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Frontiers in Photobiology ·

Volume 4 - 2026 | https://doi.org/10.3389/fphbi.2026.1911803

Quantum biology and ultraweak photon emission: an overlooked intersection?

  • Betony Adams1,2,3
  • Adriana Marais2,4*corresponding author
  • Tjaart P. J. Krüger5,6
  • Francesco Petruccione1,2,4
  1. 1Department of Physics, Stellenbosch University, Stellenbosch, South Africa
  2. 2National Institute for Theoretical and Computational Sciences (NITheCS), Stellenbosch, South Africa
  3. 3The Guy Foundation, Dorset, United Kingdom
  4. 4School of Data Science and Computational Thinking, Stellenbosch University, Stellenbosch, South Africa
  5. 5Department of Physics, University of Pretoria, Pretoria, Gauteng, South Africa
  6. 6National Institute for Theoretical and Computational Sciences (NITheCS), Pretoria, South Africa
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Abstract

Biophotons–or ultraweak photon emissions (UPE) – remain a peripheral research subject compared to conventional topics in quantum biology such as photosynthesis, avian magnetoreception, and olfaction, despite significant intersections between the fields. Re-examining UPE research through the lens of quantum biology may help address confounding questions around these light-matter interactions. One principal argument against a biologically functional role for UPE is the extremely low intensity. However, as quantum biology expands to consider non-linear optical phenomena, effects such as superradiance are attracting growing interest. These collective light–matter interactions could provide mechanisms through which initially weak photon signals become enhanced within organisms, increasing their effective biological impact. If such processes occur in living systems, they may help explain how ultraweak emissions participate in cellular communication and regulation, challenging the view that biophotons are merely incidental metabolic by-products. Another canonical topic in quantum biology is the radical pair mechanism, most famously used as a model to explain the avian compass, and more recently, applied to the production and balance of reactive oxygen species (ROS). The oxidative metabolic and stress-response processes that generate ROS are also implicated in the production of UPE. This would suggest a fruitful overlap of spin chemistry with UPE research and open a line of investigation as to whether magnetic field effects mediated through radical pairs have any influence on UPE production. In this short Perspective paper we will expand on these intersections between two fields that have had surprisingly little conversation between them.

Keywords

1 Introduction

In the first decades of the 20th century, our understanding of light–matter interactions advanced dramatically, with the emergence of quantum mechanics. Early models of atomic structure, first developed by Niels Bohr, helped establish how quantised energy levels govern the interactions of matter with radiation (Bohr, 1913; Heilbron, 2013). Bohr’s interests extended beyond inanimate matter. His 1933 lecture ‘Light and Life’ suggested that quantum mechanics might offer insights into the fundamental properties of living organisms (Bohr, 1933). Some decades earlier, the interaction of light with living matter had earned Niels Ryberg Finsen the 1903 Nobel Prize in Medicine for pioneering the use of light therapy to treat disease, particularly lupus vulgaris (Gøtzsche, 2011). While it is accepted that exogenous sources of light play an important role in biological processes such as photosynthesis, endogenously produced light has been less well studied, despite its long history. In 1923, Alexander Gurwitsch made the intriguing observation that the growing tips of one onion root increased cell division in adjacent roots despite being separated by a quartz window that blocked chemical interaction. From this, he inferred that dividing cells emit weak ‘mitogenetic radiation’ capable of stimulating mitosis in nearby cells (Gurwitsch and Gurwitsch, 1932). This suggestion that light could play a role in cellular communication founded the study of biophotons, now more commonly referred to as ultraweak photon emission (UPE) (Gurwitsch and Gurwitsch, 1932; Naumova et al., 2018; Volodyaev et al., 2023; Volodyaev and Beloussov, 2015; Mould et al., 2024).

On a separate trajectory, the field of quantum biology emerged out of early speculation–from, in addition to Bohr, founding figures of quantum mechanics such as Erwin Schrödinger and Pascual Jordan–that quantum phenomena might play a role in living systems. For much of the 20th century, however, it was met with scepticism, as the warm, wet, and noisy conditions of biology were assumed to preclude delicate quantum effects (Cao et al., 2020; Marais et al., 2018; McFadden and Al-Khalili, 2018; Kim et al., 2021; Scholes and Fleming, 2026). Despite this, quantum biology has a long experimental history, with evidence of tunnelling effects in enzymes observed as early as the 1960s (DeVault and Chance, 1966). There is also ample evidence that biological organisms are sensitive to magnetic fields, a sensitivity that includes–but may not be restricted to–magnetoreception (Hore and Mouritsen, 2016; Rodgers and Hore, 2009). While this magnetosensitivity is not necessarily indicative of quantum effects, a leading theory invokes the radical pair mechanism, which describes how magnetic fields can affect the outcomes of spin-selective chemical reactions (Zadeh-Haghighi and Simon, 2022a; van Huizen et al., 2019; Hack et al., 2021; Zhang et al., 2021; Tian et al., 2022; Xue et al., 2021; Ren et al., 2025; Yoshi et al., 2009; Zadeh-Haghighi et al., 2026; Zadeh-Haghighi and Simon, 2021; Zadeh-Haghighi and Simon, 2022b; Smith et al., 2021; Zadeh-Haghighi and Simon, 2022c; Lodesani et al., 2024; Marais et al., 2015). There is also experimental evidence that magnetic fields directly influence autofluorescence in biological systems by mediation of quantum spin (Ikeya and Woodward, 2021). Interest in quantum biology has been spurred by the development of increasingly sophisticated technologies–such as ultrafast spectroscopy–which enable researchers to probe biological processes on the relevant spatial and temporal scales (Cao et al., 2020; Marais et al., 2018). Ultrafast spectroscopy, for example, has been used to investigate whether quantum coherence might play a role in photosynthetic energy transfer (Cao et al., 2020). While the feasibility of quantum effects playing a non-trivial, functional role in biology remains a subject of debate (Cao et al., 2020; Marais et al., 2018; McFadden and Al-Khalili, 2018; Kim et al., 2021; Scholes and Fleming, 2026), quantum biology has, to a large extent, gained mainstream scientific acceptance, with a strong theoretical framework and an expanding range of biological contexts, ranging from DNA mutation, to ligand-receptor binding, to consciousness (Slocombe et al., 2022; Horsfield et al., 2017; Hoehn et al., 2015; Adams et al., 2022; Gassab et al., 2025).

Research into ultraweak photon emission (UPE) also has an established experimental history. However, despite intermittent experimental interest throughout the 20th century, UPE has remained on the margins of mainstream science, in part due to the extremely weak nature of the signals, which made them difficult to measure reliably with early instrumentation and contributed to concerns about reproducibility (Gurwitsch and Gurwitsch, 1932; Naumova et al., 2018; Volodyaev et al., 2023; Volodyaev and Beloussov, 2015; Mould et al., 2024). Advances in detection systems later confirmed that many biological systems emit ultraweak light as a byproduct of metabolic processes, particularly those involving oxidative reactions (Cifra and Pospíšil, 2014; Kobayashi, 2014; Pospíšil et al., 2014). For an excellent overview of the interdependence of ROS and UPE see Reference (Pospíšil et al., 2014). However, in contrast to quantum biology, UPE research has struggled to define a role for ultraweak radiation in biologically relevant processes, despite some suggestion that it may be involved in communication and signalling processes (Volodyaev and Beloussov, 2015; Popp et al., 1984; Cifra et al., 2015; Pospíšil and Prasad, 2026).

In recent years, UPE has begun to be invoked more often in the context of quantum biology, with the centenary of Gurwitsch’s original ideas marked by a paper reinterpreting his seminal onion experiment as a prototype for quantum biology (Babcock, 2024; Mould et al., 2024). UPE has been suggested to act as a source of endogenous photons for photo-activated processes in quantum biology, such as excitonic transport (Kurian et al., 2017) as well as possibly being implicated in the function of tubulin structures (Patwa et al., 2024; Babcock et al., 2024). The intersection between UPE and ROS is also well documented (Cifra and Pospíšil, 2014; Pospíšil et al., 2014; Hideg et al., 1991). In addition to this, magnetic fields effects have been implicated in ROS-related chemistry (Wang and Zhang, 2017), with radical pairs suggested as a possible mechanism (Zhang et al., 2021; Tian et al., 2022; Hack et al., 2021; van Huizen et al., 2019; Zadeh-Haghighi and Simon, 2022c; Zadeh-Haghighi et al., 2026; Zadeh-Haghighi and Simon, 2021; Smith et al., 2021). However, there is limited research into the effects of magnetic fields on UPE, despite evidence that magnetic fields have an effect on autofluorescence (Ikeya and Woodward, 2021). In this Perspective paper, we focus on two areas (see Figure 1) where this intersection may generate fruitful insights and help address outstanding questions: light–matter interactions–particularly the possibility of non-linear or collective effects–and spin biochemistry–especially mechanisms related to oxidative processes.

Potential intersections between quantum biology and ultraweak photon emission research
Figure 1. Potential intersections between quantum biology and ultraweak photon emission research

2 Novel light-matter interactions

At the heart of the argument against the relevance of UPE for biological function is their ultraweak nature (Cifra and Pospíšil, 2014; Kobayashi, 2014; Mould et al., 2024). Given this extremely low intensity, UPE is arguably unlikely to be reliably detected and used by biological systems and is therefore more plausibly interpreted as an incidental by-product of metabolism. There has been some attempt to counter this argument. The intensity of emission measured externally may significantly underestimate the effective photon density within tissues, where cells proliferate with light-reactive molecules–such as chromophores–that can absorb, re-emit, or otherwise interact with photons (Mould et al., 2024; Bókkon et al., 2010). In addition, scattering within biological media may redistribute photon trajectories, potentially enhancing local interactions while reducing detectable output (Bókkon et al., 2010; Piao, 2020). These considerations suggest that the internal optical environment of the cell could differ substantially from what is inferred from external measurements, allowing for the possibility that UPE may still participate in biologically relevant processes.

Another proposed resolution to the problem of low photon count is that biological systems may exploit non-linear optical mechanisms that amplify or coordinate weak signals into functionally significant effects. Although non-linear optical interactions involving low-intensity and low-coherence light have been studied since the early development of quantum optics (Zhao et al., 2020; Young et al., 2023), the invention of the laser shifted the field towards high-intensity, coherent illumination, overshadowing alternative regimes for several decades (Tamošauskas, 2011). Recent theoretical and experimental advances in quantum optics have renewed interest in non-linear processes driven by weak and incoherent light, including individual photon effects (Chang et al., 2014), coherence and two-photon excited fluorescence generated by thermal light (Shatokhin et al., 2018; Jechow et al., 2013), and second-harmonic generation under low-coherence illumination (Tamošauskas, 2011; Ji et al., 2019)]. This resurgence is particularly relevant to quantum biology, where sunlight and UPE constitute low-coherence or low-intensity light sources. For an excellent overview of non-linear optical effects as they relate to quantum biology and UPE, see Reference (Babcock, 2024).

These effects may include single photons coupling to multiple chromophores, enabling collective dynamics across molecular assemblies. In this context, concepts from quantum optics such as superradiance–where a group of emitters radiates with an intensity scaling greater than the sum of its parts–may act as potential mechanisms for enhancing otherwise weak emissions (Popp et al., 1984; Cifra et al., 2015). Biological structures rich in aromatic molecules, particularly tryptophan residues in ordered protein environments such as microtubules, have been suggested as plausible platforms for such effects (Celardo et al., 2019; Babcock et al., 2024; Gassab et al., 2026; Gassab and Craddock, 2026). Functionally relevant superradiant effects have been proposed in amyloid fibrils, where densely packed tryptophan residues may enable the absorption of high-energy, potentially damaging ultraviolet photons generated during oxidative processes, facilitating their subsequent downconversion into lower-energy, less harmful emissions (Patwa et al., 2024). While still speculative, these mechanisms suggest that biological systems may not only amplify weak photon signals but also regulate their energy, pointing toward a functional role for UPE in both signalling and photoprotection.

Research in quantum biology, particularly into the novel optical properties of chromophore architectures, may thus help explain how even very low photon numbers could retain biological relevance. At the same time, UPE research has the potential to broaden the scope of quantum biology by motivating deeper investigation into non-linear and collective optical effects in biomolecular systems. For instance, recent theoretical work has proposed mechanisms for neural synchronisation based on the endogenous entanglement of biophotons, with a focus on the optical properties of myelinated axons (Liu et al., 2024). A myelin sheath is an insulating, lipid-rich layer surrounding nerve fibres, and it has been suggested that myelin-coated axons could behave as biological microcavities, facilitating two-photon processes and generating entangled photons (Liu et al., 2024). Such proposals extend the study of light–matter interactions in living systems to include cavity quantum electrodynamics. There has been some experimental interest in the behaviour of biological materials–such as chromophores–within artificial cavities, as well as theoretical analysis of signal-enhancing phenomena like the Purcell effect in the context of bio-inspired systems (Quach et al., 2022; Kokin et al., 2022; Chung et al., 2024; Nation et al., 2023). However, little attention has been given to how biological materials themselves might function intrinsically as microcavities or resonant nanostructures. Exploring this possibility would open new avenues for understanding how light behaves within the complex, heterogeneous materials that constitute living organisms, and how effects such as signal enhancement, confinement, and strong coupling might be implicated in biological function.

3 UPE and spin chemistry

Spin chemistry was originally proposed as an explanation for avian magnetoreception, where photoexcitation of cryptochrome in the retina initiates spin-correlated radical pairs (Hore and Mouritsen, 2016; Rodgers and Hore, 2009). In this context, the requirement for light activation is satisfied by ambient illumination reaching the eye. However, extending radical pair chemistry to processes occurring 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. In this context, UPE may provide a local source of photons capable of driving light-dependent radical pair reactions. Although the intensity of UPE is generally considered too low for this purpose, proposed mechanisms of non-linear amplification could locally enhance photon availability. While this hypothesis remains speculative and awaits experimental validation, it provides a potential mechanistic bridge between endogenous photon production and light-activated quantum biological processes beyond the visual system.

Another potentially productive intersection between UPE research and quantum biology centres on reactive oxygen species (ROS) and spin chemistry. UPE is closely linked to redox processes and the dynamics of ROS in living systems (Cifra and Pospíšil, 2014; Pospíšil et al., 2014; Hideg et al., 1991; Kobayashi et al., 2014). While non-linear effects in biological systems remain a more speculative area of interest, the connection between ROS and UPE is among the most experimentally substantiated aspects of UPE research. ROS can be generated either through sequential reduction of molecular oxygen or via triplet–singlet energy transfer from excited pigments (Pospíšil et al., 2014). Stepwise oxygen reduction produces a cascade of species with distinct redox potentials and reactivities–such as superoxide, hydrogen peroxide, the highly reactive hydroxyl radical, as well as a variety of other species–while energy transfer from photosensitised pigments generates singlet oxygen. While this is the predominant pathway, singlet oxygen can also be produced in the dark during enzymatic and metabolic reactions or through multiple stress pathways (Mor et al., 2014). Although metabolic ROS generation plays vital physiological roles, unmitigated ROS can oxidise lipids, proteins, and nucleic acids into unstable species that release light upon relaxing to ground states (Pospíšil et al., 2014). Two types of UPE have been identified in the literature, dependent on whether they are produced from ROS from normal metabolic processes or stress responses (Pospíšil et al., 2014; Van Wijk et al., 2008; Kobayashi et al., 1999). In baseline metabolic processes, low-level ROS production yields a steady, low-intensity spontaneous emission. Under environmental stress–whether physical, chemical, or biological–ROS production overwhelms antioxidant defenses, driving widespread biomolecular oxidation and accelerating high-energy intermediate recombination to produce stress-induced UPE (Cifra and Pospíšil, 2014; Pospíšil et al., 2014; Hideg et al., 1991; Kobayashi et al., 2014). As a result, UPE can be considered an indirect marker of oxidative activity and metabolic state, with changes in ROS production correlating with variations in photon output. This intrinsic connection suggests that UPE may provide a non-invasive window into cellular redox dynamics (Pospíšil et al., 2014; Kobayashi et al., 2014). Interestingly, lipid peroxidation has also been shown to be magnetosensitive, a fact which has been suggested to involve spin chemistry (Sampson et al., 2019).

Beyond metabolism, ROS play a central role in stress acclimation in photosynthetic organisms. Under excess light, drought, temperature stress, or fluctuating environmental conditions, ROS can act as local and systemic signalling molecules that trigger photoprotective responses, antioxidant defences, and transcriptional reprogramming (Moustaka and Moustakas, 2025; Pospíšil, 2016; Miller et al., 2010). One such photoprotective response is known as qH, a sustained form of non-photochemical quenching (NPQ), that is activated by ROS signalling under abiotic stress conditions (Malnoë, 2018). The application of UPE to the investigation of ROS-mediated stress acclimation remains largely unexplored but could provide vital information on the spatiotemporal dynamics of photo-oxidative reactions and triplet-mediated processes 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. Central to this interest is 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. These spin dynamics directly influence reaction pathways and product yields, providing a mechanism by which weak magnetic fields can affect biochemical signatures. The radical pair mechanism has been studied predominantly in the context of avian magnetoreception (Hore and Mouritsen, 2016; Rodgers and Hore, 2009). More recently, however, this context has expanded to include how spin might be implicated in the generation and balance of reactive oxygen species (Zadeh-Haghighi and Simon, 2022a; Hack et al., 2021; van Huizen et al., 2019; Player and Hore, 2019; Karogodina et al., 2011; Usselman et al., 2016; Austvold et al., 2024; Adams et al., 2026).

There has been some scepticism around the fact that oxygen radical lifetimes appear to be too short to play any meaningful role in radical pair reactions (Player and Hore, 2019; Karogodina et al., 2011). For example, superoxide radicals in solution have very short lifetimes due to rapid spin relaxation. This arises from strong spin–orbit coupling, which efficiently links spin dynamics to molecular tumbling and total angular momentum. One way in which to mitigate this is by radical immobilisation within proteins. However this introduces inter-radical coupling interactions that can themselves reduce magnetosensitivity. Alternative approaches, such as invoking a quantum Zeno-type mechanism, have been suggested as a way in which radical pairs involving superoxide might be made feasible (Denton et al., 2024). Despite this, the radical pair mechanism has been used as a model for numerous biological effects involving reactive oxygen species, including neurogenesis (Zhang et al., 2021; Tian et al., 2022), stem cell proliferation and differentiation (Hack et al., 2021; van Huizen et al., 2019), microtubule reorganisation (Zadeh-Haghighi and Simon, 2022c; Zadeh-Haghighi et al., 2026), and even the action of psychoactive compounds such as lithium (Zadeh-Haghighi and Simon, 2021) and xenon (Smith et al., 2021). While the exact mechanism remains a subject of debate, magnetic field effects represent one of the most experimentally supported examples of possible quantum effects in biological systems. Their relevance to ROS chemistry (Wang and Zhang, 2017) raises the possibility that spin-dependent processes may play a broader role in regulating cellular redox balance and associated phenomena such as oxidative stress and biophoton emission.

Both UPE and the radical pair mechanism are also 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 (Cifra and Pospíšil, 2014; Pospíšil et al., 2014; Tsuchida et al., 2019; Kobayashi, 2014; van Wijk and van Wijk, 2005; Mould et al., 2024). In parallel, weak magnetic fields may have therapeutic applications, particularly in modulating reactive oxygen species and related signalling pathways. Studies suggest that weak static magnetic fields can influence cell proliferation and differentiation, with evidence pointing in particular to a role for superoxide in mediating these effects (Hack et al., 2021; van Huizen et al., 2019). Despite these concurrent developments, the relationship between UPE and the radical pair mechanism remains largely unexplored. 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, enabling both improved diagnostics and more targeted interventions.

4 Conclusion

Quantum biology and UPE research have largely developed along separate trajectories despite sharing important conceptual and mechanistic foundations. Closer integration between these fields has the potential to advance both. While UPE has historically been regarded as too weak to play a functional biological role, efforts to test this assumption may do more than simply establish the biological relevance of endogenous ultraweak photons. They also motivate investigation of mechanisms through which weak photonic signals may be amplified, coordinated, or confined within biological systems. At the same time, the close relationship between UPE and ROS places endogenous photon emission within the broader framework of quantum biology and spin chemistry, where the radical pair mechanism provides one of the best-established routes by which weak magnetic fields can influence chemical reactions. Understanding how radical pair dynamics regulate oxidative processes–and consequently UPE–raises the possibility that magnetic field effects may be linked to measurable optical signatures in living systems. Such a connection would have important implications for medicine, positioning UPE as a non-invasive indicator of cellular redox states while suggesting new opportunities for therapeutic magnetic field interventions informed by spin chemistry.

Several key questions have remained 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. On the theoretical side, this would involve the development of open quantum systems frameworks that describe weak-field dynamics in the context of biological materials and UPE and the radical pair mechanism (Babcock, 2024). Experimentally, systematic investigations of the effects of magnetic fields on UPE would provide a direct test of spin-dependent models, while adapting established radical pair protocols from quantum biology–such as radiofrequency experiments used in magnetoreception research–to UPE measurements could reveal whether spin-selective chemistry contributes to endogenous photon emission. Coupled measurements of ROS, UPE, and magnetic field responses, together with studies of non-linear optical mechanisms capable of amplifying endogenous photon signals, may ultimately determine whether UPE provides a biologically meaningful source of photons for photo-activated radical pair reactions in tissues beyond the eye. More broadly, such studies would stimulate the conversation between UPE and quantum biology, establishing a unified framework linking endogenous light, oxidative metabolism, spin chemistry, and magnetic field effects in living systems.

Correction note

A correction has been made to this article. Details can be found at: 10.3389/fphbi.2026.2008289.

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Footnotes

Edited by: Maurizio Benfatto, Universities and Research, Italy

Reviewed by: Ankush Prasad, Palacký University Olomouc, Czechia

Nathan Sean Babcock, Scale AI, United States

Statements

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Funding

The author(s) declared that financial support was received for this work and/or its publication. BA and TK are funded by NITheCS.

Data availability

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Author contributions

BA: Writing – review and editing, Writing – original draft. AM: Writing – original draft, Writing – review and editing. TK: Writing – original draft, Writing – review and editing. FP: Writing – original draft, Writing – review and editing.

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Adams, B., et al. (2026). Quantum biology and ultraweak photon emission: an overlooked intersection?. Frontiers in Photobiology, Volume 4. https://doi.org/
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Copyright © 2026 Adams, Marais, Krüger and Petruccione.

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*Correspondence: Adriana Marais

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