2Q

2q is a compact designation that can refer to multiple concepts depending on context. It does not denote a single universally defined entity.

In quantum information science, 2q is often shorthand for a two-qubit system or a two-qubit gate. Such systems live in a four-dimensional Hilbert space and enable entanglement, a key resource for quantum computation. Common two-qubit operations include CNOT, iSWAP, and controlled-phase gates, which, with single-qubit operations, form the basis for universal quantum computation. Realizations span superconducting qubits, trapped ions, and spin qubits, each with different fidelities and error rates.

Beyond quantum computing, 2q may appear simply as the algebraic expression meaning two times a quantity q, i.e., twice q, and is common across mathematics, physics, economics, and engineering.

As an identifier, 2q may also appear as a model number, designation, or shorthand in various technical documents, where its meaning is defined by the surrounding context.