Communication through measurements and unitary transformations

Abstract

It is generally accepted that no `faster than light signalling' (FTLS) using two entangled spin 1/2 particles is possible because of indeterminism in a quantum measurement and linearity of standard quantum mechanics. We show how in principle one bit of information could be transmitted using local measurements and a global unitary transformation of the state of two entangled spatially separated spin 1/2 particles. Assuming that the postulate of a state collapse due to measurement is valid, the no FTLS condition is saved if we do not have physical access to the required global unitary transformation. This means that the no FTLS condition is also present on the operational level, namely as imposing a physical restriction on the possible realizable unitary transformations, in this case of two entangled but spatially separated spin 1/2.