  Femtoscopic correlations between charged pions and kaons for different charge
combinations are measured in Pb$-$Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV
with ALICE at the LHC. The three-dimensional pion-kaon ($\pi-{\rm K}$)
correlation functions and double ratios in the out-side-long pair rest frame
are studied in different centrality bins. The $\pi-{\rm K}$ femtoscopic source
size parameter ($R_{\rm out}$) and emission asymmetry ($\mu_{\rm out}$) are
extracted. It is observed that the average source size of the system and the
emission asymmetry between pions and kaons increase from peripheral to central
events.
