Making bigger entangled states

We present two experiments aimed at increasing the size of entangled optical states, in which the notion of “size” is understood in two different meanings. In the first experiment, one channel of the delocalized single-photon state is subjected to a macroscopic phase-space displacement, resulting in a micro-macro entangled state, i.e. the “Schrödinger cat”. In this state, microscopic variations in the field quadrature in one channel are correlated with macroscopic variation of the particle number in the other [1,2]. In spite of its spectacular macroscopic nature, the state is obtained from a single bit of entanglement by means of a local operation, so it still contains a single ebit. In our second experiment, the situation is opposite. We apply the photon annihilation operation to the two channels of the original Einstein-Podolsky-Rosen state, thereby increasing the amount of entanglement and two-mode squeezing in that state. On the other hand, there is no significant macroscopicity increase associated with the new state. Both experiments have different yet overlapping sets of applications in quantum information and fundamental quantum physics.