Practice quantum hardware fidelity vocabulary: gate fidelity, two-qubit gate error rate, T1/T2 relaxation and dephasing times, coherence time, and fault-tolerant threshold concepts.
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What is 'gate fidelity' in quantum computing?
Gate fidelity is typically expressed as a percentage (e.g., 99.9%). A fidelity of 99.9% means 0.1% of gate operations introduce an error. Higher fidelity gates are essential for running deeper circuits and achieving fault tolerance.
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What is the 'two-qubit gate error rate' and why does it matter more than single-qubit error rates?
Two-qubit gates (like CNOT) are the most error-prone operations in quantum circuits — typically 10-100x more error-prone than single-qubit gates. Reducing two-qubit gate error rates is the primary hardware challenge for running useful quantum algorithms.
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What is T1 time in quantum computing?
T1 (longitudinal relaxation time) measures how long a qubit maintains an excited state before losing energy to its environment. Longer T1 means the qubit stays in |1⟩ longer without spontaneous decay — crucial for running longer circuits.
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What does T2 time measure in a qubit?
T2 (transverse relaxation time) measures phase coherence: how long a qubit in superposition maintains its relative phase before dephasing destroys quantum information. T2 is always ≤ 2*T1, and is typically the limiting factor for algorithm execution time.
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A researcher says a qubit is 'below the fault-tolerant threshold.' What does this mean for quantum error correction?
The fault-tolerant threshold (roughly 0.1-1% error rate depending on the error correction code) is the error rate below which quantum error correction becomes effective. Below the threshold, adding more qubits for error correction actually reduces logical error rates.