A computational model for quantum computing that consists of qubits, quantum gates applied to those qubits, and measurement operations. It is the quantum analog of a classical logic circuit.
A Quantum Circuit reads left-to-right: qubits start in a known state (usually |0⟩), gates are applied in sequence, and measurements at the end produce classical bits. Circuit depth (the number of sequential gate layers) is a critical metric: deeper circuits accumulate more error from decoherence, so quantum algorithm design tries to minimize depth. Modern quantum SDKs (Qiskit, Cirq, PennyLane) let developers express algorithms as circuits in Python.
Building a 4-qubit, depth-12 quantum circuit in Qiskit that implements a Variational Quantum Eigensolver for ground-state energy estimation of H2.
Quantum circuits are the dominant representation for quantum algorithms, and the abstraction layer at which most quantum software engineers actually work.
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