Diamond Spins Show Long-Lasting Quantum Coherence

Researchers have demonstrated a pathway towards utilising previously untapped electron spin resources within diamond, potentially revolutionising solid-state technologies. Taewoong Yoon from the Department of Physics and Astronomy, Seoul National University, along with Sangwon Oh, Junghyun Lee from the Center for Quantum Technology, Korea Institute of Science and Technology, and Hyunyong Choi, report the observation of coherent mesoscopic spin states within a disordered ensemble of nitrogen impurities in diamond. This work, a collaboration between the Department of Physics and Astronomy at Seoul National University and the Korea Institute of Science and Technology, significantly enhances spin polarisation, achieving a 740-fold improvement over thermal equilibrium, and reveals collective spin behaviour. The ability to control and observe coherence in these ‘dark’ spin ensembles establishes a crucial foundation for their application in advanced quantum sensing and many-body simulation. Future sensors and quantum computers may exploit previously untapped sources of spin within materials. Controlling these hidden states promises devices with enhanced sensitivity and computational power. New work demonstrates a pathway to use these disordered spins, achieving sustained coherence at room temperature within diamond. Scientists are increasingly focused on utilising the properties of spin systems for advanced quantum technologies. With dipolar spin environments as controllable resources presents a significant challenge, yet holds immense potential for solid-state quantum devices. Recent work details the observation of a coherent mesoscopic spin state within a disordered collection of substitutional nitrogen (P1) centres in diamond, a development that could unlock new avenues for quantum sensing and simulation. This achievement moves beyond simply identifying suitable spin systems, instead demonstrating a method to actively control and coordinate a collective spin behaviour in a typically disordered material. By controlling these systems requires overcoming inherent limitations, particularly the tendency for spin environments to introduce decoherence, the loss of quantum information. Instead of viewing these surrounding spins as detrimental, researchers have begun to explore their potential as quantum resources, envisioning their use as long-lived quantum memories or as components in analogue quantum simulators. Among these potential resources, substitutional nitrogen (P1) centres in diamond are particularly abundant, though traditionally considered an “optically dark” spin bath due to their lack of strong optical signals. By exploiting these dark spins necessitates a method for transferring polarization from brighter, more easily controlled spins, such as nitrogen-vacancy (NV) centres. Previous polarization techniques often suffered from rapid dissipation of the transferred polarization. Now, an iterative Hartmann-Hahn protocol, a specific method for transferring polarization. Has achieved a 740-fold enhancement of P1 ensemble polarization compared to room-temperature thermal equilibrium. This enhancement, revealed through a sensitive differential readout technique, allows for the observation of collective Rabi oscillations and long-lived spin-lock and Hahn-echo coherences within the P1 ensemble. Scientists identified a crossover in the saturation polarization, a point where coherent driving overcomes local disorder, offering a quantitative measure of the system’s intrinsic disorder. These results lay the groundwork for utilising these dark electron spin ensembles as dependable resources for both quantum sensing and the complex modelling of many-body quantum systems. Here, this seemingly disordered spin environments can be actively shaped into strong quantum resources, opening possibilities for more complex and powerful quantum devices. Nitrogen-vacancy and P1 centre interactions amplify diamond polarisation A 740-fold enhancement of polarization in a disordered ensemble of nitrogen-vacancy (NV) and substitutional nitrogen (P1) centres within diamond forms the basis of this effort. Experiments conducted on a hybrid electron spin ensemble embedded in an isotopically purified diamond substrate at room temperature. The sample contained high concentrations of P1 centres, approximately 6.3 ppm. NV centres, around 2.4 ppm, corresponding to roughly 10 5 spins within the probed volume. Each P1 centre possesses an optically dark electron spin subject to strong hyperfine coupling to its host 14 N nucleus and Jahn-Teller distortion. In turn, scientists applied a bias magnetic field of 446 Gauss along the NV symmetry axis to lift the spin-state degeneracy, treating the {|0⟩. | −1⟩} subspace of the NV ground state as an effective spin-1/2 system. Given the substantial Zeeman energy mismatch between the two electron spin species, the dipolar Hamiltonian reduced to a secular Ising interaction. To overcome this energy difference, researchers used spin-locking (SL) under the Hartmann-Hahn (HH) condition. Control over both coupling strength and effective disorder. The technique utilizes resonant driving to define a dressed-state energy splitting equal to the Rabi frequency. An iterative HH protocol implemented, combined with a polarization-direction-differential readout scheme to prepare and interrogate the P1 ensemble. The protocol began with optical NV initialisation, followed by interaction with the P1 ensemble under the HH condition, repeating this cycle to accumulate polarization. Microwave (MW) sources at the NV and P1 resonance frequencies drove the spins. At the same time, the P1 electron spin resonance spectrum measured via double electron-electron resonance (DEER), revealing five spectral subgroups due to hyperfine interactions and JT distortion. With a specific subgroup comprising 3/12 of the population targeted for polarization transfer. Characterising the system required careful consideration of the dipolar interactions between spins. Here, the project team focused on the Ising interaction, represented as Hint = Σ i J ij S z i S z j. Where S denotes the spin operator and J represents the dipolar coupling strength. Through driving the spins along the x-axis, the longitudinal Ising coupling converted into transverse spin flip-flop interactions within the dressed-state basis. To achieve resonant spin exchange necessitated satisfying the HH condition, where the Rabi frequencies for both NV and P1 centres were equal. Meanwhile, to quantify the saturation polarization and identify the crossover between coherent driving and local disorder, the team measured collective Rabi oscillations and characterised the spin-lock relaxation and Hahn-echo coherence times. At the same time, the NV initialisation process and subsequent interactions carefully timed and sequenced. A π/2 pulse applied to the NV centre, followed by a period of interaction with the P1 ensemble. Finally, a readout pulse to measure the polarization. This effort leveraged a dense NV network to achieve substantial P1 ensemble polarization. Coherent control and extended lifetimes of mesoscopic spin states in diamond-bound nitrogen-1 centres To achieve a 740-fold enhancement in P1 ensemble polarization over the room-temperature thermal limit defines the central result of this effort. This substantial increase revealed by a differential readout technique applied to a disordered ensemble of substitutional nitrogen (P1) centres within diamond. The project demonstrates the generation and control of a coherent mesoscopic spin state. Utilising an iterative Hartmann-Hahn protocol to transfer polarization from dense nitrogen-vacancy (NV) centres. Initial measurements indicated rapid saturation within a few transfer cycles. That relaxation channels during NV initialisation play a key role in the polarization process. Collective Rabi oscillations observed within the P1 ensemble once polarization transfer commenced. Characterisation of spin-lock relaxation yielded coherence times, alongside Hahn-echo coherence, demonstrating the longevity of the created spin state. Specifically, the saturation polarization exhibited a crossover point, where coherent driving overcame local disorder. Efficient polarization exchange between the NV and P1 centres. This crossover provides a quantitative measure of the intrinsic disorder within the system. At a bias magnetic field of 446 Gauss along the NV symmetry axis, the dipolar Hamiltonian reduced to a secular Ising interaction. The sample contained approximately 6.3 ppm of P1 centres and 2.4 ppm of NV centres, corresponding to around 105 spins within the probed volume. To address a specific P1 subgroup, representing 3/12 of the total population, facilitated polarization transfer measurements. Measurements of the Hartmann-Hahn resonance condition showed peak contrast at a P1 Rabi frequency of 3.95MHz when the NV and P1 Rabi frequencies matched. The iterative protocol, comprising optical NV initialisation followed by interaction with the P1 ensemble, allowed for accumulation of polarization. The observed saturation polarization crossover occurred as coherent driving surpassed local disorder, enabling efficient polarization exchange. By leveraging a dense NV network, The effort successfully created a strong mesoscopic polarization within the disordered P1 ensemble. Coherent control of nitrogen-15 impurities unlocks scalable quantum information processing Scientists have long sought ways to control and connect large numbers of quantum bits, or qubits, to build more powerful devices. Recent work with nitrogen-vacancy (NV) centres in diamond has shown promise. But scaling up these systems demands finding ways to utilise other, previously untapped, spin resources. This project presents a significant advance by demonstrating coherent control over a collective spin state within an otherwise disordered collection of nitrogen impurities in diamond. To achieve coherence in a disordered system is difficult because imperfections typically disrupt the delicate quantum states needed for computation and sensing. These nitrogen impurities, specifically the P1 centres, are now shown to be a controllable resource, once considered merely background noise. By transferring polarisation from bright NV centres, researchers created a mesoscopic spin ensemble exhibiting collective behaviour, including sustained oscillations and echoes. Although the observed polarisation is less than predicted by current models. The demonstration of this transfer and the ability to characterise the disorder within the ensemble represent a step forward. The discrepancy between modelled and experimental polarisation suggests our understanding of charge dynamics within the material is incomplete. Future work should explore alternative methods for initiating and sustaining coherence in these P1 ensembles. Investigations into similar disordered spin systems in other materials could broaden the scope of this approach. This effort offers a new pathway towards building more complex and capable quantum technologies, moving beyond the limitations of isolated qubits and towards collective quantum phenomena. 👉 More information 🗞 Mesoscopic Spin Coherence in a Disordered Dark Electron Spin Ensemble 🧠 ArXiv: https://arxiv.org/abs/2602.17074 Tags: