Spectroscopic evidence of cooperative (entangled) quantum states of Nd3+ ion pairs in Nd3+: LaF3 crystal

Abstract

We demonstrated the existence of entangled 4I9/2(1) x 4G5/2(1) quantum state in a pair of Nd3+ ions due to strong resonance quadrupole – quadrupole interaction between the ions. In doing so, we used narrow line (0.05 Å) tunable pulsed dye laser excitation of a 1% Nd3+: LaF3 crystal at liquid helium temperatures at the 4I9/2(1) → 4G5/2(1) transition and measured the site-selective near infrared (NIR) fluorescence spectra of different optical centers (OCs) at the 4F3/2(1) → 4I9/2(1) transition of Nd3+ with high spectral resolution (0.1 Å); the NIR fluorescence excitation spectra at the 4I9/2(1) → 4G5/2(1) transition (0.05 Å); and by varying the excitation and detection wavelengths we measured the IR fluorescence kinetics of the selected type of OCs. As a result, we observed inhomogeneous splitting within 30 cm−1 of wavenumbers for the fluorescence spectra at the 4F3/2(1) → 4I9/2(1) transition, manifested in the presence of numerous narrow inhomogeneously broadened spectral lines of different OCs. We have attributed each spectral line to a different single-site, pair, triple, or quad optical centers via analysis of the 4F3/2(1) metastable level fluorescence kinetics decay times. Each type of OCs demonstrates its own specific IR fluorescence excitation spectrum at the 4I9/2(1) → 4G5/2(1) transition detected in a 1%Nd3+: LaF3 crystal at 1.6 K. One of the spectral lines of the pair centers (with the frequency 17313.3 cm−1) exhibited the fine splitting with the value of 0.2 cm−1. The shape and the value of fine splitting are explained theoretically assuming that this line corresponds to the excitation of the entangled the 4I9/2(1) x 4G5/2(1) quantum state of the pair of Nd3+ ions located at short distance, R ~ 4.1 Å.