PT-symmetric quantum mechanics

Abstract:  The average quantum physicist on the street would say that a quantum-mechanical Hamiltonian must be Dirac Hermitian (invariant under combined matrix  transposition and complex conjugation) in order to guarantee that the energy eigenvalues are real and that time evolution is unitary. However, the Hamiltonian $H=p^2+ix^3$, for example, which is obviously not Dirac Hermitian, has a positive real discrete spectrum, generates unitary time evolution, and thus defines a fully consistent and physical quantum theory. Evidently, the axiom of Dirac Hermiticity is too restrictive. While $H=p^2+ix^3$ is not Dirac Hermitian, it is PT symmetric (invariant under combined space reflection P an time reversal T). The quantum mechanics defined by a PT-symmetric Hamiltonian is a complex generalization of ordinary quantum mechanics. When quantum mechanics is extended into the complex domain, new kinds of theories having strange and remarkable properties emerge. Some of these properties have been studied and verified in beautiful laboratory experiments. A particularly interesting PT-symmetric Hamiltonian is $H=p^2-x^4$, which has an upside-down potential. We explain in intuitive and in rigorous terms why the energy levels of this potential are real, positive, and discrete.