What does this equation mean?

The Galilei loop admits an exact, non-simulated numerical check of the whole chain, because $\Phi$[$\mathcal L_{\rm G}$] is precisely the recoil phase every stimulated-Raman atom interferometer already uses for its own calibration. Take a sodium atom, m=3.8175$\times$10^{-26}\,$\mathrm{kg}$ , driven by a two-photon stimulated-Raman transition on the 589\,$\mathrm{nm}$ line, effective wavevector $k_{\rm eff}$=2(2$\pi$/589\,$\mathrm{nm}$)=2.1335$\times$10^{7}\,$\mathrm{m^{-1}}$ [ 16 ] . The associated recoil velocity is $v_r$=$\hbar$ $k_{\rm eff}$/m=5.894\,$\mathrm{cm\,s^{-1}}$ . Hold the loop open for T=50\,$\mathrm{ms}$ , comparable to the short interrogation times of early stimulated-Raman interferometers, so the translation leg is b=$v_r$T=2.947\,$\mathrm{mm}$ . The loop area is $v_r$b=$v_r^2$T=1.737$\times$10^{-4}\,$\mathrm{m^2\,s^{-1}}$ , and the phase debt is