11.1 That’s not logical
The qubits such as those in the last two chapters are examples of logical qubits. We can use them indefinitely, they never lose state when idle, and we can apply as many gates to them as we wish without errors. qubit$logical logical qubit
When you build a quantum computer, the actual physical implementations of qubits aren’t as perfect as logical qubits. Such a qubit, called a physical qubit, starts to lose its ability to hold onto a state after its coherence time. We also say that the qubit is decohering. qubit$physical qubit$decoherence decoherence qubit$coherence time coherence time
It’s a goal of quantum computing researchers and engineers to delay the decay of a physical qubit’s quantum state as long as possible. Since the decay is inevitable, a goal of fault tolerance and error correction is to handle and fix the effects of the qubits’ decoherence throughout the execution of a circuit.
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