Minute-long quantum coherence enabled by electrical depletion of magnetic noise

Technical Summary: Minute-long quantum coherence enabled by electrical depletion of magnetic noise

Problem Statement
Solid-state spin defects, such as those in silicon carbide (SiC), are promising platforms for quantum communication and sensing due to their spin-photon interfaces and access to nuclear spin memories. However, their utility is often limited by uncontrolled electric and magnetic noise arising from defects and impurities in the host matrix. While materials engineering strategies—such as isotopic purification and growth optimization—have reduced these noise sources, they do not fully eliminate them. Consequently, existing approaches often rely on complex Hamiltonian engineering (decoupling sequences) to mitigate residual noise, which can interfere with quantum protocols. There remains a need for methods that can actively suppress both electric and magnetic noise in situ within a device structure to achieve record-breaking coherence times without relying solely on material perfection.

Methodology
The authors investigated the impact of bias control on an isotopically purified (99.85% $^{28}$Si, 99.98% $^{12}$C) 4H-SiC p-i-n diode containing embedded neutral divacancy ($VV^0$) spin defects. The experimental setup involved:

• Device Structure: A custom-grown p-i-n diode with an intrinsic region containing the $VV^0$ defect, fabricated on a 4° off-axis n-type substrate.
• Electrical Tuning: The team applied bias voltages to the diode to transition it from forward bias to reverse bias, specifically targeting the depletion regime where the electric field depletes charge carriers and traps near the defect.
• Optical Characterization: Resonant photoluminescence excitation (PLE) was used to measure Stark shifts and optical linewidths of the $VV^0$ emission.
• Spin Coherence Measurements:
  • Electron Spin: Ramsey and Hahn echo sequences were performed on the $VV^0$ electron spin ($S=1$) to measure dephasing rates ($T_2^*$ and $T_2$) under different bias conditions. Double-quantum (DQ) Ramsey measurements were utilized to isolate magnetic noise contributions.
  • Nuclear Spin: A neighboring $^{29}$Si nuclear spin ($I=1/2$) was identified and coherently controlled using dynamical decoupling (XY8 sequences) on the electron spin. The nuclear spin was initialized, manipulated, and read out via spin-to-charge conversion (SCC) on the $VV^0$.
  • Long-term Stability: Extended Ramsey measurements (up to 22.5 hours) were conducted to observe slow noise dynamics and nuclear spin relaxation ($T_1$).

Key Contributions and Results

1. Simultaneous Depletion of Electric and Magnetic Noise: The study demonstrates that operating the SiC p-i-n diode in the reverse-bias depletion regime (specifically at -80 V) significantly reduces both electric and magnetic noise sources.

   • Optical Improvement: The optical linewidth of the $VV^0$ narrowed from ~80 MHz at zero bias to 18(1) MHz in the depletion regime, approaching the natural linewidth limit.
   • Electron Spin Coherence: The electron spin Ramsey coherence time ($T_2^*$) increased by approximately 50% to 320(30) $\mu$s, and the Hahn echo time ($T_2$) improved by 25% to 1.9(1) ms. Double-quantum Ramsey measurements confirmed that this improvement is driven by a reduction in magnetic noise, not just electric noise.
   • Magnetic Noise Origin: The authors attribute the magnetic noise reduction to the electrical depletion of paramagnetic charge traps (such as nitrogen impurities and carbon vacancies) near the defect, converting them into spinless charge states.

2. Record-Breaking Nuclear Spin Coherence: By leveraging the reduced magnetic noise environment and the "frozen core" effect (where the electron spin in the $m_s = -1$ state suppresses polarization transfer to nearby spins), the authors achieved unprecedented coherence times for a single $^{29}$Si nuclear spin.

   • Relaxation: The nuclear spin relaxation time ($T_1$) doubled in the depletion regime.
   • Ramsey Coherence: The nuclear spin $T_2^*$ reached 690(40) ms when the electron spin was prepared in $m_s = -1$.
   • Hahn Echo Coherence: Most notably, the nuclear spin Hahn echo coherence time ($T_2$) was measured to have a lower bound of 105 seconds (with 99% confidence). This represents the longest reported coherence time for a single spin across any qubit platform (solid-state, atomic, or ionic) to date.

3. Scaling Law: The authors observed a scaling relation where the enhancement in nuclear spin Ramsey time correlates with the square root of the enhancement in relaxation time ($T_2^*/T_2^* \approx \sqrt{T_1/T_1}$), consistent with microscopic spin-bath models where dephasing and relaxation stem from the same ensemble of weakly interacting magnetic impurities.

Significance
The paper claims that these results demonstrate the critical importance of integrating materials control with electronic device engineering. By using a simple semiconductor diode structure to in situ deplete noise sources, the authors have surpassed the limits of material growth optimization alone. This approach enables:

• Record Coherence: Achieving minute-long coherence times for solid-state nuclear spins without the need for complex dynamical decoupling sequences or specific clock transition points.
• Scalability: Providing a pathway to integrate classical electronic control with quantum optoelectronics on wafer-scale SiC platforms, which are already mature in the high-power electronics industry.
• Quantum Networking: Establishing a foundation for highly coherent quantum nodes and networks by preserving both optical and spin properties in engineered devices, potentially overcoming the degradation of properties often seen in nanostructures.

The work suggests that active electrical engineering of the local environment is a powerful tool for enhancing solid-state qubit performance, offering a route to high-fidelity quantum information processing and sensing.