Polaritons in semiconductor microcavities offer an incomparable playground to study a great variety of physics, from applied aspects to fundamental quantum mechanical questions.
In the first part of the talk, I discuss the logical operation of an ultrafast exciton polariton transistor switch, all-optically controlled and operated by propagating Bose-Einstein exciton-polariton condensate bullets in a quasi-1D semiconductor microcavity and demonstrate that the overall operation speed of the device is limited to ∼3 GHz . I also show how spin-selective spatial filtering of these propagating condensates can be achieved using a controllable spin-dependent gating
barrier: a non-resonant laser beam provides the source of propagating polaritons, while a second circularly polarized weak beam imprints a spin dependent potential barrier, which gates the polariton flow and generates polariton spin currents .
In the second part, I illustrate that the use of optical interferometry in momentum space, avoiding any spatial overlap between two parts of a macroscopic quantum state , renders a unique way to address a long-standing question raised by P.W. Anderson: "Do two components of a condensate, which have never seen each other, possess a definitive phase?" [4,5] This issue is related to the superposition principle in quantum mechanics and it is crucial to understand how mutual coherence is acquired.
 C. Antón et al., Phys. Rev. B 89, 235312 (2014).
 T. Gao et al., Appl. Phys. Lett. 107, 011106 (2015).
 C. Antón, et al., Phys. Rev B 90, 081407(R) (2014).
 P. W. Anderson, Basic Notions of Condensed Matter Physics (Benjamin, 1984).
 L. Pitaevskii and S. Stringari, Phys. Rev. Lett. 83, 4237 (1999).