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Quantum Phase estimation with $2pi$ replaced with $2e$

Quantum Computing Asked on May 3, 2021

The QPE on IBM platform finds the eigenvalue of a unitary operator, i.e $$U|phirangle=e^{2pi itheta}|phirangle$$
and uses the rotation operators as $$U(theta)=begin{bmatrix}0 & 1
1 &e^{itheta}end{bmatrix}$$

My question is we can write $U|phirangle=e^{2pi itheta}|phirangle$ as $$U|phirangle=e^{2e itheta}|phirangle$$ where, since even then the magnitude of the eigenvalue remains same. Of course that would require more number of measurement qubits. For instance for $theta=0.5$, we had $4$, so this time we can increase the $n$ and get closer to $theta=0.5$. My question is can this be done.

One Answer

No - the key intuition of QPE is that $e^{2 pi i theta} $ holds for $theta in [0, 1) $, and the reading out the ancilla provides the binary representation of the fraction.

If you changed the operator to $e^{2 e i theta}$, $theta not in [0, 1)$ necessarily. Instead, you should just use the typical QPE approach, but then find $zeta$ where $2 e i zeta = 2 pi i theta implies zeta = frac{pi}{e}theta$, and correct for symmetries afterwards.

Correct answer by C. Kang on May 3, 2021

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