⚛️ quantum physics

Optical Detection of Coherent Nuclear Spin Control in a Molecular System at Room Temperature

This study demonstrates the first optical detection of nuclear magnetic resonance and coherent control of $^{14}$ N nuclear spins in the photoexcited triplet state of the organic molecule 6,13-diazapentacene at room temperature, utilizing strong hyperfine coupling to enable fast Rabi oscillations for advanced molecular quantum sensing.

Original authors: Eoin O. Irving, Ziqiu Huang, Max Attwood, Sam L. Bayliss, Sarah K. Mann

Published 2026-10-14

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CC BY 4.0

Original authors: Eoin O. Irving, Ziqiu Huang, Max Attwood, Sam L. Bayliss, Sarah K. Mann

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Quantum sensing is a field dedicated to measuring the physical world with extreme precision, often by using the smallest possible building blocks of nature. At the heart of this work are tiny particles called spins, which act like microscopic magnets. Some of these spins belong to electrons, while others belong to the nuclei at the center of atoms. Electrons are excellent for sensing because they react strongly to magnetic fields and can be controlled with light, but they are also fragile and lose their information quickly. Atomic nuclei, by contrast, are incredibly stable and can hold onto information for much longer, making them ideal for memory. For years, scientists have sought to combine these two strengths: using the electron to read and write information, and the nucleus to store it. While this has been achieved in solid materials like diamonds, doing the same thing with flexible, custom-made molecules at room temperature has remained a significant challenge.

A team of researchers has now cleared this hurdle, demonstrating for the first time that they can use light to control the nuclear spins inside a molecule while it sits at room temperature. The scientists worked with a specific organic molecule called 6,13-diazapentacene, which was embedded in a crystal made of a different organic substance. When they shine a laser on this molecule, it absorbs the light and enters a special excited state where its electrons become polarized, meaning they are all aligned in a specific direction. This alignment is the starting point for their experiment. The molecule contains nitrogen atoms, and the nuclei of these nitrogen atoms possess their own spins. Because the electrons and these nuclei are so close together, they interact strongly with each other. The researchers used this connection to their advantage, using radio waves to nudge the nuclear spins and light to see what happened.

The process begins with a laser pulse that prepares the molecule, followed by a microwave pulse that targets the electron spins. This microwave pulse acts as a switch, transferring the state of the electron to the nucleus. Once the nucleus is in a specific state, the researchers apply a radio frequency pulse to make the nuclear spin rotate or flip. Finally, they shine another laser pulse to read the result. If the radio frequency pulse successfully changed the nuclear spin, it alters how the electron behaves, which in turn changes the amount of light the molecule emits. By measuring this change in brightness, the team could confirm that they had successfully detected and controlled the nuclear spin. This technique, known as optically detected nuclear magnetic resonance, allowed them to see the specific energy transitions of the nitrogen nuclei without needing the extreme cold usually required for such delicate measurements.

The results showed that the researchers could not only detect these nuclear spins but also make them oscillate in a controlled, rhythmic pattern, a phenomenon known as Rabi oscillations. These oscillations occurred at a frequency of several million cycles per second, which is remarkably fast for nuclear spins. The speed was possible because the strong interaction between the electron and the nucleus amplified the effect of the radio waves, allowing the nuclei to respond much more quickly than they would on their own. This speed is crucial for quantum technologies, as it means information can be processed and stored rapidly. The team also mapped out the specific energy levels of the nitrogen nuclei, finding that their behavior matched theoretical predictions for how these atoms should act within the molecule's structure.

This achievement marks a significant step forward in the development of molecular quantum sensors. By proving that these complex interactions can be managed at room temperature, the researchers have opened the door to using custom-designed molecules as versatile tools for sensing. The ability to chemically engineer these molecules means that scientists can potentially tune their properties to suit specific needs, such as detecting magnetic fields in biological tissues or measuring pressure with high precision. The work also suggests that these molecules could serve as memory units in future quantum computers, where the stable nuclei store data while the electrons handle the processing. While the current study focuses on a single type of molecule, the principles demonstrated here provide a foundation for creating more advanced systems that combine the best features of electrons and nuclei for the next generation of quantum technology.

Technical Summary: Optical Detection of Coherent Nuclear Spin Control in a Molecular System at Room Temperature

Problem Statement
Optically addressable electron spins in molecular systems offer versatile platforms for quantum sensing, combining tunable qubit properties with molecular-scale spatial control. While coupled nuclear spins provide a critical resource for enhancing sensing performance due to their long lifetimes (acting as quantum memories), accessing and controlling these nuclear spins at room temperature within molecular systems has remained unexplored. Previous demonstrations of optically detected nuclear magnetic resonance (OD-NMR) and coherent nuclear spin control have been limited to cryogenic temperatures or solid-state defects (e.g., nitrogen-vacancy centers in diamond). For molecular systems to realize the full potential of electron-nuclear registers for quantum sensing, two specific requirements must be met: (i) optical detection of nuclear magnetic resonance and (ii) optically detected coherent control of nuclear spin states, both at room temperature.

Methodology
The authors utilized a 6,13-diazapentacene (DAP) molecule, a 14N-substituted pentacene derivative, doped into a para-terphenyl (PTP) crystal host. The system relies on the photoexcited triplet state of DAP, where optical excitation induces spin-selective intersystem crossing (ISC) from the singlet ground state ( $|S_0\rangle$ ) to the triplet manifold ( $|T_1\rangle$ ), creating a spin-polarized electron spin.

The experimental setup involved:

Optical Addressability: A 520 nm laser was used to initialize the triplet state and read out the spin state via photoluminescence (PL) intensity changes, which depend on spin-dependent relaxation pathways.
Microwave (MW) Control: MW pulses were applied to drive transitions between electron spin sublevels ( $|X\rangle$ , $|Y\rangle$ , $|Z\rangle$ ) to manipulate electron spin populations.
Radio Frequency (RF) Control: RF pulses were used to drive nuclear spin transitions of the $I=1$ $^{14}\text{N}$ nuclei.
OD-ENDOR Sequence: The team employed an optically detected Davies ENDOR pulse sequence. This involved:
1. Optical initialization.
2. A selective MW $\pi$ -pulse to create nuclear spin polarization within a specific electron sublevel.
3. An RF pulse to drive transitions between nuclear spin states.
4. A second MW $\pi$ -pulse to map changes in nuclear spin populations back to electron spin populations.
5. Optical readout of the resulting PL intensity difference.

Key Results

Room-Temperature OD-NMR: The authors successfully demonstrated the first optical detection of NMR in a molecular system at room temperature. By sweeping the RF frequency while applying a selective MW pulse on the $|X\rangle \leftrightarrow |Z\rangle$ transition, they observed three distinct resonances at 0.60, 2.91, and 3.51 MHz. These resonances correspond to transitions between the quadrupolar eigenstates of the $^{14}\text{N}$ nuclei within the longer-lived $|Z\rangle$ electronic manifold. Fitting the spectrum yielded a quadrupolar coupling constant $|e^2qQ/h| = 4.28$ MHz and an asymmetry parameter $\eta = 0.28$ , values consistent with density functional theory calculations and structurally related systems.
Coherent Nuclear Spin Control: The study achieved optically detected coherent control of $^{14}\text{N}$ nuclear spins. By selecting resonant MW and RF frequencies associated with the $|X\rangle \leftrightarrow |Y\rangle$ transition and varying the RF pulse duration, the authors observed clear Rabi oscillations of the nuclear spins.
Hyperfine-Enhanced Rabi Frequencies: The observed nuclear Rabi frequencies were on the order of MHz. This represents an enhancement of approximately three orders of magnitude compared to the expected direct nuclear drive frequency (~330 Hz). This enhancement is attributed to strong hyperfine mixing between the electron and nuclear spins within the $|X\rangle$ and $|Y\rangle$ manifolds, mediated by second-order perturbation terms ( $A_{zz}^2/2E$ ). This mixing imparts partial electronic character to the nuclear transitions, allowing them to couple strongly to the applied RF field.
Power and Detuning Dependence: The nuclear Rabi frequency exhibited a linear dependence on the square root of the RF power, consistent with coherent driving. Detuning the RF frequency from resonance resulted in the expected increase in oscillation frequency, confirming the coherent nature of the control.

Significance and Claims
The paper claims to present the first demonstration of both OD-NMR and optically detected coherent nuclear spin control in a molecular system at room temperature. The work establishes that molecular systems can support optically readable nuclear-spin qubits under ambient conditions.

The authors emphasize that the strong hyperfine coupling in this system facilitates fast coherent nuclear-spin rotations, which is crucial for reducing the overhead time in electron-nuclear spin sensing schemes. They posit that this work lays the foundation for leveraging nuclear spins as memories to enhance molecular-based quantum sensing. Furthermore, the synthetic tunability of molecular systems offers a unique pathway to engineer nuclear spin environments, optimize hyperfine couplings, and balance gate times against decoherence. The ability to switch the hyperfine interaction on and off via optical excitation (due to the spin-free ground state of photoexcited triplets) is highlighted as a potential mechanism for extending nuclear spin coherence while retaining fast gate operations and optical readout capabilities.

The study does not claim to have achieved single-shot readout or specific nanoscale sensing applications yet, but rather establishes the fundamental capability to initialize, coherently control, and read out nuclear spins in a molecular platform at room temperature, a prerequisite for such advanced applications.

Technical Summary: Optical Detection of Coherent Nuclear Spin Control in a Molecular System at Room Temperature

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