Dark states can be much better excitation carriers

When we study for the first time the structure of the atom in high school we encounter for the first time the concept of quantum excitation. We learn that excitation is the process in which a nucleus, electron, atom, ion, or molecule acquires energy that raises it to a quantum state (the excited state) higher than that of its ground state. And that’s it.

But what we do not learn in high school is that, because atoms are not usually isolated, that they group in huge bunches, there exists also collective excitations, that is, quantized modes that are a consequence of the cooperation and interactions of the individual atoms in a collective. Examples of these collective excitations are plasmons, for quantized oscillations of the electrons in a metal, or phonons, a quantum of crystal lattice vibrational energy.

Some more exotic collective excitations exist, such as polaritons. Polaritons are the result of a strong coupling of electromagnetic waves with an electric or magnetic dipole-carrying excitation, and, if we consider them quasiparticles, these represent somehow an hybrid state between light and matter.

The ability to create and engineer hybrid light-matter states can bring together the most advantageous properties of both worlds, such as the high speed and delocalization of photons together with the stability and interacting character of matter excitations. Besides fundamental prospects, polaritonic systems are interesting for many applications that cover, among others, future quantum technologies, both light harvesting and transport of energy and charge in organic materials, and even control of chemical reactions.

In order to create such hybrid light-matter states, it is usually necessary to reach the so-called collective strong coupling (CSC) between a light field and an ensemble of quantum emitters. This CSC regime is characterized by the coupling of the electromagnetic field to a set of states in the ensemble (the bright states) forming the polaritons. However, many states of the QEs stay uncoupled to the photons and are thus called dark states.

Despite the great deal of attention received by polaritons, the uncoupled dark states have often been ignored as they are assumed not to benefit from the light-matter coupling. Indeed, these pure matter states are considered only a source of losses for polaritons, their potential applications being limited to passive operations such as qubit storage. But this may not be the case.

Now, a team of researchers from IFIMAC, coordinated by Francisco J. García-Vidal (DIPC), demonstrates ¹ that dark states can be much better excitation carriers than their polariton counterparts.

Actually, the team shows that, whereas this customary picture of dark states being strongly localized works for photonic structures in which the electromagnetic spectrum is continuous, dark states display a delocalized character, similar to that exhibited by polaritons, in systems that support a discrete electromagnetic spectrum. In the case the main loss mechanism resides within the electromagnetic modes, then dark states become better carriers.

The researchers elaborate a simple model that is able to capture the basic ingredients of the interaction of an ensemble of quantum emitters with a photonic structure that displays a discrete electromagnetic spectrum. In this model they neglect dipole-dipole coupling and only consider a single electromagnetic mode and assume that the electromagnetic mode is resonant with the excitations within the quantum emitters.

Using this model they find that in the collective strong coupling regime of an electromagnetic field to an ensemble of emitters, not only the polaritons but also the dark states can feature a delocalized behavior across the system. This unforeseen result, given the fact that dark states are uncoupled to light, is of a very general nature, requiring only the discrete character of the relevant electromagnetic spectrum.

Thanks to this different perspective on the properties of strongly coupled systems, resonant structures with low to moderate quality factors could thus find a broad range of applications in, among others, excitonic circuits, energy transport, and quantum circuitry.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance