A quantum computer includes a unit including thin films A, B and C each containing a physical-system group A, B and C formed of physical systems A, B and C, the films A, B and C being alternately stacked in an order of A, B, C, A, . . . , each of the systems A, B and C having three-different-energy states |0>x, |1>x , |e>x, a quantum bit being expressed by a quantum-mechanical-superposition state of |0>x and |1>x , a light source generating light beams having angular frequencies ωA(E), ye, g, ωA(E), ye, e, ωx, ye, gg, ωx, ye, ge, ωx, ye, eg and ωx, ye, ee, ωA(E), ye, g, a unit controlling frequencies and intensities of the beams, and a unit measuring intensity of light emitted from or transmitted through physical-system group A(E) contained in a lowest one of the thin films A to detect a quantum state of the group A(E).