A quantum computer includes: N (where N is an integer of at least 2) physical systems having five states |0>, |1>, |2>, |3> and |4> in which transitions between three lower states |0>, |1> and |2> and two upper states |3> and |4> are optically allowed; and an optical resonator in which the N physical systems are disposed. A transition frequency of a transition between |2> and |3> in all physical systems is equal to a resonant frequency of the optical resonator, a distribution width of a transition frequency between |3> and |4> is at least N times greater than a maximum value of transition frequencies between the three lower states, and light which is resonant with a transition between |0> and |4>, between |1> and |4>, or between |2> and |4> in a certain physical system is sufficiently off-resonant with all optical transitions in other physical systems.