<P> The ground state of a quantum mechanical system is its lowest - energy state; the energy of the ground state is known as the zero - point energy of the system . An excited state is any state with energy greater than the ground state . In the quantum field theory, the ground state is usually called the vacuum state or the vacuum . </P> <P> If more than one ground state exists, they are said to be degenerate . Many systems have degenerate ground states . Degeneracy occurs whenever there exists a unitary operator which acts non-trivially on a ground state and commutes with the Hamiltonian of the system . </P>

What does ground state and excited state mean
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