Quantum Circuit
A sequence of quantum gates applied to qubits to carry out a quantum computation.
Definition
A quantum circuit is the standard model of quantum computation: qubits are initialized, a sequence of quantum gates transforms their joint state, and a final measurement reads out classical bits.
Today's machines are described as noisy intermediate-scale quantum devices, with tens to hundreds of imperfect qubits and no error correction. Algorithms for this era must be shallow enough that noise does not overwhelm the signal, which sharply limits what they can do.
Because every gate adds error and every qubit decoheres over time, useful results on current hardware require circuits shallow enough to finish before noise dominates. This constraint shapes the whole field of near-term algorithms, which seek to extract value from limited-depth circuits, often by offloading part of the work to a classical optimizer in a hybrid loop. The circuit model itself, however, remains the target for future fault-tolerant machines.
Circuit depth (the number of sequential gate layers) and width (the number of qubits) measure a computation's size and its vulnerability to noise.
Structure
- Initialization to a known state, usually all zeros.
- Gate layers that create superposition and entanglement.
- Measurement that collapses the state to classical output.
Why it matters
The circuit model is how quantum algorithms are expressed and compiled to hardware. Because every gate adds noise, minimizing circuit depth is critical on today's error-prone machines, which motivates error correction.
Fusion connection
Variational quantum circuits are among the near-term approaches explored for simulating small quantum systems relevant to materials science.