Decoherence-protected entangling gates in a silicon carbide quantum node

Solid-state color centers are promising candidates for nodes in quantum network architectures. However, realizing scalable and fully functional quantum nodes, comprising both processor and memory qubits with high-fidelity universal gate operations, remains a central challenge in this field. Here, we demonstrate a fully functional quantum node in silicon carbide, where electron spins act as quantum processors and nuclear spins serve as quantum memory. Specifically, we design a pulse sequence that combines dynamical decoupling with hyperfine interactions to realize decoherence-protected universal gate operations between the processor and memory qubits. Leveraging this gate, we deterministically prepare entangled states within the quantum node, achieving a fidelity of 90%, which exceeds the fault-tolerance threshold of certain quantum network architectures. These results open a pathway toward scalable and fully functional quantum nodes based on silicon carbide.

💡 Research Summary

This paper reports the realization of a fully functional quantum node in silicon carbide (4H‑SiC) that integrates both a quantum processor and a quantum memory within a single solid‑state platform. The authors use a shallow PL6 color center as the electron‑spin processor and a nearby strongly coupled ^29Si nuclear spin as the memory qubit. The PL6 defect possesses a spin‑triplet ground state (S = 1) that can be optically initialized and read out, and its electron spin can be driven with microwave (MW) pulses on a nanosecond timescale. Because the natural isotopic composition of the SiC crystal contains about 4.7 % ^29Si (I = ½), a subset of nuclear spins lies within a few nanometers of the defect and exhibits hyper‑fine couplings on the order of 10 MHz. By selecting the electron sub‑levels |m_s = 0⟩ and |m_s = −1⟩ together with the nuclear states |↑⟩ and |↓⟩, the authors define a four‑level hybrid register.

Key experimental steps include: (1) Polarization of the nuclear spin to >99 % using electron‑assisted dynamic nuclear polarization (DNP) at the excited‑state level‑anticrossing (ES‑LAC) around 330 G; (2) Characterization of the hyper‑fine spectrum via optically detected magnetic resonance (ODMR) and optically detected nuclear magnetic resonance (ODNMR), revealing a hyper‑fine splitting of 12.4 MHz for a Si IIa lattice site; (3) Measurement of nuclear Rabi oscillations (π‑pulse ≈ 2.33 µs) and Ramsey coherence (T₂* ≈ 143 µs), confirming that the nuclear memory retains coherence for orders of magnitude longer than the electron spin (T₂* ≈ 2.04 µs).

A central challenge is that conventional two‑qubit gates (e.g., CNOT) require gate times comparable to or longer than the electron dephasing time, limiting entanglement fidelity to ~70 %. To overcome this, the authors develop a composite pulse sequence that interleaves dynamical decoupling (DD) microwave π‑pulses with radio‑frequency (RF) pulses—a technique they term DDRF (Dynamical Decoupling + RF). The DD pulses continuously refocus electron‑spin dephasing, while the RF pulses enact conditional nuclear rotations. This approach enables both electron‑controlled‑NOT (C_eNOT_n) and nuclear‑controlled‑NOT (C_nNOT_e) gates with total durations well below the electron T₂*, typically under 6 µs.

Using the DDRF‑protected gates, the authors prepare a Bell state between the processor and memory qubits. The protocol begins with optical initialization to |0↑⟩, a MW π/2 pulse creates (|0↑⟩ + |−1↑⟩)/√2, an RF π pulse flips the nuclear spin conditionally, yielding (|0↑⟩ + |−1↓⟩)/√2. Quantum state tomography performed at room temperature (no cryogenic cooling) reconstructs the density matrix via maximum‑likelihood estimation, with statistical uncertainties evaluated by Monte‑Carlo simulations that incorporate Poissonian photon‑counting noise. The resulting entangled state exhibits a fidelity of 90 % ± 3 %, surpassing the fault‑tolerance thresholds required by several quantum‑network architectures.

The significance of this work lies in three major contributions: (i) Demonstration that a strongly coupled electron‑nuclear pair in SiC can serve as a high‑fidelity processor‑memory pair; (ii) Introduction of a decoherence‑protected universal two‑qubit gate that leverages DD and RF control to mitigate the short electron coherence time; (iii) Achievement of entanglement fidelity well above the error‑correction threshold, establishing SiC as a viable platform for scalable, distributed quantum information processing. The authors discuss prospects for scaling to multiple nuclear memories, integrating the node with nanophotonic cavities or waveguides for efficient photon‑mediated networking, and further extending coherence times via low‑temperature operation or isotopic purification. Overall, the paper provides a concrete pathway toward practical quantum network nodes built from silicon carbide color centers.