arXiv subjects

Dominic M. Rouse

Publications and source records attributed to Dominic M. Rouse.

5 recordsLinked to original sources

arXiv2025

The Effect of Permanent Dipoles on Dark States in Molecular Dimers

Many organic molecules possess large permanent dipole moments that differ depending on the electronic state. These permanent dipoles influence both intermolecular coupling and interactions with the optical fields, yet they are often neglected in typical theoretical quantum optics treatments. Here, we investigate the optical properties and their effect on dark states of dimers possessing such permanent dipoles. We show that when monomers have excitation-dependent permanent dipoles, optical transitions between the bright and dark states of the dimer are enabled. We investigate how permanent dipoles allow for the existence of static driving terms between the ground and excited states of each monomer. In turn, these can cause the excited states of the monomers to couple indirectly to the zero excitation state of the dimer. This leads to interference between permanent and transition dipoles and can result in the formation of dark states that are entirely localised. Furthermore, dark states formed through indirect coupling exhibit enhanced robustness against energy level fluctuations, which may improve the efficiency of the design of photovoltaic devices.

quant-ph↗

arXiv2022

Theory of photon condensation in an arbitrary gauge condensed matter cavity model

We derive an arbitrary-gauge criterion under which condensed matter within an electromagnetic field may transition to a photon condensed phase. Previous results are recovered by selecting the Coulomb-gauge wherein photon condensation can only occur for a spatially-varying field and can be interpreted as a magnetic instability. We demonstrate the gauge-invariance of our description directly, but since matter and photons are gauge-relative concepts we find more generally that photon condensation can occur within a spatially uniform field, and that the relative extent to which the instability is both magnetic and electric versus purely magnetic depends on the gauge.

quant-ph↗

arXiv2021

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↗

arXiv2021

Environmentally improved coherent light harvesting

Coherence-enhanced light harvesting has not been directly observed experimentally, despite theoretical evidence that coherence can significantly enhance light-harvesting performance. The main experimental obstacle has been the difficulty in isolating the effect of coherence in the presence of confounding variables. Recent proposals for externally controlling coherence by manipulating the light's degree of polarization showed that coherent efficiency enhancements would be possible, but were restricted to light-harvesting systems weakly coupled to their environment. Here, we show that increases in system-bath coupling strength can amplify coherent efficiency enhancements, rather than suppress them. This result dramatically broadens the range of systems that could be used to conclusively demonstrate coherence-enhanced light harvesting or to engineer coherent effects into artificial light-harvesting devices.

physics.chem-ph↗

arXiv2021

Avoiding gauge ambiguities in cavity QED

Systems of interacting charges and fields are ubiquitous in physics. Recently, it has been shown that Hamiltonians derived using different gauges can yield different physical results when matter degrees of freedom are truncated to a few low-lying energy eigenstates. This effect is particularly prominent in the ultra-strong coupling regime. Such ambiguities arise because transformations reshuffle the partition between light and matter degrees of freedom and so level truncation is a gauge dependent approximation. To avoid this gauge ambiguity, we redefine the electromagnetic fields in terms of potentials for which the resulting canonical momenta and Hamiltonian are explicitly unchanged by the gauge choice of this theory. Instead the light/matter partition is assigned by the intuitive choice of separating an electric field between displacement and polarisation contributions. This approach is an attractive choice in typical cavity quantum electrodynamics situations.

quant-ph↗