Inl993, using a two-photon interferometer, american professor chiao and his colleagues measured the time delay for single photon to tunnel cross a barrier consisting of a 1-urn-thick id photonic band-gap material, then they gained the result that the photon tunnels across the barrier by speed of ( 1.7 + 0.2 ) c 1993年,美國的chiao教授使用雙光子干涉儀測量了單光子通過一段1um厚的勢壘的時間延遲,進而計算出光子通過勢壘的速度為(1.70.2)c。