains a formidable theoretical challenge. One popular and well-established method of approximation in these circumstances is provided by the polaron master equation (PME). In this work we reevaluate and extend the validity of the PME to capture the impact of non-Markovian polaron dressing, induced by non-equilibrium open system dynamics. By comparing with numerically exact techniques, we confirm that while the standard PME successfully predicts the dynamics of system observables that commute with the polaron transformation (e.g. populations in the Pauli z-basis), it can struggle to fully capture those that do not (e.g. coherences). This limitation stems from the mixing of system and environment degrees of freedom inherent to the polaron transformation, which affects the accuracy of calculated expectation values within the polaron frame. Employing the Nakajima-Zwanzig projection operator formalism, we introduce correction terms that provide an accurate description of observables that do not commute with the transformation. We demonstrate the significance of the correction terms in two cases, the canonical spin-boson model and a dissipative time-dependent Landau-Zener protocol, where they are shown to impact the system dynamics on both short and long timescales.

quant-ph

On-demand heralded MIR single-photon source using a cascaded quantum system

We propose a novel mechanism for generating single photons in the mid-Infrared (MIR) using a solid-state or molecular quantum emitter. The scheme utilises cavity QED effects to selectively enhance a Frank-Condon transition, deterministically preparing a single Fock state of a polar phonon mode. By coupling the phonon mode to an antenna, the resulting excitation is then radiated to the far field as a single photon with a frequency matching the phonon mode. By combining macroscopic QED calculations with methods from open quantum system theory, we show that optimal parameters to generate these MIR photons occur for modest light-matter coupling strengths, which are achievable with state-of-the-art technologies. Combined, the cascaded system we propose provides a new quasi-deterministic source of heralded single photons in a regime of the electromagnetic spectrum where this previously was not possible.

physics.optics

Nanocavity enhanced photon coherence of solid-state quantum emitters operating up to 30 K

Solid-state emitters such as epitaxial quantum dots have emerged as a leading platform for efficient, on-demand sources of indistinguishable photons, a key resource for many optical quantum technologies. To maximise performance, these sources normally operate at liquid helium temperatures ($\sim 4~\mathrm{K}$), introducing significant size, weight and power requirements that can be impractical for proposed applications. Here we experimentally resolve the two distinct temperature-dependent phonon interactions that degrade indistinguishability, allowing us to demonstrate that coupling to a photonic nanocavity can greatly improve photon coherence at elevated temperatures up to $30~\mathrm{K}$ that are compatible with compact cryocoolers. We derive a polaron model that fully captures the temperature-dependent influence of phonons observed in our experiments, providing predictive power to further increase the indistinguishability and operating temperature of future devices through optimised cavity parameters.

quant-ph

Combining experiments on luminescent centres in hexagonal boron nitride with the polaron model and ab initio methods towards the identification of their microscopic origin

The two-dimensional material hexagonal boron nitride (hBN) hosts luminescent centres with emission energies of 2 eV which exhibit pronounced phonon sidebands. We investigate the microscopic origin of these luminescent centres by combining ab initio calculations with non-perturbative open quantum system theory to study the emission and absorption properties of 26 defect transitions. Comparing the calculated line shapes with experiments we narrow down the microscopic origin to three carbon-based defects: $\mathrm{C_2C_B}$, $\mathrm{C_2C_N}$, and $\mathrm{V_NC_B}$. The theoretical method developed enables us to calculate so-called photoluminescence excitation (PLE) maps, which show excellent agreement with our experiments. The latter resolves higher-order phonon transitions, thereby confirming both the vibronic structure of the optical transition and the phonon-assisted excitation mechanism with a phonon energy 170 meV. We believe that the presented experiments and polaron-based method accurately describe luminescent centres in hBN and will help to identify their microscopic origin.

cond-mat.mtrl-sci

Quantum work statistics at strong reservoir coupling

Determining the statistics of work done on a quantum system while strongly coupled to a reservoir is a formidable task, requiring the calculation of the full eigenspectrum of the combined system and reservoir. Here we show that this issue can be circumvented by using a polaron transformation that maps the system into a new frame where weak-coupling theory can be applied. Crucially, this polaron approach reproduces the Jarzynski fluctuation theorem, thus ensuring consistency with the laws of stochastic thermodynamics. We apply our formalism to a system driven across the Landau-Zener transition, where we identify clear signatures in the work distribution arising from a non-negligible coupling to the environment. Our results provide a new method for studying the stochastic thermodynamics of driven quantum systems beyond Markovian, weak-coupling regimes.

quant-ph

Vibronic effects on the quantum tunnelling of magnetisation in Kramers single-molecule magnets

Single-molecule magnets are among the most promising platforms for achieving molecular-scale data storage and processing. Their magnetisation dynamics are determined by the interplay between electronic and vibrational degrees of freedom, which can couple coherently, leading to complex vibronic dynamics. Building on an ab initio description of the electronic and vibrational Hamiltonians, we formulate a non-perturbative vibronic model of the low-energy magnetic degrees of freedom in monometallic single-molecule magnets. Describing their low-temperature magnetism in terms of magnetic polarons, we are able to quantify the vibronic contribution to the quantum tunnelling of the magnetisation, a process that is commonly assumed to be independent of spin-phonon coupling. We find that the formation of magnetic polarons lowers the tunnelling probability in both amorphous and crystalline systems by stabilising the low-lying spin states. This work, thus, shows that spin-phonon coupling subtly influences magnetic relaxation in single-molecule magnets even at extremely low temperatures where no vibrational excitations are present.

quant-ph

Signatures of Non-Markovianity in Cavity-QED with Color Centers in 2D Materials

Light-matter interactions of defects in two dimensional materials are expected to be profoundly impacted by strong coupling to phonons. In this work, we combine ab initio calculations of a defect in hBN, with a fully quantum mechanical and numerically exact description of a cavity-defect system to elucidate this impact. We show that even at weak light-matter coupling, the dynamical evolution of the cavity-defect system has clear signatures of non-markovian phonon effects, and that linear absorption spectra show the emergence of hybridised light-matter-phonon states in regimes of strong light-matter coupling. We emphasise that our methodology is general, and can be applied to a wide variety of material/defect systems.

quant-ph

Optical Spin Initialisation and Readout with a Cavity-Coupled Quantum Dot in an In-Plane Magnetic Field

The spin of a charged semiconductor quantum dot (QD) coupled to an optical cavity is a promising candidate for high fidelity spin-photon interfaces; the cavity selectively modifies the decay rates of optical transitions such that spin initialisation, manipulation, and readout are all possible in a single magnetic field geometry. By performing cavity QED calculations, we show that a cavity with a single, linearly-polarised mode can simultaneously support both high-fidelity optical spin initialisation and readout in a single, in-plane (Voigt geometry) magnetic field. Furthermore, we demonstrate that single mode cavities always outperform bi-modal cavities in experimentally favourable driving regimes. Our analysis, when combined with established methods of control in a Voigt geometry field, provides optimal parameter regimes for high-fidelity initialisation and readout, and coherent control in both cavity configurations, providing insights for the design and development of QD spin-photon interfaces as the basis of quantum network nodes and for the generation of photonic graph states.

quant-ph

Tailoring solid-state single-photon sources with stimulated emissions

The coherent interaction of electromagnetic fields with solid-state two-level systems can yield deterministic quantum light sources for photonic quantum technologies. To date, the performance of semiconductor single-photon sources based on three-level systems is limited mainly due to a lack of high photon indistinguishability. Here, we tailor the cavity-enhanced spontaneous emission from a ladder-type three-level system in a single epitaxial quantum dot (QD) through stimulated emission. After populating the biexciton (XX) of the QD through two-photon resonant excitation (TPE), we use another laser pulse to selectively depopulate the XX state into an exciton (X) state with a predefined polarization. The stimulated XX-X emission modifies the X decay dynamics and yields improved polarized single-photon source characteristics such as a source brightness of 0.030(2), a single-photon purity of 0.998(1), and an indistinguishability of 0.926(4). Our method can be readily applied to existing QD single-photon sources and expands the capabilities of three-level systems for advanced quantum photonic functionalities.

physics.optics

Exact dynamics of non-additive environments in non-Markovian open quantum systems

When a quantum system couples strongly to multiple baths then it is generally no longer possible to describe the resulting system dynamics by simply adding the individual effects of each bath. However, capturing such multi-bath system dynamics has up to now required approximations that can obscure some of the non-additive effects. Here we present a numerically-exact and efficient technique for tackling this problem that builds on the time-evolving matrix product operator (TEMPO) representation. We test the method by applying it to a simple model system that exhibits non-additive behaviour: a two-level dipole coupled to both a vibrational and an optical bath. Although not directly coupled, there is an effective interaction between the baths mediated by the system that can lead to population inversion in the matter system when the vibrational coupling is strong. We benchmark and validate multi-bath TEMPO against two approximate methods - one based on a polaron transformation, the other on an identification of a reaction coordinate - before exploring the regime of simultaneously strong vibrational and optical coupling where the approximate techniques break down. Here we uncover a new regime where the quantum Zeno effect leads to a fully mixed state of the electronic system.

quant-ph

Photon Statistics of Filtered Resonance Fluorescence

Spectral filtering of resonance fluorescence is widely employed to improve single photon purity and indistinguishability by removing unwanted backgrounds. For filter bandwidths approaching the emitter linewidth, complex behaviour is predicted due to preferential transmission of components with differing photon statistics. We probe this regime using a Purcell-enhanced quantum dot in both weak and strong excitation limits, finding excellent agreement with an extended sensor theory model. By changing only the filter width, the photon statistics can be transformed between antibunched, bunched, or Poissonian. Our results verify that strong antibunching and a sub-natural linewidth cannot simultaneously be observed, providing new insight into the nature of coherent scattering.

quant-ph

Few-photon Transport in Fano-resonance waveguide geometries

We present a theoretical study of Fano interference effects in few-photon transport. Under appropriate conditions, a local defect in an optical waveguide induces a highly asymmetric transmission lineshape, characteristic of Fano interference. For a two-level emitter placed adjacent to such a defect, here modeled as a partially transmitting element, we find an analytical expression for the full time evolution of single-photon wavepackets and the emitter excitation probability. We show how the partially transmitting element affects the emitter lifetime and shifts the spectral position of the effective system resonances. Using input-output formalism, we determine the single and two-photon $ S $-matrices for both a two-level emitter and a cavity-emitter system coupled to a waveguide with a partially transmitting element. We show how the Fano interference effect can be exploited for the implementation of a Hong-Ou-Mandel switch in analogy with a tunable linear or nonlinear beam splitter.

quant-ph