Experimental simulation of charge conservation violation and Majorana   dynamics
  Unphysical particles are commonly ruled out from the solution of physical equations, as they fundamentally cannot exist in any real system and, hence, cannot be examined experimentally in a direct fashion. One of the most celebrated equations that allows unphysical solutions is the relativistic Majorana equation\cite{Majorana} which might describe neutrinos and other exotic particles beyond the Standard Model. The equation's physical solutions, the Majorana fermions, are predicted to be their own anti-particles and as a consequence they have to be neutrally charged; the charged version however (called Majoranon) is, due to charge non-conservation, unphysical and cannot exist. On the other hand, charge conservation violation has been contemplated in alternative theories associated with higher spacetime dimensions or a non-vanishing photon mass; theories whose exotic nature makes experimental testing with current technology an impossible task. In our work, we present an experimental scheme based on optics with which we simulate the dynamics of a Majoranon, involving the implementation of unphysical charge conjugation and complex conjugation. We show that the internal dynamics of the Majoranon is fundamentally different from that of its close cousin, the Dirac particle, to illustrate the nature of the unphysical operations. For this we exploit the fact that in quantum mechanics the wave function itself is not a measurable quantity. Therefore, wave functions of real physical particles, in our case Dirac particles with opposite masses, can be superposed to a wave function of an unphysical particle, the Majoranon. Our results open a new front in the field of quantum simulations of exotic phenomena, with possible applications in condensed matter physics, topological quantum computing, and testing theories within and beyond the Standard Model with existing technology. 
R. Keil, C. Noh, A. Rai, S. St\"utzer, S. Nolte, D. G. Angelakis, and   A. Szameit
quant-ph