Fig. 3: Spectral hole burning (SHB) in the ⁵D₀ → ⁷F₀ transition of [Eu₂].

a An inhomogeneously (Γ_inh) broadened line is composed of narrow homogeneously (Γ_h) broadened lines. Selective optical excitation of one or several homogeneously broadened lines is used to burn spectral holes with implications for QIP applications. b Simplified mechanism of SHB. In the diagram, ±1/2_gr, ±3/2_gr, ±5/2_gr, and ±1/2_ex, ±3/2_ex, and ±5/2_ex correspond to nuclear spin levels associated with the ground ⁷F₀ and excited ⁵D₀ states, respectively. (i) A laser scan reveals an inhomogeneously broadened absorption spectrum, like the one shown in (a), due to the excitation of multiple ions ensembles with infinitesimally small energy difference. (ii) Selective and continuous laser pumping transfers the population from one of the ground-state spin levels to an excited-state spin level. This is illustrated for ions with the ±5/2_gr (⁷F₀) to ±1/2_ex (⁵D₀) transition resonant with the laser. (iii) Population relaxation from level ±1/2_ex to level ±3/2_gr, under the ambit of level ±3/2_gr having sufficiently longer lifetime than levels ±1/2_gr and ±5/2_gr, results in a decreased population in level ±5/2_gr. Since the spin level splittings are much lower than the optical inhomogeneous broadening, the laser will excite other ions along with transitions from the ±3/2_gr or ±1/2_gr levels, which will therefore be emptied. This finally results in a hole in the inhomogeneously broadened absorption spectrum. c Spectral hole burned in the ⁵D₀ → ⁷F₀ transition of [Eu₂]. A Lorentzian fit reveals an FWHM of 43 ± 2 MHz, corresponding to Γ_h = 22 ± 1 MHz. A T_2opt = ~14.5 ns is calculated using the relation Γ_h = (1/πT_2opt). d The decay of the hole depth as a function of delay time before readout, showing the relaxation rate of the nuclear spin levels in the binuclear complex. a and b are partially reproduced with permission from ref. ²³.