Cospin

Cospin is a term used in some theoretical and experimental works to describe a composite spin-like degree of freedom that emerges when two spin subsystems are coherently coupled. Unlike ordinary spin, which refers to the intrinsic angular momentum of a single particle, cospin refers to an effective two-level system that encodes the joint state of two coupled spins, a pseudospin, or a spin and an orbital or sublattice degree of freedom. In this sense, cospin can be framed as a two-state space with basis states that denote distinct configurations of the coupled system.

In many models, the cospin is described by Pauli matrices acting on the cospin space; the full Hamiltonian includes exchange-like coupling between the spins, a Zeeman term for external magnetic fields acting on the real spins, and a transverse cospin field that can be controlled by gates or fields. External control can manipulate the cospin through gate voltages, magnetic fields, or electric fields, enabling coherent control and readout via spin-resolved spectroscopy, transport measurements, or optical assays.

Experimental relevance arises in systems such as coupled quantum dots, bilayer or twisted materials where a layer, valley, or orbital index acts as a two-level degree of freedom, and in certain qubit architectures where spin and another degree of freedom are deliberately coupled to form a two-level cospin. The concept is related to pseudospin and isospin, but its usage is not standardized across fields.

Status and usage vary by context. The term is primarily found in theory discussions and some experiments, and its meaning may differ between authors. See also spin, pseudospin, isospin, quantum dots, bilayer graphene.