Stephen P. Walborn is nearing the end of his doctoral research in quantum optics at the Universidade Federal de Minas Gerais (UFMG) in Belo Horizonte, Brazil. Address: Departamento de Física, UFMG, Caixa Postal 702, Belo Horizonte, MG 30123-970, Brazil. Internet: swalborn@fisica.ufmg.br