The usefulness of the photon concept has been debated since its original proposal by Einstein in 1905. The photon picture has evolved via the “fuzzy-ball” and stochastic to the entangled, intrinsically quantum mechanical entity. However, in spite of the recent progress, questions about the nature of light still arise and often cause confusion from misinterpretations of various light-matter interaction phenomena. Questions are frequently asked about the existence of a `wave function for the photon’ and about the photon localization. Many light-matter interactions may be described using semiclassical theory and the question is when and where the quantum theory of radiation is needed to provide the best description? Here, we extend the discussion of these issues from several previous works on this subject. We use the definition of the photon wave function described by Scully and coworkers to discuss the photon detection in the context of single atoms and “giant atom” ensembles. We review the role of virtual photons, and discuss the effects of cooperative virtual processes on light harvesting efficiency of photodetectors and photosynthetic biological systems. Noise-induced quantum coherence may increase the performance of these quantum heat engines and the efficiency of photon harvesting.
The usefulness of the photon concept has been debated since its original proposal by Einstein in 1905. The photon picture has evolved via the “fuzzy-ball” and stochastic to the entangled, intrinsically quantum mechanical entity. However, in spite of the recent progress, questions about the nature of light still arise and often cause confusion from misinterpretations of various light-matter interaction phenomena. Questions are frequently asked about the existence of a `wave function for the photon’ and about the photon localization. Many light-matter interactions may be described using semiclassical theory and the question is when and where the quantum theory of radiation is needed to provide the best description? Here, we extend the discussion of these issues from several previous works on this subject. We use the definition of the photon wave function described by Scully and coworkers to discuss the photon detection in the context of single atoms and “giant atom” ensembles. We review the role of virtual photons, and discuss the effects of cooperative virtual processes on light harvesting efficiency of photodetectors and photosynthetic biological systems. Noise-induced quantum coherence may increase the performance of these quantum heat engines and the efficiency of photon harvesting.
The idea of the photon has stirred the imaginations of physicists ever since 1905 when Einstein originally proposed the use of light quanta to explain the photoelectric effect. This concept is formalized in the quantum theory of radiation, which has had unfailing success in explaining the interaction of electromagnetic radiation with matter, seemingly limited only by the ability of physicists to perform the indicated calculations. Nevertheless, it has its conceptual problems—various infinities and frequent misinterpretations. Consequently an increasing number of workers are asking, “to what extent is the quantized field really necessary and useful?” In fact the experimental results of the photoelectric effect were explained by G. Wentzel in 1927 without the quantum theory of radiation. Similarly most electro-optic phenomena such as stimulated emission, reaction of the emitted field on the emitting atom, resonance fluorescence, and so on, do not require the quantization of the field for their explanation. As we will see, these processes can all be quantitatively explained and physically understood in terms of the semiclassical theory of the matter–field interaction in which the electric field is treated classically while the atoms obey the laws of quantum mechanics. The quantized field is fundamentally required for accurate descriptions of certain processes involving fluctuations in the electromagnetic field: for example, spontaneous emission, the Lamb shift, the anomalous magnetic moment of the electron, and certain aspects of blackbody radiation. (The Compton effect also fits here, but see later under references 8b and c.) Here we will outline how the photon concept originated and developed, where it is not required and is often misused, and finally where it plays an essential role in the understanding of physical phenomena. In our discussion we will attempt to give a logically consistent definition of the word “photon”—a statement far more necessary than one might think, for so many contradictory uses exist of this elusive beast. In particular consider the original coining of the word by G. N. Lewis: “[because it appears to spend] only a minute fraction of its existence as a carrier of radiant energy, while the rest of the time it remains an important structural element within the atom…, I therefore take the liberty of proposing for this hypothetical new atom which is not light but plays an essential part in every process of radiation, the name photon!” (our exclamation point). Clearly the present usage of the word is very different.
Networks of interacting molecular optical dipoles play an important role in photosynthetic light harvesting, and also hold significant promise for future artificial technologies. This manuscript gives a brief overview of questions addressed by (our) recent theoretical work with a focus on taking inspiration from biological structures and processes to come up with designs for utilising bio-inspired collective quantum optical effects. These could, for instance, enable quantum-enhanced light harvesting, the charging of Dicke quantum batteries through superabsorption of light, and achieving efficient long-range energy transport. Particular attention is given to the role of inter-emitter quantum coherence as an enabler of these effects.
Abstract Efficient devices for light harvesting and photon sensing are fundamental building blocks of basic energy science and many essential technologies. Recent efforts have turned to biomimicry to design the next generation of light-capturing devices, partially fueled by an appreciation of the fantastic efficiency of the initial stages of natural photosynthetic systems at capturing photons. In such systems extended excitonic states are thought to play a fundamental functional role, inducing cooperative coherent effects, such as superabsorption of light and supertransfer of photoexcitations. Inspired by this observation, we design an artificial light-harvesting and photodetection device that maximally harnesses cooperative effects to enhance efficiency. The design relies on separating absorption and transfer processes (energetically and spatially) in order to overcome the fundamental obstacle to exploiting cooperative effects to enhance light capture: the enhanced emission processes that accompany superabsorption. This engineered separation of processes greatly improves the efficiency and the scalability of the system.
The centrality of the photon concept in modern physics is strongly evident in wide spheres of photonics and nanophotonics. Despite the resilience and persistence of earlier classical representations, there are numerous optical features and phenomena that only truly photon-based descriptions of theory can properly address. It is crucial to cast theory in terms of observables, and in this respect the quantum theory of light engages most directly and pragmatically with experiment. No other theory adequately reconciles the discreteness in energy of optical quanta, with their characteristic quantum mechanical delocalization in space. Examples of the distinctiveness of a photonic representation are to be found in nonclassical optical correlations; intensity fluctuations and phase; polarization, spin, and information content; measures of optical chirality; near-field interactions; and plasmonics. Increasingly, links between these fundamental properties and features are proving significant in the context of nanoscale interactions. Yet, even as new technologies are being built on the framework of modern photonics, a number of difficult questions surrounding the nature of the photon still remain. Both in its flourishing applications and in matters of fundamental entity, the photon is still a subject very much at the heart of current research.
concept is one of the most debated issues in the history of physical science. Some thirty years ago, we published an article in Physics Today entitled The Concept of the Photon, 1 in which we described the photon as a classical electromagnetic field plus the fluctuations associated with the vacuum. However, subsequent developments required us to envision the as an intrinsically quantum mechanical entity, whose basic physics is much deeper than can be explained by the simple 'classical wave plus vacuum fluctuations' picture. These ideas and the extensions of our conceptual understanding are discussed in detail in our recent quantum optics book. 2 In this article we revisit the concept based on examples from these sources and more.
We review and sharpen the concept of a photon wave function based on the quantum theory of light. We argue that a point-like atom serves as the archetype for both the creation and detection of photons. Spontaneous emission from atoms provides a spatially localized source of photon states that serves as a natural wave packet basis for quantum states of light. Photodetection theory allows us to give operational meaning to the photon wave function which, for single photons, is analogous to the electric field in classical wave optics. Entanglement between photons, and the uniquely quantum phenomena that result from it, are exemplified by two-photon wave functions.