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arXiv:2512.15213

Quantum Dynamics of a Nanorotor Driven by a Magnetic Field

V. N. Binhi

physics.bio-phquant-phSubmitted Dec 17, 2025• Announced Dec 18, 2025• 0 citations
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Abstract

A molecular rotor mechanism is proposed to explain weak magnetic field effects in biology. Despite being nanoscale (1 nm), this rotor exhibits quantum superposition and interference. Analytical modeling shows its quantum dynamics are highly sensitive to weak, but not strong, magnetic fields. Due to its enhanced moment of inertia, the rotor maintains quantum coherence relatively long, even in a noisy cellular environment. Operating at the mesoscopic boundary between quantum and classical behavior, such a rotor embedded in cyclical biological processes could exert significant and observable biological influence.

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A molecular rotor mechanism is proposed to explain weak magnetic field effects in biology. Despite being nanoscale (1 nm), this rotor exhibits quantum superposition and interference. Analytical modeling shows its quantum dynamics are highly sensitive to weak, but not strong, magnetic fields. Due to its enhanced moment of inertia, the rotor maintains quantum coherence relatively long, even in a noisy cellular environment. Operating at the mesoscopic boundary between quantum and classical behavior, such a rotor embedded in cyclical biological processes could exert significant and observable biological influence.
A mechanism for the biological effects of the hypomagnetic field is proposed, based on the rotational motion of a molecule as a whole within a cavity in an enzyme. It has been shown previously that a molecular rotator about 1 nm in size can have a decoherence time of up to tens of milliseconds. On shorter timescales, the nanorotator exists in a state of quantum superposition and exhibits interference effects. The statics and dynamics of the nanorotator in a magnetic field are analyzed using the Schrödinger and Liouville-von Neumann equations, taking into account chemical kinetics and thermal relaxation. It is demonstrated that the small-scale interference of the nanorotator is highly sensitive to weak magnetic fields. The quantum nanorotator shows magnetic effects ranging from a few to several tens of percent over a wide range of realistic decoherence rates. If the nanorotator constitutes an adequate model of rotating amino acid residues in the active sites of certain enzymes involved in protein synthesis, then weak magnetic fields can exert significant biological effects, leading to the observed phenomena.
The rotation of a nanoparticle in a quantum system has many applications, from theory to experiments. This paper will treat nanoparticle rotational dynamics for spin-embedded nanorotors. We will model it as a rigid body that properly treats the rotation in the coframe of the nanorotor in the presence of external fields. Besides rotation, we will further investigate how to create large spatial superpositions in the inhomogeneous external magnetic field, such as in the case of the Stern-Gerlach apparatus. The spin-embedded nanorotors play a crucial role in creating matter-wave interferometers through their spin and external magnetic field interaction Hamiltonian. We aim to provide a holistic interpretation of the dynamics of three Euler angles, their quantum evolution, and the nanorotor's spatial motion in a Stern-Gerlach-type setup, where we will consider one-full-loop interferometry. We will then study how the quantum evolution of all the Euler angles leads to a spin coherence loss upon interference and what manifests the Einstein-de Haas effect in an external magnetic field. In particular, we show that by imparting rotation along the direction of the magnetic field, we can stabilize the nanorotor's libration mode. We will also extend our analysis to a case where the initial state of the libration mode is thermal and discuss the contrast loss due to interference of the nanorotor upon one-loop completion.
Classical rotations of asymmetric rigid bodies are unstable around the axis of intermediate moment of inertia, causing a flipping of rotor orientation. This effect, known as the tennis racket effect, quickly averages to zero in classical ensembles since the flipping period varies significantly upon approaching the separatrix. Here, we explore the quantum rotations of rapidly spinning thermal asymmetric nanorotors and show that classically forbidden tunneling gives rise to persistent tennis racket dynamics, in stark contrast to the classical expectation. We characterize this effect, demonstrating that quantum coherent flipping dynamics can persist even in the regime where millions of angular momentum states are occupied. This persistent flipping offers a promising route for observing and exploiting quantum effects in rotational degrees of freedom for molecules and nanoparticles.
Coherent superposition states of a mesoscopic quantum object play a major role in our understanding of the quantum to classical boundary, as well as in quantum-enhanced metrology and computing. However, their practical realization and manipulation remains challenging, requiring a high degree of control of the system and its coupling to the environment. Here, we use dysprosium atoms-the most magnetic element in its ground state-to realize coherent superpositions between electronic spin states of opposite orientation, with a mesoscopic spin size J = 8. We drive coherent spin states to quantum superpositions using non-linear light-spin interactions, observing a series of collapses and revivals of quantum coherence. These states feature highly non-classical behavior, with a sensitivity to magnetic fields enhanced by a factor 13.9(1.1) compared to coherent spin states-close to the Heisenberg limit 2J = 16-and an intrinsic fragility to environmental noise.
Quantum biology is the study of quantum effects on biochemical mechanisms and biological function. Quantum physics describes the behaviour of the nanoscale particles that make up all matter including living organisms, but it was generally believed that the ‘warm, wet and noisy environment’ in living organisms would result in decoherence making it impossible for meaningful quantum effects to occur. However, there is now a substantial body of work supporting that nature has actually adapted in such a way as to exploit quantum properties to enhance cellular functioning and is believed to have role in a diverse range of key processes in living organisms ranging from maintaining the stability of DNA to neuron function to conscious cognition; from light harvesting in photosynthesis to avian magnetic field based navigation. This review aims to summarize the various mechanisms and functions in living organisms believed to utilize quantum mechanics to purposefully and effectively enhance their performance, and to explore the potential this could hold in diagnosing and treating various medical conditions.
Radical pairs and the dynamics they undergo are prevalent in many chemical and biological systems. Specifically, it has been proposed that the radical-pair mechanism results from a relatively strong hyperfine interaction with its intrinsic nuclear spin environment. While the existence of this mechanism is undisputed, the nanoscale details remain to be experimentally shown. Here, we analyze the role of a quantum sensor in detecting the spin dynamics (non-Markovian) of individual radical pairs in the presence of a weak magnetic field. We show how quantum control methods can be used to set apart the dynamics of the radical-pair mechanism at various stages of the evolution. We expect these findings to have implications to the understanding of the physical mechanism in magnetoreception and other biochemical processes with a microscopic detail.