Round 13: Tossup 5

For a controlled operation, the value of this quantity applied to the second input is “kicked back” to the first input. Order-finding in Shor’s algorithm involves a subproblem in which this quantity of an eigenvalue is estimated to solve the task of finding an eigenvalue from the eigenstate of a unitary operator. The azimuthal angle on a Bloch sphere represents this quantity, which is shifted by rotations (10[1])around its z-axis. The relative value of this quantity of a qubit is physically relevant unlike its global value, since the latter only contributes a factor of “the exponential of i theta” to the wave function that does not affect probability. If this quantity is the same for two waves, they are coherent and constructively interfere. For 10 points, name this quantity representing the angular (10[1])shift at which a wave begins. ■END■

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