Invited Speakers

Professor Dave Binks, University of Manchester

'The potential of singly-doped quantum dots as a platform for optically-addressed qubits'

Colloidal quantum dots (CQDs) are semiconductor nanocrystals produced by wet-chemistry methods, which benefit from a suite of synthetic techniques developed over the last 30+ years that now allow precise control of their size, structure and composition. They are manufactured commercially by a number of companies worldwide and used in mass-market electronics, most notably in display technologies. CQDs can also be produced so that they can contain a single transition metal dopant, making them suitable as spin-photon interfaces for quantum technology (QT). In this talk, the properties of such singly-doped CQDs will be surveyed and their potential as a platform for optically-addressed qubits assessed.

Dr Moritz Cygorek, Heriot-Watt University

'Process tensors for quantum dots: Numerically exact simulation of spectra, off-resonant excitation, and cooperative emission'

The strong coupling of quantum dots to phonons is known to strongly affect their functionality and behaviour under optical excitation.

While the impact of non-Markovian phonon effects for real-world applications [1] has been very successfully modelled using numerically exact real-time path integral methods for over a decade, numerical convergence still posed limits on the applicability of such methods. The recently developed process tensor formalism [2,3] removes many of the limitations of earlier path integral approaches.

Here, I summarise current developments in process tensors, focusing on how they can be employed to tackle topical and challenging problems in quantum dot dynamics:

A novel divide-and-conquer scheme to construct periodic process tensors [4] enables fast simulations involving millions of time steps. This makes it possible to simulate emission spectra obtained after strong time-dependent (pulsed) driving [5] or to resolve the fast oscillations involved in strongly off-resonant driving protocols like SUPER [6].

Moreover, process tensors can be used to model multi-emitter systems with multiple environments. This facilitates investigations of non-additive cross-interactions between, e.g., optical and vibrational environments [3]. But it is also useful for studying how cooperative emission and superradiance in systems composed of multiple quantum dots can persist over long times despite the interaction with local phonon baths [7].

These features make process tensors ideal easy-to-use general-purpose tools for simulating the dynamics in solid state quantum emitters.

[1] M. Cosacchi, F. Ungar, M. Cygorek, A. Vagov, and V. M. Axt, Phys. Rev. Lett. 123, 017403 (2019)

[2] M.R. Jørgensen, F.A. Pollock, Phys. Rev. Lett. 123, 240602 (2019)

[3] M. Cygorek, M. Cosacchi, A. Vagov, V. M. Axt, B. W. Lovett, J. Keeling, E. M. Gauger, Nature Physics 18, 662 (2022)

[4] M. Cygorek, J. Keeling, B. W. Lovett, E. M. Gauger, arXiv:2304.05291 [quant-ph] (2023)

[5] K. Boos et al., arXiv:2305.15827 [quant-ph] (2023)

[6] T. K. Bracht, M. Cygorek, T. Seidelmann, V. M. Axt, D. E. Reiter, arXiv:2307.00304 [cond-mat.mes-hall] (2023)

[7] J. Wiercinski, E. M. Gauger, M. Cygorek, Phys. Rev. Research 5, 013176 (2023)

Professor Ruth Oulton, Quantum Engineering and Technology Labs, University of Bristol

'Quantum dot micropillars for quantum information processing'

Quantum dot micropillars are an established device technology for producing on-demand single photons, entangled pair photons, cluster states and non-linear interactions such as photon number sorting and spin-based switching. I will present the prospects for quantum dot micropillars as near-perfect devices for photon sources and non-linear interactions.

By experimentally performing coherent scattering measurements we are able to demonstrate deterministic interaction of narrowband light with a single QD in a micropillar. Moreover, we demonstrate a novel light-matter interaction: the imprinting of the evolving state of a carrier spin in a QD as a phase modulation of the scattered narrowband light. This demonstration paves the way for achieving deterministic entanglement of the spin with photons that are bandwidth mismatched to the QD by several orders of magnitude.

I will also discuss and demonstrate the prospects for QDs as near perfect sources of single photons (and therefore biexciton cascade entangled photons and cluster states). I will present designs of low Q-factor micropillar cavities that show exceptional figures of merit: >95% overall efficiency for moderate Q-factor of 2500. Such cavities have an inherent phonon sideband suppression of over an order of magnitude, which gives the potential for single photon indistinguishabilities of >0.999. These low Q-factor designs are undemanding to manufacture compare to other very high brightness designs proposed, and promise the prospect of using QDs widely as sources, switches and cluster state generators. The spectrally broad cavity is also capable of supporting high brightness biexciton cascade entangled pair sources: pair efficiencies of 75% are possible and the broad cavity shows no linear mode splitting that would otherwise destroy the polarization entanglement of the pair.

This work demonstrates that there is a very good prospect of deterministic sources with generation rates of 10GHz, efficiencies of >95% or more, and near-perfect indistinguishabilities comparable or surpassing the workhorse spontaneous parametric down-conversion sources and we should expect to see such devices in action in large scale quantum photonic circuits in the next 2-3years.

Dr. Hannah Stern, University of Cambridge

'A quantum coherent spin in a two-dimensional material at room temperature'

Quantum networks and sensing require solid-state spin-photon interfaces that combine single-photon generation and long-lived spin coherence with scalable device integration, ideally at ambient conditions. Despite rapid progress reported across several candidate systems, those possessing quantum coherent single spins at room temperature remain extremely rare. In this talk I will present new results of quantum coherent control under ambient conditions of a single-photon emitting defect spin in a two-dimensional material, hexagonal boron nitride. I will show that the carbon-related defect has a spin-triplet electronic ground-state manifold and that the spin coherence is governed predominantly by coupling to only a few proximal nuclei and is prolonged by decoupling protocols.