Quantum stochastic resonance in symmetric systems

We investigate the low-temperature quantum stochastic resonance (QSR) pheno menon in a two-level system (TLS) which is coupled to an Ohmic heat bath. I n contrast to common belief we find that QSR occurs also for symmetric (i.e ., unbiased) TLS's if the viscous friction parameter alpha exceeds a critic al value: We demonstrate that with respect to the spectral power amplificat ion measure QSR always occurs for alpha>1; in contrast, the output signal-t o-noise ratio exhibits an amplification only for alpha>3/2. [S1063-651X(99) 12305-5].