= Solution
<Quantum tunnelling> is transmission through a region where the particle's <energy> is below the classical <potential energy>. The stationary <wavefunction> decays within that region but need not vanish at its far boundary, so matching can leave a nonzero transmitted <probability current>.
This does not require a particle to borrow <energy> temporarily. <Energy> is conserved for a time-independent barrier; the classically forbidden region is described by an evanescent wave, with the same stationary <energy> as the incident and transmitted waves.
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