Zwier, O'Shea, Onur and van der Wal (Sci. Rep. 5, 10931, 2015) demonstrated the first fully optical coherent population trapping (CPT) with basal-plane divacancy spin ensembles in 4H-SiC, but the CPT dips were broadened by a ground-state dephasing time of only 42±842\pm8 ns under continuous-wave driving — about 30 times shorter than microsecond-scale electron-spin-resonance values — attributed to a fluctuating Stark effect from laser-driven charge motion. They left open whether this penalty can be suppressed, whether kHz-resolution CPT spectroscopy, laser-frequency-selective bleaching as spectral hole burning, CPT-based magnetometry, and on-chip integration can be realized. The same group's 2022 follow-up resolved parts of this: electromagnetically induced transparency (EIT) in c-axis ensembles with ensemble T2∗=(4.3±0.5)T_2^*=(4.3\pm0.5) µs (no significant optically induced dephasing) and sub-kHz detuning accuracy, though full transparency would need ~600 mW in bulk geometry. The surviving problem is to reach full-contrast CPT/EIT at practical optical powers — the 2015 proposal of bleaching-based narrowing of the ~140 GHz inhomogeneous broadening remains untested — to demonstrate characterized microwave-free (vector) magnetometry, and to integrate the control on chip.

Background

Silicon carbide (SiC) is a wide-bandgap semiconductor with mature wafer-scale device processing that also hosts optically addressable spin defects. The best-studied among these is the divacancy: two adjacent missing atoms, one silicon and one carbon, whose unpaired electrons form a spin-1 (S=1S=1) triplet ground state with optical zero-phonon lines (ZPLs, the narrow lines of the defect's optical spectrum) near 1.1 µm, close to the telecom bands. The 4H polytype hosts several inequivalent divacancies, distinguished by whether the defect axis lies along the crystal c-axis or in the basal plane; because their spin states can be optically initialized and read out, they are candidates for quantum sensing, quantum memories, and spin-photon interfaces, in analogy with the nitrogen-vacancy (NV) center in diamond.

Coherent population trapping (CPT) is a quantum-interference effect in which two phase-coherent lasers, each resonant with one of two optical transitions sharing a common excited state (a so-called Λ system), drive the spin into a 'dark' coherent superposition of two ground spin states that no longer absorbs light. The resulting dip in absorption (or photoluminescence excitation, PLE) at two-photon resonance is a purely optical probe of the ground-state spin splitting — no microwave fields are needed — which underlies microwave-free ('RF-free') magnetometry and light-storage quantum memories; in ensemble optics the same physics is known as electromagnetically induced transparency (EIT).

Coherent control of divacancy spin ensembles was first established with electron-spin-resonance (ESR, microwave-driven) techniques, which found microsecond-scale ensemble inhomogeneous dephasing times T2∗T_2^* at room temperature (Koehl et al. 2011) and millisecond spin coherence under dynamical decoupling, i.e. refocusing spin dephasing with trains of microwave control pulses (Christle et al. 2015). Zwier et al. (2015) then demonstrated the first all-optical CPT with divacancy ensembles (basal-plane defects in 4H-SiC): they mapped the full spin-triplet structure of the ground and excited states and resolved CPT dips in PLE spectra. However, the CPT dips yielded a ground-state dephasing time of only 42±842\pm8 ns under continuous-wave driving — about 30 times shorter than the microsecond-scale ESR ensemble values — which the authors attributed to extra dephasing from a fluctuating Stark effect (electric-field-induced shifts of the spin levels) caused by charge moving in and out of localized traps under strong laser illumination, and noted that pulsed control or reduced powers should avoid it without demonstrating this. Their two-laser detuning was determined only to 1 MHz, whereas the projected ultimate accuracy for CPT-dip frequencies, given optimized geometry and the known spin coherence times, was at the kHz level. They further observed that resonant excitation 'bleaches' divacancies by changing their charge state, that this bleaching is laser-frequency selective, persists for hours, and is reversed within seconds by a 685 nm repump laser — and proposed, without testing, its use as persistent spectral hole burning (writing narrow holes in the ensemble absorption profile) to remove optical inhomogeneity. They identified unidirectional sub-ensembles (groups of divacancies whose symmetry axes all point along one crystal direction, so that their CPT response is sensitive to field direction) as promising for CPT-based field sensing (magnitude sensitivity about 30 MHz/mT; alignment sensitivity 15–25 MHz per degree at 70 mT, decreasing at lower fields) and left integration into on-chip SiC quantum devices as an open engineering challenge.

The same group's follow-up (Zwier et al. 2022) established EIT with high visibility in c-axis divacancy ensembles despite about 140 GHz of inhomogeneous optical broadening, with an ensemble-averaged ground-state coherence of T2∗=(4.3±0.5)T_2^*=(4.3\pm0.5) µs — slightly longer than microwave experiments on similar ensembles — showing that simultaneous resonant optical driving does not by itself add substantial spin dephasing, and with the two-photon detuning determined to sub-kHz accuracy via an electro-optic modulator. However, complete EIT (full transparency) was estimated to require a control-laser Rabi frequency (the rate at which the control field coherently drives the optical transition) Ωc≈180\Omega_c\approx180 MHz, corresponding to roughly 600 mW in their bulk geometry, and was not reached; ensemble narrowing by selective bleaching has not been attempted, no CPT/EIT-based sensing has been demonstrated, and no on-chip all-optical ensemble control exists.

Problem Statement

Turn partial-visibility coherent population trapping into a functional all-optical spin-photon interface with divacancy spin ensembles in 4H-SiC. This is one coherent objective — making the CPT/EIT control demonstrated in 2015 and 2022 complete and usable — whose three parts were left open by the source work and are not resolved by existing literature:

(1) Full-contrast CPT/EIT at practical optical powers. Reach the complete-transparency regime in a divacancy ensemble at milliwatt-scale laser powers, for example by narrowing the ensemble's ~140 GHz inhomogeneous optical broadening — the 2015 work proposed, without testing it, that persistent, laser-frequency-selective charge-state bleaching (reversible with a 685 nm repump) can serve as spectral hole burning that removes optical inhomogeneity without extinguishing the optically active population — or by waveguide/cavity intensity enhancement, while keeping the ensemble ground-state spin coherence under driving in the realized regime at the intrinsic ESR-level (microsecond-scale or better) value, so that the laser-degraded 42±842\pm8 ns dephasing of the 2015 basal-plane experiment does not reappear under the narrowed or intensity-enhanced driving. Full contrast here means complete EIT in the sense of the source work: vanishing probe absorption at two-photon resonance, which for an ensemble with inhomogeneous broadening beyond the homogeneous optical linewidth requires Ωc2>ΔIγg∗\Omega_c^2>\Delta_I\gamma_g^*, with Ωc\Omega_c the control-laser Rabi frequency, ΔI\Delta_I the FWHM inhomogeneous optical broadening and γg∗\gamma_g^* the ensemble ground-state dephasing rate (Zwier et al. 2022, Eq. (1)).

(2) Microwave-free sensing. Demonstrate and characterize CPT/EIT-based magnetometry with SiC divacancy ensembles: the achievable sensitivity, the vector-field capability of unidirectional sub-ensembles, and the trade-off between magnetic-field strength and alignment sensitivity as the field is lowered.

(3) On-chip integration. Reproduce in an integrated SiC photonic device (waveguide or resonator) the all-optical ensemble control and spin coherence of part (1) — the regime in which the 2022 estimate shows full EIT becomes reachable at practical power. A part-(1) full-contrast demonstration that is itself realized in such an integrated device also satisfies this part; one realized in a bulk/non-integrated geometry does not.

The problem is complete when (1) a divacancy ensemble exhibits full-contrast CPT/EIT — complete EIT, vanishing probe absorption at two-photon resonance — at milliwatt-scale laser powers while its ensemble ground-state spin coherence under driving remains at the intrinsic ESR-level (microsecond-scale or better) value; (2) this control performs a characterized microwave-free measurement of magnetic fields, reporting the achievable sensitivity, the vector-field capability of unidirectional sub-ensembles, and the trade-off between magnetic-field strength and alignment sensitivity as the field is lowered; and (3) the same all-optical ensemble control and spin coherence are reproduced in an on-chip SiC photonic structure.

References

  1. Olger V. Zwier, Danny O'Shea, Alexander R. Onur, Caspar H. van der Wal, 'All-optical coherent population trapping with defect spin ensembles in silicon carbide', Scientific Reports 5, 10931 (2015). DOI: 10.1038/srep10931; arXiv:1411.1366; https://doi.org/10.1038/srep10931
  2. Olger V. Zwier, Tom Bosma, Carmem M. Gilardoni, Xu Yang, Alexander R. Onur, Takeshi Ohshima, Nguyen T. Son, Caspar H. van der Wal, 'Electromagnetically induced transparency in inhomogeneously broadened divacancy defect ensembles in SiC', Journal of Applied Physics 131, 094401 (2022). DOI: 10.1063/5.0077112; arXiv:2203.09869; https://doi.org/10.1063/5.0077112
  3. William F. Koehl, Bob B. Buckley, F. Joseph Heremans, Greg Calusine, David D. Awschalom, 'Room temperature coherent control of defect spin qubits in silicon carbide', Nature 479, 84-87 (2011). DOI: 10.1038/nature10562; https://doi.org/10.1038/nature10562
  4. David J. Christle, Abram L. Falk, Paolo Andrich, Paul V. Klimov, Jawad Ul Hassan, Nguyen T. Son, Erik Janzén, Takeshi Ohshima, David D. Awschalom, 'Isolated electron spins in silicon carbide with millisecond coherence times', Nature Materials 14, 160-163 (2015). DOI: 10.1038/nmat4144; https://doi.org/10.1038/nmat4144
  5. Christopher P. Anderson, Elena O. Glen, Cyrus Zeledon, Alexandre Bourassa, Yu Jin, Yizhi Zhu, Christian Vorwerk, Alexander L. Crook, Hiroshi Abe, Jawad Ul-Hassan, Takeshi Ohshima, Nguyen T. Son, Giulia Galli, David D. Awschalom, 'Five-second coherence of a single spin with single-shot readout in silicon carbide', Science Advances 8, eabm5912 (2022). DOI: 10.1126/sciadv.abm5912; arXiv:2110.01590; https://doi.org/10.1126/sciadv.abm5912
  6. Cyrus Zeledon, Benjamin Pingault, Jonathan C. Marcks, Mykyta Onizhuk, Yeghishe Tsaturyan, Yu-xin Wang, Benjamin S. Soloway, Hiroshi Abe, Misagh Ghezellou, Jawad Ul-Hassan, Takeshi Ohshima, Nguyen T. Son, F. Joseph Heremans, Giulia Galli, Christopher P. Anderson, David D. Awschalom, 'Minute-long quantum coherence enabled by electrical depletion of magnetic noise', arXiv:2504.13164 (2025). https://arxiv.org/abs/2504.13164
  7. Tom Bosma, Joop Hendriks, Misagh Ghezellou, Nguyen T. Son, Jawad Ul-Hassan, Caspar H. van der Wal, 'Broadband single-mode planar waveguides in monolithic 4H-SiC', Journal of Applied Physics 131, 025703 (2022). DOI: 10.1063/5.0077164; arXiv:2202.10932; https://doi.org/10.1063/5.0077164
  8. Tongyuan Bao, Qi Luo, Ailun Yi, Bo Liang, Yao Zhang, Hai-Bo Hu, Shen Lai, Zhengtong Liu, Shumin Xiao, Xin Ou, Yu Zhou, Qinghai Song, 'Tunable cavity coupling to spin defects in a 4H-silicon-carbide-on-insulator platform', ACS Photonics 12, 2988-2996 (2025). DOI: 10.1021/acsphotonics.4c02574; arXiv:2412.20039; https://doi.org/10.1021/acsphotonics.4c02574
  9. S. Castelletto, C. T-K. Lew, Wu-Xi Lin, Jin-Shi Xu, 'Quantum systems in silicon carbide for sensing applications', Reports on Progress in Physics 87, 014501 (2024). DOI: 10.1088/1361-6633/ad10b3; https://doi.org/10.1088/1361-6633/ad10b3
  10. S. Castelletto, A. Boretti, 'Chip-scale quantum sensors based on an integrated silicon carbide photonic platform', Journal of Physics: Photonics 7, 041001 (2025). DOI: 10.1088/2515-7647/ae0bd4; https://doi.org/10.1088/2515-7647/ae0bd4
  11. D. Simin, V. A. Soltamov, A. V. Poshakinskiy, A. N. Anisimov, R. A. Babunts, D. O. Tolmachev, E. N. Mokhov, M. Trupke, S. A. Tarasenko, A. Sperlich, P. G. Baranov, V. Dyakonov, G. V. Astakhov, 'All-optical dc nanotesla magnetometry using silicon vacancy fine structure in isotopically purified silicon carbide', Physical Review X 6, 031014 (2016). DOI: 10.1103/PhysRevX.6.031014; https://doi.org/10.1103/PhysRevX.6.031014
  12. A. Csóré, I. G. Ivanov, N. T. Son, A. Gali, 'Fluorescence spectrum and charge state control of divacancy qubits via illumination at elevated temperatures in 4H silicon carbide', Physical Review B 105, 165108 (2022). DOI: 10.1103/PhysRevB.105.165108; https://doi.org/10.1103/PhysRevB.105.165108
  13. Zhen-Xuan He, Ji-Yang Zhou, Wu-Xi Lin, Qiang Li, Rui-Jian Liang, Jun-Feng Wang, Xiao-Lei Wen, Zhi-He Hao, Wei Liu, Shuo Ren, Hao Li, Li-Xing You, Jian-Shun Tang, Jin-Shi Xu, Chuan-Feng Li, Guang-Can Guo, 'Robust single divacancy defects near stacking faults in 4H-SiC under resonant excitation', arXiv:2402.12999 (2024). https://arxiv.org/abs/2402.12999
  14. Alexey Lyasota, Joshua Bader, Shao Qi Lim, Brett C. Johnson, Jeffrey C. McCallum, Qing Li, Sven Rogge, Stefania Castelletto, 'Narrow magneto-optical transitions of erbium implanted into silicon carbide-on-insulator', Communications Materials 7, 154 (2026). DOI: 10.1038/s43246-026-01154-5; arXiv:2511.00400; https://doi.org/10.1038/s43246-026-01154-5