⚛️ quantum physics

Dense pentacene cocrystal demonstrates room-temperature coherent control

This paper reports the creation of a dense 6,13-dihydropentacene/pentacene cocrystal that achieves a record-breaking spin-site density while maintaining room-temperature coherent control and microsecond spin coherence by leveraging specific molecular packing and coformer energetics to suppress detrimental dipolar interactions and triplet migration.

Original authors: Noella D'Souza, Guangzhao Chen, Madhur Parashar, Wern Ng, Brandon Wallace, Tatiana Lindahl, Tanner S. Volek, Lam Lam, Joseph Garrett, Emanuel Druga, Sean T. Roberts, Jeffrey Reimer, Harpreet Singh, Li

Published 2026-09-25

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CC BY 4.0

Original authors: Noella D'Souza, Guangzhao Chen, Madhur Parashar, Wern Ng, Brandon Wallace, Tatiana Lindahl, Tanner S. Volek, Lam Lam, Joseph Garrett, Emanuel Druga, Sean T. Roberts, Jeffrey Reimer, Harpreet Singh, Liang Z. Tan, Riccardo Montis, Ashok Ajoy

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

Imagine a world where we could build sensors so sensitive they detect the faintest magnetic whispers of a single atom, or measure temperature changes smaller than a breath of air. This is the promise of quantum sensing, a field that relies on tiny, spinning particles called "spins" to act as microscopic probes. For these sensors to work well, scientists need to pack as many of these spinning particles as possible into a small space; the more they have, the clearer the signal. However, there is a stubborn problem: when you pack these particles too tightly, they begin to bump into one another, interfering with their delicate spins and causing the signal to fade away almost instantly. It is a classic trade-off where increasing the number of sensors destroys the very quality needed to read them.

For years, researchers have tried to solve this by diluting the sensors, mixing just a few of them into a sea of inactive material to keep them apart. While this works, it limits how strong the signal can ever be. A team of scientists has now found a way to break this rule. They have created a new type of crystal where the sensors are packed incredibly tightly—more than a hundred times denser than previous records—yet they still manage to keep their spins coherent and readable at room temperature. They achieved this not by keeping the sensors apart, but by carefully arranging them in a specific pattern that stops them from interfering with each other.

The researchers, working with a team from the University of California, Berkeley, and other institutions, focused on a molecule called pentacene. Pentacene is excellent for sensing because its spins can be controlled and read using light, but it usually suffers from the packing problem. When too many pentacene molecules are placed near each other, they start to share energy in ways that scramble their spins. To fix this, the team grew a special crystal made of two different types of molecules: pentacene and a related molecule called 6,13-dihydropentacene. They grew these crystals by heating pentacene in a controlled environment, causing it to transform and reassemble into a needle-like structure where the two molecules sit in a precise two-to-one ratio.

In this new crystal, the pentacene molecules are packed at a density of roughly 33 percent, meaning one out of every three spots in the crystal lattice is occupied by a sensor. This is an enormous concentration, far exceeding what has been possible with other materials like diamonds or previously doped crystals. The team measured the distance between these packed molecules and found them to be incredibly close, separated by only about four angstroms. In a typical setup, such closeness would cause the sensors to collide and lose their quantum properties almost immediately. Yet, when the researchers shined a green laser on the crystals, they found that the pentacene molecules still glowed with a distinct red light and, more importantly, their spins remained stable for microseconds.

To understand why this dense packing did not destroy the signal, the team looked closely at how the molecules interact. They used a technique called transient electron paramagnetic resonance, which acts like a high-speed camera for magnetic spins, to see how the molecules behave after being hit by light. They discovered that the spins were being generated in a specific, orderly way that avoided the chaotic energy transfers that usually happen in dense groups. Furthermore, they used computer simulations to map out the energy landscape of the crystal. The simulations revealed that the unique geometry of the crystal acts as a barrier. The neighboring molecules are positioned in such a way that their electronic clouds do not overlap significantly, and the energy required for a spin to jump from one molecule to another is too high. This effectively traps the spin on its original molecule, preventing the chain reaction of interference that usually ruins the signal.

The results were confirmed through a series of precise measurements. The team demonstrated that they could manipulate the spins of the entire dense group using microwave pulses, making them oscillate in a controlled rhythm. They measured how long these spins could stay in sync, finding that they maintained their coherence for about 0.87 microseconds, and up to 1.6 microseconds with special techniques to cancel out noise. While this time is short in human terms, it is a significant achievement for such a dense material and is comparable to much more dilute systems. They also measured the crystal's ability to detect magnetic fields, estimating a sensitivity that, while not yet optimized, proves the concept works.

This work suggests that the key to building better quantum sensors lies not just in the material itself, but in how it is arranged. By using a "coformer"—a second molecule that helps build the crystal structure—the researchers were able to engineer a environment where high density and high performance coexist. The crystal acts as a scaffold that holds the sensors in a perfect, non-interfering alignment. This approach opens the door to creating a new class of materials where sensors can be packed as densely as possible without losing their ability to communicate. It transforms the challenge of quantum sensing from a game of keeping things apart to a game of arranging them just right, offering a promising path toward more powerful and compact sensors for measuring the invisible forces of our world.

Technical Summary: Dense Pentacene Cocrystal Demonstrates Room-Temperature Coherent Control

Problem Statement
Quantum sensors based on optically addressable spins rely on maximizing the number of addressable spins ( $N$ ) within a fixed volume to improve sensitivity, which theoretically scales as $N^{-1/2}$ . However, increasing spin-site density typically introduces detrimental effects: stronger dipolar interactions broaden resonance lines and shorten coherence times ( $T_2$ ), while dense chromophore packing facilitates excited-state transport (e.g., triplet migration, singlet fission, and triplet-triplet annihilation) that quenches optical readout. This creates a fundamental trade-off where high-performance solid-state sensors, such as nitrogen-vacancy (NV) centers in diamond or pentacene-doped $p$ -terphenyl (PDP), must operate at low concentrations (typically a few to tens of ppm) to preserve coherence. The central challenge is to increase spin-site density without sacrificing spin coherence or optical addressability.

Methodology
The authors investigate a 2:1 cocrystal of 6,13-dihydropentacene (DHP) and pentacene (Pc) as a solution to this density-coherence trade-off. The material was synthesized via physical vapor transport of pentacene under an inert atmosphere, inducing thermal disproportionation to generate DHP in situ. The resulting pink needle-like crystals were characterized using:

Structural Analysis: Single-crystal X-ray diffraction (XRD) and scanning electron microscopy (SEM) to determine lattice parameters and morphology.
Optical Spectroscopy: Steady-state UV/Vis absorption and fluorescence measurements to assess intermolecular coupling and Davydov splitting.
Spin Dynamics: Transient electron paramagnetic resonance (trEPR) to identify triplet generation pathways (intersystem crossing vs. singlet fission) and sublevel polarization. Transient absorption microscopy (TAM) and time-correlated single-photon counting (TCSPC) were used to resolve ultrafast excited-state dynamics.
Coherent Control: Room-temperature optically detected magnetic resonance (ODMR) experiments employing Rabi oscillations, Ramsey interferometry, Hahn-echo, and CPMG-4 dynamical decoupling sequences to measure coherence times ( $T_2^*$ and $T_2$ ).
Theoretical Modeling: Density-functional theory (DFT) calculations were performed to evaluate electronic coupling between nearest-neighbor pentacene molecules, orbital energy splitting (Davydov splitting), and energetic barriers for triplet migration.

Key Contributions and Results

Record-Breaking Spin Density: The DHP/Pc cocrystal achieves a pentacene concentration of 33.3 mol% ( $3.3 \times 10^5$ ppm), corresponding to a volumetric spin-site density of $9.51 \times 10^{20}$ cm $^{-3}$ . This exceeds the density of benchmark systems (NV-diamond and PDP) by more than two orders of magnitude.
Preserved Coherence and Optical Readout: Despite the extreme density, the cocrystal retains resolved optical transitions and exhibits room-temperature ODMR. The system demonstrates:
- A ground-state recovery time of 74.19 $\mu$ s.
- A free-induction decay time ( $T_2^*$ ) of 80 ns.
- A Hahn-echo coherence time ( $T_2$ ) of 0.874 $\mu$ s, extendable to 1.555 $\mu$ s with CPMG-4 dynamical decoupling.
- These coherence times are comparable to those of dilute PDP systems, defying the expectation that high density would cause rapid dephasing.
Triplet Generation Mechanism: Time-resolved measurements (trEPR and TAM) confirm that the long-lived triplet population is generated predominantly via intersystem crossing (ISC), not singlet fission (SF). The EPR spectra show an emissive/absorptive pattern (eeeaaa) characteristic of ISC, and TAM transients lack the rapid dynamics associated with SF. The absence of SF is attributed to the spatial separation of pentacene molecules, which suppresses the orbital overlap required for singlet fission.
Mechanistic Insights from DFT: Theoretical calculations reveal two critical design features enabled by the cocrystal geometry:
- Suppressed Electronic Coupling: The native orientation of pentacene molecules in the cocrystal places them near a global minimum in orbital energy splitting (Davydov splitting) for both singlet and triplet states. This minimizes intermolecular electronic coupling, preventing the broadening of optical transitions and the delocalization of excitons.
- Triplet Migration Barrier: The triplet energy of the DHP coformer (2.98 eV) is significantly higher than that of pentacene (0.78 eV). This large energy offset creates a substantial uphill barrier (~2.2 eV) that effectively blocks triplet migration from pentacene to DHP sites, localizing the triplet excitons.

Significance
The paper demonstrates that cocrystallization is a viable design strategy for creating dense, optically addressable spin materials for room-temperature quantum sensing. By engineering the crystal lattice to include a coformer with higher triplet energy and specific molecular orientations, it is possible to decouple spin-site density from the loss of coherence. The DHP/Pc system proves that molecular packing and coformer energetics can be tuned as complementary parameters to preserve spin coherence even at stoichiometric densities where traditional doping strategies would fail. This approach opens a pathway to developing organic quantum materials with high sensor-site densities, potentially enhancing the sensitivity of ensemble quantum sensors for magnetic field and temperature measurements. The authors note that while the current DC sensitivity estimate is 157 nT/ $\sqrt{\text{Hz}}$ , significant improvements are possible through optimized optical and microwave driving and thermal management.

Technical Summary: Dense Pentacene Cocrystal Demonstrates Room-Temperature Coherent Control

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