In the field of quantum sensing, quantum entangled states have the potential to beat the standard quantum limit (SQL) and to approach the Heisenberg limit (HL)1. Since the nitrogen-vacancy (NV) center is a qubit at room temperature, the entangled state among an NV array will offer new possibilities for quantum sensing and computing. However, considering decoherence, it is not clear whether entangled sensing can achieve sensitivity beyond the SQL under ambient conditions2,3. We have previously evaluated the decoherence of a shallow NV-NV pair. At Quantum Innovation (QI) 2024, we characterized the decoherence using noise spectroscopy and observed two types of noise with autocorrelation times on the ~μs and ~ns scales. Our results are consistent with previous studies, which attributed these noise sources to the surface electron spin bath and the surface-modified phonons4. As demonstrated by Y. Matsuzaki et al.3, the autocorrelation time is required to be comparable to the evolution time (~μs) for the entangled state. Therefore, we showed that the ratio of the coupling constants to the two types of noise determines whether the entangled state between NV-NV pair beats the SQL. Compared to the temperature-dependent phonon effects, the electron spin density is more readily controlled by surfacetermination techniques. Surface electron spins are a promising target for designing the decoherence environment surrounding NV-NV entangled sensors. In this study, to determine the optimal surface electron spin density for NV-NV entangled sensing, we calculated the flip-flop rate among surface electron spins, as well as the coupling constant to the NV center.
In the field of quantum sensing, quantum entangled states have the potential to beat the standard quantum limit (SQL) and to approach the Heisenberg limit (HL)1. Since the nitrogen-vacancy (NV) center is a qubit at room temperature, the entangled state among an NV array will offer new possibilities for quantum sensing and computing. However, considering decoherence, it is not clear whether entangled sensing can achieve sensitivity beyond the SQL under ambient conditions2,3. We have previously evaluated the decoherence of a shallow NV-NV pair. At Quantum Innovation (QI) 2024, we characterized the decoherence using noise spectroscopy and observed two types of noise with autocorrelation times on the ~μs and ~ns scales. Our results are consistent with previous studies, which attributed these noise sources to the surface electron spin bath and the surface-modified phonons4. As demonstrated by Y. Matsuzaki et al.3, the autocorrelation time is required to be comparable to the evolution time (~μs) for the entangled state. Therefore, we showed that the ratio of the coupling constants to the two types of noise determines whether the entangled state between NV-NV pair beats the SQL. Compared to the temperature-dependent phonon effects, the electron spin density is more readily controlled by surfacetermination techniques. Surface electron spins are a promising target for designing the decoherence environment surrounding NV-NV entangled sensors. In this study, to determine the optimal surface electron spin density for NV-NV entangled sensing, we calculated the flip-flop rate among surface electron spins, as well as the coupling constant to the NV center.
Quantum entangled states have a potential to beat the standard quantum limit (SQL) and approach the Heisenberg limit (HL). We previously demonstrated that phase shifts induced by magnetic fields are coherently accumulated in a Ramsey protocol using an entangled Nitrogen-Vacancy (NV) pair. However, the sensitivity is limited to the order of ~µT/√Hz due to the various sources of error and noise, including classical readout noise, spin manipulation and initialization errors, and decoherence error. One of the most critical problems is that decoherence of entangled sensing proceeds more rapidly than ensemble-based sensing. When decoherence follows a Markovian process (i.e., when the environment noises behave as white noise without autocorrelation), the sensitivity of entangled sensing is limited to SQL3,4. Therefore, enhancement from entanglement cannot be achieved under the white noise conditions. To realize entanglement-enhanced sensing to beat SQL, it is essential to characterize the environment noise and prepare a suitable condition. In this study, we conducted noise spectroscopy to evaluate the environmental noise and demonstrated that diamond NV centers have the potential to beat SQL.
Nitrogen vacancy centers (NV) in proximity to diamond surfaces are promising nanoscale quantum sensors. However, their coherence properties are negatively affected by magnetic and electric surface noise, whose origin and detailed impact have remained elusive. Using atomistic models of diamond surfaces derived with density functional theory, together with decoherence time calculations with cluster correlation expansion methods, we quantify the effects of surface crystallographic orientation and functionalization, and of the density of unpaired electrons on the NV Hahn-echo time T2. We determine a crossover depth at which T2 ceases to be limited by surface nuclear spins and recovers the bulk-limited value. We find that for static surface-electron baths, the ratio between the NV depth and the separation between surface electron spins determines a transition from fast-fluctuating to quasi-static noise, leading to a dependence of T2 on orientation for specific surfaces. We also find that the modulation of T2 by spin-phonon relaxations leads to motional-narrowing at sub-microsecond relaxation times. Importantly, our calculations show that it is only when accounting for surface-spin in-sequence hopping that measured T2 values as a function of depth can be reproduced, thus highlighting the importance of hopping-mediated models to describe the surface spin noise affecting NV sensors. Overall, our work provides clear guidelines for engineering diamond surfaces to achieve enhanced NV coherence for quantum sensing and information processing applications.
The precision measurements provided by quantum sensing plays a significant role in fields of science. However, the sensitivity is limited by the standard quantum limit (SQL). Quantum entangled state have a potential to beat the SQL and to approach the Heisenberg limit (HL). In particular, entanglement sensors based on Nitrogen-Vacancy (NV) centers provide high spatial resolution at room temperature. In 2021, it was reported that the multi-spin sensor constituted by the NV electron spin (S=1), the 14N nuclear spin (I=1), and proximate 13C nuclear spin (I=1/2) beat the SQL1 . However, since the nuclear gyromagnetic ratio is ~MHz/T, it does not lead to a dramatic improvement of sensitivity. For enhancement of quantum sensing, it is required to utilize the entangled state among dipole coupled NV centers.
Because of their high sensitivity, quantum materials are extremely promising for new technologies. The main challenge lies in maintaining a sufficiently quantum coherence times for quantum computation or quantum sensing systems. NV centers in diamond which are point defects consisting of a substitutional nitrogen atom neighboring an atomic vacancy, where an electron pair can reside in a triplet state – are ideal candidates for the development of industrial-scale quantum systems. These spin states exhibit ms-long coherence times at room temperature and can be manipulated optically, hence opening interesting perspectives for sending magnetic fields with high sensitivity1.At HiQuTe Diamond, we take up the challenge of mastering the entire nitrogen-doped diamond production chain synthesized by Microwave Plasma-Assisted Chemical Vapor Deposition (MPACVD). We aim at creating affordable diamonds specially engineered to achieve long coherence times with high NV density and high doping uniformity. This quest require to overcome the impact of several external factors that contribute to electron spin decoherence. First, the chemical composition of the diamond, specifically the types and concentrations of impurities within the diamond matrix (such as substitutional nitrogen NS, NV centres, mono-vacancies and clusters, hydrogen, etc.), as well as their electrical charge states (NS⁰, NS⁺, NV⁰, NV⁻) should be carefully controlled. Thanks to extensive scientific research conducted over the past decade, combined with our own R&D efforts, we are now able to quantify these parameters using experimental techniques compatible with industrial applications (Raman and UV-Vis spectroscopy) and compare our results with key performance indicators2. Second, we study the relationship between coherence (T2*) and strain inhomogeneities in our diamond layers, which we analyse using birefringence measurements. Our findings demonstrate our capability to produce CVD diamond layers with reduced strain and extended defect and confirm that strain could be a critical limiting factor for T2*.In this poster, I will review the efforts carried out at HiQuTe Diamond to develop specific material that is well suited for quantum sensing applications.References1.Barry, J. F., Schloss, J. M., Bauch, E., Turner, M. J., Hart, C. A., Pham, L. M., & Walsworth, R. L. (2020). Sensitivity optimization for NV-diamond magnetometry. Reviews of Modern Physics, 92(1), 015004.2.Langer, J. (2022). Quantum-grade diamond for cavity-based solutions. Fraunhofer Verlag.
Quantum coherence serves as a crucial quantum resource for achieving high-sensitivity quantum sensing. Because of its long coherence time at room temperature, the nitrogen-vacancy (NV) center has emerged as a quantum sensor in various fields in recent years. While nanoscale quantum sensing at room temperature has been demonstrated for NV centers, noise on the diamond surface severely limits its further development at a higher sensitivity. Here, we utilize the hybridization between graphene and diamond surfaces to directly deplete surface unpaired electron spins, thereby achieving roughly two-fold enhancement in coherence. Through the combination of electron spin resonance spectra and first-principle calculations, we explain that this phenomenon arises from a significant reduction in electron spin density on the diamond surface due to interface electron orbital hybridization. Our research presents a new approach for solid-state quantum sensors to reach the desired sensitivity level and offers a new pathway for future studies on material interfaces.
The nitrogen vacancy (NV) center in diamond, a well-studied, optically active spin defect, is the prototypical system in many state of the art quantum sensing and communication applications. In addition to the enticing properties intrinsic to the NV center, its diamond host's nuclear and electronic spin baths can be leveraged as resources for quantum information, rather than considered solely as sources of decoherence. However, current synthesis approaches result in stochastic defect spin positions, reducing the technology's potential for deterministic control and yield of NV-spin bath systems, as well as scalability and integration with other technologies. Here, we demonstrate the use of theoretical calculations of electronic central spin decoherence as an integral part of an NV-spin bath synthesis workflow, providing a path forward for the quantitative design of NV center-based quantum sensing systems. We use computationally generated coherence data to characterize the properties of single NV center qubits across relevant growth parameters to find general trends in coherence time distributions dependent on spin bath dimensionality and density. We then build a maximum likelihood estimator with our theoretical model, enabling the characterization of a test sample through NV T2* measurements. Finally, we explore the impact of dimensionality on the yield of strongly coupled electron spin systems. The methods presented herein are general and applicable to other qubit platforms that can be appropriately simulated.