Electron-phonon decoupling due to strong light-matter interactions

Phonon interactions in solid-state photonics systems cause intrinsic quantum decoherence and often present the limiting factor in emerging quantum technology. Due to recent developments in nanophotonics, exciton-cavity structures with very strong light-matter coupling rates can be fabricated. We show that in such structures, a new regime emerges, where the decoherence is completely suppressed due to decoupling of the dominant phonon process. Using a numerically exact tensor network approach, we perform calculations in this non-perturbative, non-Markovian dynamical regime. Here, we identify a strategy for reaching near-unity photon indistinguishability and also discover an interesting phonon-dressing of the exciton-cavity polaritons in the high-Q regime, leading to multiple phonon sidebands when the light-matter interaction is sufficiently strong.Comment: Accepted for publication in Physical Review

Protocol for generating multiphoton entangled states from quantum dots in the presence of nuclear spin fluctuations

Multi-photon entangled states are a crucial resource for many applications in quantum information science. Semiconductor quantum dots offer a promising route to generate such states by mediating photon-photon correlations via a confined electron spin, but dephasing caused by the host nuclear spin environment typically limits coherence (and hence entanglement) between photons to the spin $T_2^*$ time of a few nanoseconds. We propose a protocol for the deterministic generation of multi-photon entangled states that is inherently robust against the dominating slow nuclear spin environment fluctuations, meaning that coherence and entanglement is instead limited only by the much longer spin $T_2$ time of microseconds. Unlike previous protocols, the present scheme allows for the generation of very low error probability polarisation encoded three-photon GHZ states and larger entangled states, without the need for spin echo or nuclear spin calming techniques

Collective Quantum Memory Activated by a Driven Central Spin

Coupling a qubit coherently to an ensemble is the basis for collective quantum memories. A single driven electron in a quantum dot can deterministically excite low-energy collective modes of a nuclear spin ensemble in the presence of lattice strain. We propose to gate a quantum state transfer between this central electron and these low-energy excitations—spin waves—in the presence of a strong magnetic field, where the nuclear coherence time is long. We develop a microscopic theory capable of calculating the exact time evolution of the strained electron-nuclear system. With this, we evaluate the operation of quantum state storage and show that fidelities up to 90% can be reached with a modest nuclear polarization of only 50%. These findings demonstrate that strain-enabled nuclear spin waves are a highly suitable candidate for quantum memory.We thank E. Chekhovich for helpful discussions. This work was supported by the ERC PHOENICS grant (617985), the EPSRC Quantum Technology Hub NQIT (EP/M013243/1), and the Royal Society (RGF/EA/181068). D. A. G. acknowledges support from St. John’s College Title A Fellowship. E. V. D. and J. M. acknowledge funding from the Danish Council for Independent Research (Grant No. DFF-4181-00416). C. L. G. acknowledges support from a Royal Society Dorothy Hodgkin Fellowship

Quantum light-matter interaction and controlled phonon scattering in a photonic Fano cavity

The Fano effect arises from the interference between a continuum of propagating modes and a localised resonance. By using this resonance as one of the mirrors in an optical cavity, a localised mode with a highly asymmetric line shape is obtained. Placing a single quantum emitter inside the cavity leads to a new regime of cavity quantum electrodynamics, where the light-matter interaction dynamics is fundamentally different from that observed in a conventional cavity with Lorenztian lineshape. Furthermore, when the vibrational dynamics of the emitter is taken into account, an intricate phonon-photon interplay arises, and the optical interference induced by the Fano mirror significantly alters the leakage of energy into vibrational modes. We demonstrate that this control mechanism improves the maximum attainable indistinguishability of emitted photons, as compared to an equivalent cavity with a conventional mirror

Cavity-induced exciton localization and polariton blockade in two-dimensional semiconductors coupled to an electromagnetic resonator

Recent experiments have demonstrated strong light-matter coupling between electromagnetic nanoresonators and pristine sheets of two-dimensional semiconductors, and it has been speculated whether these systems can enter the quantum regime operating at the few-polariton level. To address this question, we present a first-principles microscopic quantum theory for the interaction between excitons in an infinite sheet of two-dimensional material and a localised electromagnetic resonator. We find that the light-matter interaction breaks the symmetry of the otherwise translation-invariant system and thereby effectively generates a localised exciton mode, which is coupled to an environment of residual exciton modes. This dissipative coupling increases with tighter lateral confinement, and our analysis reveals this to be a potential challenge in realising nonlinear exciton-exciton interaction. Nonetheless, we predict that polariton blockade due to nonlinear exciton-exciton interactions is well within reach for nanoresonators coupled to transition-metal dichalcogenides, provided that the lateral resonator mode confinement can be sufficiently small that the nonlinearity overcomes the polariton dephasing caused by phonon interactions

Quantum theory of two-dimensional materials coupled to electromagnetic resonators

We present a microscopic quantum theory of light-matter interaction in pristine sheets of two-dimensional semiconductors coupled to localized electromagnetic resonators such as optical nanocavities or plasmonic particles. The light-matter interaction breaks the translation symmetry of excitons in the two-dimensional lattice, and we find that this symmetry-breaking interaction leads to the formation of a localized exciton state, which mimics the spatial distribution of the electromagnetic field of the resonator. The localized exciton state is in turn coupled to an environment of residual exciton states. We quantify the influence of the environment and find that it is most pronounced for small lateral confinement length scales of the electromagnetic field in the resonator, and that environmental effects can be neglected if this length scale is sufficiently large. The microscopic theory provides a physically appealing derivation of the coupled oscillator models widely used to model experiments on these types of systems, in which all observable quantities are directly derived from the material parameters and the properties of the resonant electromagnetic field. As a consistency check, we show that the theory recovers the results of semiclassical electromagnetic calculations and experimental measurements of the excitonic dielectric response in the linear excitation limit. The theory, however, is not limited to linear response, and in general describes nonlinear exciton-exciton interactions in the localized exciton state, thereby providing a powerful means of investigating the nonlinear optical response of such systems

Cavity-waveguide interplay in optical resonators and its role in optimal single-photon sources

Interfacing solid-state emitters with photonic structures is a key strategy for developing highly efficient photonic quantum technologies. Such structures are often organised into two distinct categories: nanocavities and waveguides. However, any realistic nanocavity structure simultaneously has characteristics of both a cavity and waveguide, which is particularly pronounced when the cavity is constructed using low-reflectivity mirrors in a waveguide structure with good transverse light confinement. In this regime, standard cavity quantum optics theory breaks down, as the waveguide character of the underlying dielectric is only weakly suppressed by the cavity mirrors. By consistently treating the photonic density of states of the structure, we provide a microscopic description of an emitter including the effects of phonon scattering over the full transition range from waveguide to cavity. This generalised theory lets us identify an optimal regime of operation for single-photon sources in optical nanostructures, where cavity and waveguide effects are concurrently exploited