NullfeldZustand

NullfeldZustand is a term used in physics, primarily in German-speaking literature, to denote the quantum state of a system when external fields are absent or negligibly small. It functions as a reference or baseline condition in which the Hamiltonian reduces to its zero-field form. The concept appears in areas such as condensed matter physics, quantum optics, and spintronics, where distinguishing field-free states from field-dressed ones clarifies theoretical and experimental analyses.

For a system described by a field-free Hamiltonian H0, a NullfeldZustand |ψ> is an eigenstate of H0 with eigenvalue E0, i.e., H0 |ψ> = E0 |ψ>. When external fields B, E, or strain are introduced, the state generally evolves into a field-dressed state. Practically, the NullfeldZustand is used as a reference state and can be prepared by turning off the external fields slowly (adiabatic deactivation) or by shielding the system from residual fields.

Typical examples include a spin-1/2 particle in the absence of a magnetic field, electrons in a quantum dot without gate bias, or superconducting qubits operated at a sweet spot where the effective field vanishes. The exact form of a NullfeldZustand depends on the internal interactions of the system, so degeneracies or symmetry considerations may play a crucial role.

See also: zero field, ground state, field-free Hamiltonian, reference state. Notes: The term is not universally standardized and may be used differently across subfields; in English literature it is often referred to as a "zero-field state" or "field-free state."