Room-Temperature Quantum Gate Fidelity Above the Surface Code Threshold: A Multi-Channel Decoherence Analysis for NV-Diamond Quantum Processors
The nitrogen-vacancy (NV) centre in diamond is the only solid-state qubit that operates at room temperature, yet its viability for fault-tolerant quantum computing has been dismissed on the grounds that decoherence times are too short. Here we show, through a calibrated twelve-channel decoherence model spanning 1.3 K to 600 K (60+ experimental anchors, R² = 0.953), that single- and two-qubit gate error rates fall below the surface code threshold of 1% at 295 K for bulk ultra-pure diamond under dynamical decoupling with N ≥ 2048 pi-pulses. The key enabler is the recently demonstrated T₂ = 2T₁ limit (4.34 ms at room temperature), achieved by filtering the spin-lattice Han noise channel above 1 MHz via high-order CPMG. The model now includes ¹³C nuclear spectral diffusion (dominant in ultra-pure bulk at high B-field, validated against Wrachtrup QV=8 at -5.4%) and a magnetic-field-dependent T₁ correction. For standard-grade bulk diamond, we identify a Peltier-cooled sweet spot at 200–270 K that brings all gates below threshold at a cooling cost of ~200 EUR. We further show that the XZZX surface code, exploiting the strongly Z-biased noise of NV circuits (effective bias η_eff ≈ 834 from idle-dominated architecture), reduces the physical qubit overhead by more than 95% compared to the standard surface code. Combined with soft-information decoding using analog photoluminescence counts, this establishes a concrete path from the current QV=8 (Wrachtrup 2026) to QV > 1000. Decoherence is no longer the bottleneck for NV-based quantum computing at room temperature. UK Patent Applications GB2606382.6 and GB2606639.9 cover methods described herein.