Realization of four-state qudits using biphotons
  The novel experimental realization of four-level optical quantum systems (ququarts) is presented. We exploit the polarization properties of frequency non-degenerate biphoton field to obtain such systems. A simple method that does not rely on interferometer is used to generate and measure the sequence of states that can be used in quantum key distribution (QKD) protocol. 
E. V. Moreva, G. A. Maslennikov, S. S. Straupe, S.P. Kulik