Potential of electron paramagnetic resonance to study Einstein-Podolsky-Rosen-Bohm pairs

The potential of electron paramagnetic resonance (EPR) methods to study the correlation of the states of two noninteracting spins prepared in the singlet state (Einstein-Podolsky-Rosen-Bohm EPRB] pairs) is discussed. EPR methods with a selective excitation of spins in the EPRB pairs allow one, in principle, to reveal this correlation of spin states if single-spin measurements are performed. However, it is illustrated that the conventional ensemble EPR experiments, when the average values of projections of the spin moments are observables, fail in studying the correlation of spins in EPRB pairs. An exploitation of the EPR phenomenon to study the correlation of spins for ensembles of EPRB pairs needs some modifications of the experimental approach: either the indirect detection of EPR signals (new observables) should be used or the EPRB pairs should be transferred to another state when the spin-spin interaction becomes essential, while EPR observables manifest the spin correlation in the precursor EPRB pair state. In this respect it appears that in spin chemistry many results were already obtained which demonstrate that it is a reality that two spins might occupy the “entangled” (correlated) state, when there is no interaction between them. The results obtained in spin chemistry confirm the quantum mechanical predictions for spin-correlated pairs of spins which can be considered as a realization of EPRB pairs.