Jake Iles-Smith

Energy transfer in structured and unstructured environments: Master equations beyond the Born-Markov approximations

We explore excitonic energy transfer dynamics in a molecular dimer system coupled to both structured and unstructured oscillator environments. By extending the reaction coordinate master equation technique developed by Iles-Smith et al. [Phys. Rev. A 90, 032114 (2014)], we go beyond the commonly used Born-Markov approximations to incorporate system-environment correlations and the resultant non-Markovian dynamical effects. We obtain energy transfer dynamics for both underdamped and overdamped oscillator environments that are in perfect agreement with the numerical hierarchical equations of motion over a wide range of parameters. Furthermore, we show that the Zusman equations, which may be obtained in a semiclassical limit of the reaction coordinate model, are often incapable of describing the correct dynamical behaviour. This demonstrates the necessity of properly accounting for quantum correlations generated between the system and its environment when the Born-Markov approximations no longer hold. Finally, we apply the reaction coordinate formalism to the case of a structured environment comprising of both underdamped (i.e., sharply peaked) and overdamped (broad) components simultaneously. We find that though an enhancement of the dimer energy transfer rate can be obtained when compared to an unstructured environment, its magnitude is rather sensitive to both the dimer-peak resonance conditions and the relative strengths of the underdamped and overdamped contributions.

Environmental dynamics, correlations, and the emergence of noncanonical equilibrium states in open quantum systems

Quantum systems are invariably open, evolving under surrounding influences rather than in isolation. Standard open quantum system methods eliminate all information on the environmental state to yield a tractable description of the system dynamics. By incorporating a collective coordinate of the environment into the system Hamiltonian, we circumvent this limitation. Our theory provides straightforward access to important environmental properties that would otherwise be obscured, allowing us to quantify the evolving system-environment correlations. As a direct result, we show that the generation of robust system-environment correlations that persist into equilibrium (heralded also by the emergence of non-Gaussian environmental states) renders the canonical system steady state almost always incorrect. The resulting equilibrium states deviate markedly from those predicted by standard perturbative techniques and are instead fully characterized by thermal states of the mapped system-collective coordinate Hamiltonian. We outline how noncanonical system states could be investigated experimentally to study deviations from canonical thermodynamics, with direct relevance to molecular and solid-state nanosystems.

Phonon scattering inhibits simultaneous near-unity efficiency and indistinguishability in semiconductor single-photon sources

Semiconductor quantum dots have recently emerged as a leading platform to efficiently generate highly indistinguishable photons [1-3], and this work addresses the timely question of how good these solid-state sources can ultimately be. Based on a microscopic theory, we establish the crucial impact that lattice relaxation has in these systems, which gives rise to a broad phonon sideband in bulk quantum dot emission spectra, as seen in Fig. (1) a. We show how both the indistinguishability and efficiency of a single photon source based on such a quantum dot in a modified photonic environment depends on the way in which this incoherent sideband is removed from the spectra [4].

Limits to coherent scattering and photon coalescence from solid-state quantum emitters

The desire to produce high-quality single photons for applications in quantum information science has lead to renewed interest in exploring solid-state emitters in the weak excitation regime. Under these conditions it is expected that photons are coherently scattered, and so benefit from a substantial suppression of detrimental interactions between the source and its surrounding environment. Nevertheless, we demonstrate here that this reasoning is incomplete, as phonon interactions continue to play a crucial role in determining solid-state emission characteristics even for very weak excitation. We find that the sideband resulting from non-Markovian relaxation of the phonon environment is excitation strength independent. It thus leads to an intrinsic limit to the fraction of coherently scattered light and to the visibility of two-photon coalescence at weak driving, both of which are absent for atomic systems or within simpler Markovian treatments.

Quantum correlations of light and matter through environmental transitions

One aspect of solid-state photonic devices that distinguishes them from their atomic counterparts is the unavoidable interaction between system excitations and lattice vibrations of the host material. This coupling may lead to surprising departures in emission properties between solid-state and atomic systems. Here we predict a striking and important example of such an effect. We show that in solid-state cavity quantum electrodynamics, interactions with the host vibrational environment can generate quantum cavity–emitter correlations in regimes that are semiclassical for atomic systems. This behavior, which can be probed experimentally through the cavity emission properties, heralds a failure of the semiclassical approach in the solid state, and challenges the notion that coupling to a thermal bath supports a more classical description of the system. Furthermore, it does not rely on the spectral details of the host environment under consideration and is robust to changes in temperature. It should thus be of relevance to a wide variety of photonic devices.

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

Phonon limit to simultaneous near-unity efficiency and indistinguishability in semiconductor single photon sources

T_2^*

Probing electron-phonon interaction through two-photon interference in resonantly driven semiconductor quantum dots

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

Probing emitter-cavity dressed states through environmental transitions

T_2

Intrinsic and environmental effects on the interference properties of a high-performance quantum dot single-photon source

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.