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Quantum physics meets biology
Markus Arndt, Thomas Juffmann, Vlatko Vedral
TL;DR
The paper addresses how quantum physics may contribute to biology as both fields extend toward molecular and increasingly complex systems, despite limited evidence for biologically relevant non-trivial quantum effects. It provides a guided synthesis of quantum phenomena, biological applications, experiments, and proposed future directions. The review concludes that experimental demonstrations of quantum coherence in biology remain limited to a few molecules, while evidence and theoretical understanding remain incomplete.
Problem
The paper examines what role non-trivial quantum phenomena—such as long-lived coherence, entanglement, and single-quanta effects—play in biological processes, where interpretation of findings remains unsettled.
Method
The article recapitulates elementary quantum phenomena, discusses decoherence and dephasing, and surveys theories and experiments at the interface of quantum physics and biology.
Results
Experimental demonstrations of quantum coherence in biology remain limited to a few molecules, including quantum chemistry, tunneling, coherent excitation transport, and local spin effects.
Takeaways & Limitations
The review presents quantum biology as a growing research interface whose hypotheses range from exploratory and visionary to speculative, very likely, or simply wrong.
Takeaways & Limitations
Experimental facts are largely missing and theoretical understanding remains incomplete, limiting precise classification of proposed quantum-biology hypotheses.
Abstract
from arXiv · showhide
Quantum physics and biology have long been regarded as unrelated disciplines, describing nature at the inanimate microlevel on the one hand and living species on the other hand. Over the last decades the life sciences have succeeded in providing ever more and refined explanations of macroscopic phenomena that were based on an improved understanding of molecular structures and mechanisms. Simultaneously, quantum physics, originally rooted in a world view of quantum coherences, entanglement and other non-classical effects, has been heading towards systems of increasing complexity. The present perspective article shall serve as a pedestrian guide to the growing interconnections between the two fields. We recapitulate the generic and sometimes unintuitive characteristics of quantum physics and point to a number of applications in the life sciences. We discuss our criteria for a future quantum biology, its current status, recent experimental progress and also the restrictions that nature imposes on bold extrapolations of quantum theory to macroscopic phenomena.
1 Introduction
The article examines how quantum physics connects to biology as both fields increasingly address molecular and complex systems. It distinguishes established quantum phenomena and applications from hypotheses whose biological relevance remains uncertain.
- Modern biology, pharmacology, and medicine increasingly explain macroscopic phenomena through molecular interactions and organic information processing.
- The article asks what role quantum physics can play in biology as the size scales studied by both fields increasingly approach one another.
- The discussion begins with established physics, treats decoherence and dephasing as central transition mechanisms, and then reviews experiments, theories, and open speculations.
- It surveys genuine quantum phenomena in biomolecules, including photon anti-bunching, matter-wave delocalization, and elementary quantum algorithms in nucleotides.
- It defines non-trivial quantum biology around long-ranged, long-lived, or multiparticle coherences, entanglement, and single-quanta effects that may trigger macroscopic phenomena.
2 A brief review of elementary quantum phenomena
Quantum physics encompasses phenomena that are unusual because they conflict with everyday classical expectations. Its scope therefore includes behaviors that require concepts beyond ordinary intuition.
- Quantum physics includes a wide variety of phenomena rather than a single defining effect.
- Many quantum phenomena are regarded as unusual because they violate everyday expectations about how nature should behave.
- The section frames elementary quantum phenomena as departures from classical intuition.
2.1 Quantum discreteness
Quantum discreteness means that some physical properties take countable rather than continuous values. In biology, discrete spectral energies provide fingerprints for chemical substances across nanomatter.
- Quantum physics derives its name from the discreteness of nature, with some physical properties restricted to countable values.
- Electronic energies in atoms and vibrational energies in molecules are examples of quantized physical properties.
- Quantized spectral lines are used in the life sciences as fingerprints of chemical substances.
2.2 Quantum superposition
Quantum superposition, interference, tunneling, spin, and coherent energy states extend physical behavior beyond classical alternatives and support several life-science applications. The evidence ranges from imaging and structural analysis to biomolecular spectroscopy and photosynthetic excitation transfer.
- 2.2 Quantum superposition: Quantum states can describe coexistence of mutually exclusive classical possibilities, forming the basis of quantum superposition.
- 2.2.1 Wave-particle duality of light: Single photons display wave-particle duality: they propagate in a delocalized manner but are detected as localized energy packets E = hν.
- 2.2.2 Wave nature of matter: Matter-wave behavior supports high-resolution electron microscopy, neutron analysis of crystallized proteins, and diffraction experiments with entire C60 molecules.
- 2.2.3 Quantum tunneling: Quantum tunneling permits particles to cross barriers that classical mechanics forbids, with probability depending exponentially on barrier properties, mass, and kinetic energy.
- 2.2.4 Spin: Spin can exist in superpositions and contributes to molecular magnetism, while proton spins enable structural and functional imaging of biological tissue.
- 2.2.5 Quantum superposition of energy states: Femtosecond excitation can create coherent superpositions of electronic and vibrational energy states, and excitonic coherence has been reported across several photosynthetic pigment molecules.
2.3 Quantum statistics
Quantum statistics links particle spin to ensemble behavior, producing major physical consequences but requiring specialized conditions to observe. Although quantum degeneracy has been observed at 19 K in exciton-polaritons, macroscopic biological evidence remains absent.
- Bosons tend to occupy the same quantum state, whereas fermions avoid each other under otherwise identical conditions.The distinction follows from integer versus half-integer spin quantum numbers.
- Fermion statistics underlie Pauli’s exclusion principle, the stability of matter, and neutron stars.
- Boson statistics produce macroscopic effects including lasers and Bose-Einstein condensates.
- Observing quantum statistics with atoms requires highly specialized environments such as µK temperatures and ultra-high vacuum.
- 19 K exciton-polariton quantum degeneracy has been observed in condensed matter, but quantum statistics have not been experimentally demonstrated on a macroscopic biological scale.
2.4 Quantum entanglement and quantum information
Quantum entanglement is an inseparable non-classical correlation that can link particles across long distances and times, but environmental interactions strongly constrain its biological realization. Experiments have demonstrated elementary quantum algorithms in biomolecular nuclear spins, while meaningful biological quantum information processing remains experimentally unestablished.
- Entanglement is an inseparable, non-classical correlation between two objects or properties that can theoretically persist across long distances and times.External interactions and measurements can perturb the connection.
- Polarization-entangled photons have individually undefined polarizations before measurement but are predicted with certainty to have orthogonal polarizations.The state is represented as ψ ∝|H1⟩|V2⟩± |V1⟩|H2⟩.
- A nonlinear crystal can convert photons into lower-energy pairs whose polarizations are quantum correlated.
- NMR experiments demonstrated first quantum-computing circuits and elementary algorithms using a few nuclear spins within an individual biomolecule.The experiments used a condensed-matter environment at temperatures of a few kelvin.
- Dense-ensemble NMR quantum computing is hampered by ground-state initialization difficulty and rapid randomization of quantum phases by the host matrix.
- In macroscopic biological systems, molecular superpositions rapidly become entangled with the environment, and experimental evidence for meaningful quantum information processing remains absent.
3 The quantum-to-classical transition
The transition from quantum to classical behavior reflects the reduced observability of quantum effects through scale, phase averaging, decoherence, and environmental interactions. Biological conditions are especially restrictive, although macroscopic observables may still reveal entanglement in complex systems.
- 3.1 Some general insights: The smallness of Planck’s constant helps explain why quantum effects are difficult to observe in macroscopic biological phenomena.The paper illustrates this with an extremely short de Broglie wavelength for an adult man walking.
- 3.1 Some general insights: Decoherence, phase averaging, and related mechanisms gradually reduce quantum-effect observability without introducing a qualitative quantum-to-classical transition.Objective-collapse models instead postulate an abrupt change in dynamics.
- 3.2 Can quantumness survive in biological environments?: Quantum coherence is highly sensitive to environmental information: scattering a single visible photon per molecule can destroy interference through a 250 nm-separated diffraction grating.
- 3.2 Can quantumness survive in biological environments?: Biomolecular matter-wave coherence has nevertheless been observed at internal temperatures exceeding 690 K.
- 3.2 Can quantumness survive in biological environments?: Biology operates in open systems far from thermal equilibrium, motivating hypotheses that living systems may provide local cooling or repeatedly refresh entanglement.These ideas are presented as hypotheses rather than established biological mechanisms.
- 3.3 How can quantum entanglement be revealed in mesoscopic systems?: Macroscopic thermodynamic variables such as magnetic susceptibility or heat capacity may serve as entanglement witnesses in complex condensed-matter environments.The proposed strategy is to identify observables that can be reliably measured experimentally.
4 Life science research with an interface to quantum physics
Quantum effects appear in several life-science processes, from single-photon detection and molecular tunneling to photosynthetic excitation transfer, retinal isomerization, and magnetic orientation. Evidence includes molecular-scale quantum behavior and short-lived coherence, while the biological relevance of some proposed mechanisms remains unsettled.
- Single-photon phenomena: Between two and seven photons usually suffice for perception by a dark-adapted human observer.Photon counting reliability can be limited mainly by quantum shot noise.
- Single-photon phenomena: Single-molecule fluorescence reveals discrete molecular energy states and photon anti-bunching rather than thermal-light bunching.An excited molecule generally cannot absorb a second identical-wavelength photon before decay.
- Quantum tunneling: Electron tunneling has been identified in photosynthesis, cellular respiration, and electron transport along DNA.Early cytochrome evidence showed large, temperature-independent reaction rates below 100 K, inconsistent with thermal activation alone.
- Photosynthesis: Photosynthetic complexes may involve coherent excitonic transfer, quantum tunneling, or matter-wave interference alongside largely non-coherent processes.Their pigment arrangement enables efficient excitation transfer toward the reaction center.
- Photosynthesis: 77 K FMO experiments observed exciton delocalization and coherence extending several nanometers for a few hundred femtoseconds.Two-dimensional Fourier transform spectroscopy probed excitation-transfer pathways on femtosecond timescales.
- Photosynthesis: The interplay of coherent exciton transfer, decoherence, and dephasing may produce robust short-time behavior even under ambient conditions.This balance is presented as central to the efficiency of the modeled natural reaction.
- Conformational states: Pulse-shaped femtosecond excitation modulated retinal photoisomerization yield by ±20%, with laser-phase dependence indicating quantum interference.A single photon can initiate the all-trans–13-cis retinal transition, although later rearrangement interacts with the thermal environment.
- Magnetic orientation: Migratory-animal orientation depends on visible light above a threshold energy and can be disrupted by oscillating magnetic fields at 0.1-10 MHz.A proposed explanation involves light-induced radical-pair formation in the eye.
5 Speculations on quantum information and biology on the large scale
Large-scale quantum-biology proposals extend quantum information concepts to consciousness and the origin of self-replication. The article emphasizes that these hypotheses lack experimental justification or face severe decoherence and feasibility constraints.
- Scope of large-scale proposals: Quantum-biology proposals at larger scales sometimes require quantum information as a defining ingredient.The article explicitly describes these hypotheses as experimentally unsupported and theoretically disputed.
- Quantum physics and the human mind: The Penrose–Hameroff model treats two microtubule conformations as quantum-bit values and links consciousness to gravity-induced wave-function collapse.The proposal combines quantum theory with general relativity to address consciousness.
- Quantum physics and the human mind: No useful coherent macroscopic superposition of two macromolecular conformations has yet been prepared and characterized, even in the laboratory.This is identified as a hard feasibility bound for the microtubule proposal.
- Origin of self-replication: A proposed macroscopic quantum-sorting mechanism would explain how a self-replicating molecule could emerge from random molecular trials.The idea addresses the origin of the molecular complexity required for self-replication and evolution.
6 Conclusions
Quantum biology is an expanding interface, but experimentally demonstrated biological quantum coherence remains limited to a few molecules. The conclusion therefore distinguishes established molecular phenomena from speculative macroscopic extrapolations.
- Conclusions: Experimental demonstrations of quantum coherence in biology remain limited to the scale of a few molecules.Examples include quantum chemistry, tunneling, coherent excitation transport, and local spin effects.
- Conclusions: Research at the interface between quantum physics and the life sciences is growing rapidly.The article presents quantum biology as a field attracting increasing scientific interest.
- Conclusions: The current status supports molecular quantum effects while leaving broader biological extrapolations subject to experimental constraints.This conclusion follows the passage’s contrast between demonstrated few-molecule coherence and the wider ambitions of quantum biology.