Quantum Gate
A reversible operation that transforms qubit states, the building block of quantum circuits.
Definition
A quantum gate is a unitary operation acting on one or more qubits, rotating their state within the space of superpositions. Unlike many classical gates, quantum gates are reversible, since unitary operations preserve information.
Gates are never perfect: each application introduces a small error, and errors accumulate with circuit depth. Gate fidelity, the probability a gate acts as intended, is a headline metric for quantum hardware, and pushing it above the fault-tolerance threshold is a central engineering goal.
Two-qubit gates, which create entanglement, are typically the noisiest and slowest operations and thus the limiting factor for circuit quality. Compiling an algorithm to a specific machine means translating it into that machine's native gate set and its particular qubit connectivity, often inserting extra operations that add noise. Minimizing this overhead is a key job of a quantum compiler and a major determinant of what a device can actually run.
Common gates
- Pauli X, Y, Z: single-qubit rotations, X is the quantum NOT.
- Hadamard: creates equal superposition.
- CNOT: two-qubit gate that creates entanglement.
- Phase and T gates: manipulate relative phase.
Universality
A small set of gates, such as Hadamard, T, and CNOT, is universal: any quantum computation can be approximated by combining them. This mirrors how classical logic reduces to a few gate types.
Why it matters
Gates are the instructions of a quantum computer; algorithms are sequences of gates forming a quantum circuit.
Fusion connection
Gate-based quantum simulation of physical Hamiltonians is the model most relevant to eventually studying fusion-related quantum many-body problems.