Experimental demonstration of decoherence-induced spontaneous symmetry breaking

Abstract

We experimentally investigate the variations of exchange-symmetry properties of the four Bell states in an exchange-symmetric pure dephasing process with a two-photon system generated from spontaneous parametric down-conversion (SPDC). Experiment results show that under such an exchange-symmetric local-noise Hamiltonian, the exchange-symmetry property remains unchanged for two of the three symmetric Bell states, i.e., the states $|\ensuremath{\Phi}\ensuremath{\rangle}{}^{\ifmmode\pm\else\textpm\fi{}}=\frac{1}{\sqrt{2}}(|00\ensuremath{\rangle}\ifmmode\pm\else\textpm\fi{}|11\ensuremath{\rangle})$. For the antisymmetric Bell state $|\ensuremath{\Psi}\ensuremath{\rangle}{}^{\ensuremath{-}}=\frac{1}{\sqrt{2}}(|01\ensuremath{\rangle}\ensuremath{-}|10\ensuremath{\rangle})$, the exchange-symmetry property increases and achieves a maximum value of 0.5 at the asymptotic limit. However, for the third exchange-symmetric Bell state $|\ensuremath{\Psi}\ensuremath{\rangle}{}^{+}=\frac{1}{\sqrt{2}}(|01\ensuremath{\rangle}+|10\ensuremath{\rangle})$, the exchange-symmetry property breaks, surviving with a probability of 0.5 at the asymptotic limit, which provides some evidence supporting such decoherence-induced spontaneous-symmetry-breaking phenomena.