Quantum biology and ultraweak photon emission: an overlooked intersection?

Every living organism emits light. Not the dramatic glow of fireflies or jellyfish, but an ultraweak shimmer of photons, so faint that a single cell may release only a handful per second. These ultraweak photon emissions, known as UPE or biophotons, have hovered at the margins of mainstream science for a century. Now a team of physicists from Stellenbosch University, the University of Pretoria and the National Institute for Theoretical and Computational Sciences in South Africa argues that this peripheral subject may be hiding in plain sight at the heart of one of science’s most fashionable fields: quantum biology.

The new Perspective article, published in Frontiers in Photobiology by Betony Adams, Adriana Marais, Tjaart P. J. Krüger and Francesco Petruccione, examines why two research communities that share deep conceptual foundations have had surprisingly little conversation with each other. Quantum biology, which investigates whether quantum phenomena play functional roles in living systems, has flourished around canonical topics such as photosynthesis, avian magnetoreception and olfaction. UPE research, by contrast, has struggled to define a biologically relevant role for the light it studies, despite an experimental history stretching back to 1923, when Alexander Gurwitsch observed that growing onion roots separated by a quartz window, which blocked chemical signals but transmitted light, stimulated cell division in one another. He inferred that dividing cells emit weak mitogenetic radiation capable of triggering mitosis in nearby cells, founding the study of biophotons.

The principal objection against a functional role for UPE is brutally simple: the intensity is extraordinarily low. External measurements suggest photon densities far too weak for biological systems to detect and use reliably, making UPE look like an incidental metabolic by-product. But the authors argue that this reasoning may rest on a flawed assumption, namely that the external measurement reflects the internal optical environment of tissue. Cells are packed with light-reactive molecules such as chromophores that can absorb, re-emit or otherwise interact with photons, and scattering within biological media may redistribute photon trajectories, potentially enhancing local interactions while reducing detectable output. The internal photon density within tissues could therefore differ substantially from what external detectors infer.

More provocatively, the authors point to recent advances in quantum optics suggesting that biological systems may exploit non-linear optical mechanisms that amplify or coordinate weak signals into functionally significant effects. The invention of the laser shifted quantum optics towards high-intensity, coherent illumination for several decades, overshadowing alternative regimes. But renewed interest in non-linear processes driven by weak and incoherent light, including individual photon effects, two-photon excited fluorescence generated by thermal light, and second-harmonic generation under low-coherence illumination, is particularly relevant to quantum biology, where sunlight and UPE constitute low-coherence or low-intensity light sources.

Among the most tantalising concepts is superradiance, a collective light-matter interaction in which a group of emitters radiates with an intensity scaling greater than the sum of its parts. Biological structures rich in aromatic molecules, particularly tryptophan residues in ordered protein environments such as microtubules, have been proposed as plausible platforms for such effects. Recent theoretical work has suggested that densely packed tryptophan networks in amyloid fibrils may enable the absorption of high-energy, potentially damaging ultraviolet photons generated during oxidative processes, facilitating their downconversion into lower-energy, less harmful emissions. If such mechanisms operate in living systems, they could explain how ultraweak emissions participate in cellular communication and regulation, challenging the view that biophotons are merely incidental metabolic waste.

The authors also explore how UPE might extend quantum biology’s reach beyond the visual system. The radical pair mechanism, the leading theory of avian magnetoreception, requires photoexcitation of cryptochrome in the retina, where ambient illumination satisfies the need for light activation. But extending radical pair chemistry to processes deeper within the body or brain raises the question of how sufficient photons are generated to activate photosensitive molecules in tissues largely shielded from external light. UPE may provide a local source of endogenous photons capable of driving light-dependent radical pair reactions, though the authors acknowledge this hypothesis remains speculative and awaits experimental validation.

The most experimentally substantiated intersection between UPE and quantum biology centres on reactive oxygen species, or ROS. UPE is closely linked to redox processes and ROS dynamics in living systems. ROS can be generated either through sequential reduction of molecular oxygen, producing a cascade of species with distinct redox potentials and reactivities such as superoxide, hydrogen peroxide and the highly reactive hydroxyl radical, or via triplet-singlet energy transfer from excited pigments, generating singlet oxygen. Unmitigated ROS oxidise lipids, proteins and nucleic acids into unstable species that release light upon relaxing to ground states. Two types of UPE have been identified: a steady, low-intensity spontaneous emission from baseline metabolic ROS production, and stress-induced UPE produced when environmental stress overwhelms antioxidant defenses, driving widespread biomolecular oxidation.

This intrinsic connection suggests that UPE may provide a non-invasive window into cellular redox dynamics. In photosynthetic organisms, ROS play a central role in stress acclimation, acting as local and systemic signalling molecules that trigger photoprotective responses, antioxidant defenses and transcriptional reprogramming under excess light, drought, temperature stress or fluctuating conditions. The application of UPE to investigating ROS-mediated stress acclimation remains largely unexplored but could provide vital information on the spatiotemporal dynamics of photo-oxidative reactions in vivo, potentially revealing how photoprotective states suppress or redirect ROS-generating pathways during stress acclimation.

ROS are also a growing subject of interest in quantum biology, particularly in the context of spin chemistry. The radical pair mechanism, in which pairs of radicals are formed with correlated electron spins that can interconvert between singlet and triplet states under the influence of internal interactions and external magnetic fields, directly influences reaction pathways and product yields. This provides a mechanism by which weak magnetic fields can affect biochemical signatures. The mechanism has been used as a model for numerous biological effects involving reactive oxygen species, including neurogenesis, stem cell proliferation and differentiation, microtubule reorganisation, and even the action of psychoactive compounds such as lithium and xenon. Sceptics note that oxygen radical lifetimes appear too short to play a meaningful role in radical pair reactions, owing to rapid spin relaxation from strong spin-orbit coupling. But radical immobilisation within proteins, or alternative approaches invoking a quantum Zeno-type mechanism, have been suggested as ways to make superoxide-containing radical pairs feasible.

Both UPE and the radical pair mechanism are of interest in medical contexts. UPE has been explored as a non-invasive diagnostic indicator, with changes in photon emission linked to metabolic activity and oxidative stress, offering potential insight into disease states and physiological imbalance. In parallel, weak static magnetic fields may have therapeutic applications, particularly in modulating reactive oxygen species and related signalling pathways, with evidence pointing to a role for superoxide in mediating effects on cell proliferation and differentiation. lipid peroxidation has also been shown to be magnetosensitive, a fact which has been suggested to involve spin chemistry. Despite these concurrent developments, the relationship between UPE and the radical pair mechanism remains largely unexplored.

The authors propose that a more integrated understanding of how spin-dependent radical processes influence oxidative reactions, and consequently UPE, could help unify diagnostic and therapeutic perspectives, linking measurable optical outputs with controllable magnetic field effects. Several key questions remain largely unexplored, including whether non-linear optical phenomena enhance the biological relevance of UPE, whether endogenous photons generated during ROS-mediated reactions can provide a sufficient source of excitation for light-activated radical pair processes beyond the visual system, and whether spin-dependent radical pair reactions contribute directly to photon emission or its modulation by magnetic fields. Addressing these questions will require complementary theoretical and experimental approaches, including the development of open quantum systems frameworks describing weak-field dynamics in biological materials, systematic investigations of magnetic field effects on UPE, and adapting established radical pair protocols from quantum biology, such as radiofrequency experiments used in magnetoreception research, to UPE measurements. Coupled measurements of ROS, UPE and magnetic field responses may ultimately determine whether UPE provides a biologically meaningful source of photons for photo-activated radical pair reactions in tissues beyond the eye, establishing a unified framework linking endogenous light, oxidative metabolism, spin chemistry and magnetic field effects in living systems.

Subject of Research: Quantum Biology

Article Title: Quantum biology and ultraweak photon emission: an overlooked intersection?

Article References: Adams, B., Marais, A., Krüger, T. P. J., & Petruccione, F. (2026). Quantum biology and ultraweak photon emission: an overlooked intersection?. Frontiers in Photobiology, 4, Article 1911803. https://doi.org/10.3389/fphbi.2026.1911803

Image Credits: AI Generated

DOI: 10.3389/fphbi.2026.1911803

Keywords: quantum biology, ultraweak photon emission, biophotons, superradiance, radical pair mechanism, reactive oxygen species, spin chemistry, magnetic field effects, oxidative stress, Frontiers in Photobiology, Quantum, biology