Polarization ququarts
  We discuss the concept of polarization states of four-dimensional quantum systems based on frequency non-degenerate biphoton field. Several quantum tomography protocols were developed and implemented for measurement of an arbitrary state of ququart. A simple method that does not rely on interferometric technique is used to generate and measure the sequence of states that can be used for quantum communication purposes. 
Yu. I. Bogdanov, E. V. Moreva, G. A. Maslennikov, R. F. Galeev, S. S.   Straupe, and S. P. Kulik