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

AI summary

Overview

Research area: Quantum physics — optically addressable molecular electron–nuclear spin systems, optically detected magnetic resonance (ODMR/ENDOR), and room-temperature quantum sensing.

Technical level: Advanced. The paper assumes familiarity with spin Hamiltonians, zero-field splitting, hyperfine and quadrupolar couplings, and pulsed ODMR/ENDOR sequences.

Scope: The paper reports the first room-temperature optical detection of nuclear magnetic resonance (OD-NMR) and the first optically detected coherent nuclear spin control (nuclear Rabi oscillations) in a molecular system, using the ¹⁴N nuclei of 6,13-diazapentacene (DAP) in a para-terphenyl (PTP) crystal host.

What This Paper Is About

Nuclear spins have long lifetimes and make attractive quantum memories, while electron spins couple strongly to fields and can be initialised and read out with light. Combining the two in a molecule — rather than a solid-state defect such as the NV⁻ centre in diamond — would add chemical tunability, but room-temperature optical detection and coherent control of nuclei in molecular systems had not been demonstrated. The authors set out to show both capabilities in a single molecular platform, using the two I = 1 ¹⁴N nuclei of the organic chromophore DAP in its photoexcited triplet state.

Key Contributions

1. First room-temperature OD-NMR in a molecular system. Using optically detected Davies ENDOR, the authors resolve ¹⁴N nuclear magnetic resonance transitions in DAP:PTP at room temperature and zero applied external magnetic field.
2. Extraction of ¹⁴N quadrupolar parameters from the optical signal. Fitting the OD-ENDOR spectrum yields the quadrupolar eigenvalues and the derived coupling constant and asymmetry parameter, benchmarked against a structurally related molecule (phenazine-d₈) and against density functional theory.
3. First optically detected coherent nuclear spin control in a molecule at room temperature. Nuclear Rabi oscillations are driven and read out optically, with the Rabi frequency scaling linearly with RF amplitude.
4. Quantification of hyperfine-enhanced nuclear driving. The measured nuclear Rabi frequencies are approximately three orders of magnitude larger than the direct nuclear drive estimated from the electron Rabi frequency and the ratio of gyromagnetic ratios, an enhancement attributed to hyperfine mixing within the |X⟩ and |Y⟩ manifolds.

Main Findings

• System and spin parameters: DAP in PTP shows zero-field splitting parameters D = 1390.4 MHz and E = −84.9 MHz, with initial triplet populations P_X : P_Y : P_Z ≃ 0.743 : 0.143 : 0.114. The authors note that ¹⁴N substitution of pentacene to form DAP previously improved optical-spin contrast from ∼20 % to 40 %.
• Three OD-ENDOR resonances: With a selective MW π-pulse on the |X⟩ ↔ |Z⟩ transition and a 20 µs RF pulse, three resonances are observed at 0.60, 2.91 and 3.51 MHz (assigned in the text as 0.61, 2.92 and 3.51 MHz). Because the RF duration exceeds the |X⟩ lifetime (1/k_X = 4 µs), the spectrum originates from splittings of the longer-lived |Z⟩ sublevel, and the signal shows negative contrast.
• Transition assignments: First-order hyperfine vanishes for the pure triplet eigenstates, and second-order terms in the |Z⟩ manifold are ∼A_xx²/D and A_yy²/D, i.e. ∼1 kHz — weak compared with the ∼4 MHz quadrupole interaction. The resonances are therefore assigned to quadrupolar eigenstate transitions ε_x − ε_z, ε_y − ε_x and ε_y − ε_z, using DFT hyperfine values A_xx, A_yy, A_zz = −0.793, −0.988, 23.2 MHz.
• Quadrupolar parameters: The spectral fit gives ε_x, ε_y, ε_z = −0.77, 2.14, −1.37 MHz, hence |e²qQ/h| = 2ε_y = 4.28 MHz and η = (ε_x − ε_z)/ε_y = 0.28. These closely match phenazine-d₈ (4.36 MHz, η = 0.21) and agree reasonably with DFT (4.44 MHz, η = 0.11), though the calculated asymmetry parameter underestimates the measured asymmetry.
• A much faster nuclear drive near |X⟩ ↔ |Y⟩: A two-dimensional OD-ENDOR map of MW and RF frequency requires only a 720 ns RF pulse, versus the 20 µs needed in the |Z⟩ manifold, and shows pronounced diagonal structures attributed to multiple overlapping proton transitions.
• Nuclear Rabi oscillations: Coherent nuclear Rabi oscillations within the |X⟩ and |Y⟩ manifolds are observed and fitted to a single decaying sinusoid, with Rabi frequencies on the order of ∼MHz.
• Power dependence: Rabi frequencies extracted by FFT give linear fits with slopes of 0.8 MHz W^−1/2 (nuclear) and 2.2 MHz W^−1/2 (electronic, on |X⟩ ↔ |Y⟩).
• Size of the enhancement: Using a measured electronic Rabi frequency of Ω_e = 3.0 MHz and the ¹⁴N-to-electron gyromagnetic ratio, the expected direct nuclear drive is Ω_¹⁴N = Ω_e γ_¹⁴N/γ_e ≈ 330 Hz — roughly three orders of magnitude below what is observed. The enhancement arises from second-order hyperfine mixing of the |X⟩ and |Y⟩ manifolds via A_zz Ŝ_z Î_z, of order A_zz²/2E, which gives nominally nuclear transitions partial electronic character.
• Detuning behaviour: Detuning the RF below resonance increases the oscillation frequency as expected, with an asymmetry that reflects the OD-ENDOR line shape.

Methodology in Plain English

The authors dissolve a small amount (0.05 %) of DAP into PTP crystals grown by the Bridgman method, so that individual DAP molecules sit isolated in a transparent host. A green (520 nm) laser pulse excites a DAP molecule into a spin-polarised triplet state; the spin state is then read out by the brightness of the photoluminescence that follows, because different triplet sublevels relax back to the ground state at different rates.

To address the nuclei, they use a pulsed sequence in the style of Davies ENDOR. A selective microwave π-pulse (350 ns, centred at 1.4771 GHz for |X⟩ ↔ |Z⟩; 340 ns at 171.3 MHz for |X⟩ ↔ |Y⟩) transfers spin population in a way that depends on the nuclear state; a radio-frequency pulse then attempts to flip the ¹⁴N nucleus; a second microwave π-pulse converts any nuclear change into a change in electron spin population, which is read out optically after a delay. Comparing signals with and without the RF pulse gives the OD-ENDOR contrast, measured with a lock-in amplifier. By sweeping RF frequency they record NMR spectra; by sweeping RF pulse length or amplitude they record Rabi oscillations. All measurements are at room temperature and in zero applied external field.

Why This Matters

The work shows that the two prerequisites for a molecular room-temperature electron–nuclear spin register — optical detection of NMR and optically detected coherent nuclear control — can both be met in one chemically tunable material. That shifts nuclear-spin-enhanced sensing from a solid-state-defect specialty toward designer molecules, where the nucleus, its placement, and its hyperfine coupling can in principle be chosen by synthesis. The hyperfine enhancement also shortens nuclear gate times, directly reducing sensor overhead in electron–nuclear sensing protocols, at the acknowledged cost of faster decoherence.

Real-world applications (as framed by the paper):

• Nanoscale NMR, including nuclear-spin correlation spectroscopy whose resolution is no longer limited by the electron-spin lifetime.
• Enhanced quantum sensing protocols that use nuclei as quantum memories, following the NV⁻ precedent.
• Room-temperature dynamic nuclear polarisation, building on pentacene-based platforms that are efficient DNP agents.
• Optical readout of ENDOR-based spin-label distance measurements, and integration into thin films, metal-organic frameworks, or molecules scaffolded on 2D material surfaces.

Industry relevance: Room-temperature operation and zero applied field remove cryogenics and superconducting magnets from the instrumentation envelope, and molecular synthesis is compatible with scalable, solution-processed or thin-film fabrication routes — attractive for sensing, materials characterisation, and molecular-scale metrology.

Future Directions

• Chemical tuning of the spin environment: Selective nuclear substitution to tune hyperfine coupling and balance fast gate times against nuclear coherence, plus sample deuteration to reduce proton-driven decoherence suggested by the proton features in the OD-ENDOR map.
• Exploiting the spin-free ground state: Because DAP's ground state is a singlet (S = 0), the hyperfine interaction can be switched off by relaxation, potentially preserving nuclear coherence between experiments and enabling memory protocols.
• Multi-nucleus registers: Multiple distinct optically addressable nuclear spins per molecule for greater storage capacity and for more advanced sensing and error-correction protocols.
• Detection of nuclei outside the host molecule: Extending OD-NMR to target spins in proximity to the sensor, and exploring the strong-driving regime and time-optimised quantum control that the MHz-scale Rabi rates now make accessible.

Target Audience

Quantum sensing and quantum information researchers working with optically addressable spins, particularly those in molecular spintronics, ENDOR/ODMR spectroscopy, and pentacene-family chromophores. It is also relevant to chemists designing spin-bearing molecules, and to research groups working on nanoscale NMR or dynamic nuclear polarisation who want an ambient-condition, chemically tunable alternative to diamond defect platforms.

Authors’ abstract

Molecular systems with optically addressable electron spins are emerging as versatile quantum sensing platforms, offering tunable qubit properties and molecular-scale spatial control. Owing to their longer lifetimes, coupled nuclear spins provide an attractive resource for enhancing sensing performance. For molecular spin systems, the ability to initialise, coherently control and readout nuclear spins at room temperature is crucial for realising such benefits. Here, we present the first optical detection of nuclear magnetic resonance and coherent nuclear spin control with a molecular system at room temperature. Specifically, we demonstrate optically detected Rabi oscillations of $^{14}$N nuclear spins in the photoexcited triplet state of the organic chromophore 6,13-diazapentacene. The strong hyperfine coupling between the electronic spin and $^{14}$N nuclei enhances the nuclear Rabi frequency, enabling fast coherent nuclear-spin rotations. This work lays the foundation for leveraging nuclear spins as memories for enhanced molecular-based quantum sensing.