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mean qubit lifetime

Mean qubit lifetime (often referred to as coherence time or $T_1$/$T_2$ times) is the average duration a qubit maintains its quantum state before decoherence caused by environmental noise or internal defects. It is a critical metric for determining the fault-tolerance threshold of a quantum computer and the number of error-correction cycles possible before information loss.

Key Metrics

• Relaxation time ($T_1$): Time scale for energy decay (spontaneous emission from $|1⟩$ to $|0⟩$).
• Dephasing time ($T_2$): Time scale for loss of phase coherence between superposition states.
• Relation: $T_2\le 2T_1$, with equality only in ideal conditions without pure dephasing noise.

Relevance to Topological Quantum Computing

In topological quantum computing, qubits are protected by topological properties, theoretically offering significantly longer lifetimes compared to conventional superconducting or trapped-ion qubits. The robustness against local perturbations is a primary advantage claimed by platforms utilizing Majorana zero modes.

Recent Developments and Critical Analysis

• Microsoft Majorana 2 Chip: In mid-2026, Microsoft announced the “Majorana 2” quantum chip, claiming breakthroughs in topological protection.
  • Independent critical reviews highlight skepticism regarding these claims, suggesting that observed stability may not yet demonstrate true topological protection required for fault-tolerant computing Microsoft Majorana 2 Quantum Chip: Critical Review of Topological Claims.
  • Analysis suggests that while coherence times may appear improved, they might not meet the stringent criteria for topological error correction without further verification of non-Abelian statistics.

Challenges

• Material Defects: Impurities in nanowires or superconducting interfaces can create parasitic states that reduce effective lifetime.
• Measurement Backaction: The act of measuring topological qubits often introduces noise that competes with the protective benefits.
• Scalability vs. Coherence: Increasing qubit count typically introduces crosstalk, potentially reducing individual qubit lifetimes.

References

• Microsoft Majorana 2 Quantum Chip: Critical Review of Topological Claims — Sabine Hossenfelder, YouTube (2026).