Quantum Computing Asked by Psanfi on August 20, 2021
I found an answer that shows how to detect the phase in cases like $0$, $pi/8$, $pi/2$, $pi/4$ or $pi$ for circuit to prepare state as H P, where P is a phase gate like $I$, $U1(pi/8)$, $S$, $T$ or $Z$.
But in my case the circuit to prepare state is $X H P$, where $P$ is $X$ gate (conditionally phase $pi$) or $ID$ (conditionally phase 0).
This circuit in Qasm with conditionally phase equal to $pi$:
x q[0];
h q[0];
x q[0];
with conditionally phase equal to $0$:
x q[0];
h q[0];
id q[0];
Appending $H$ gate (as in the above answer) don’t detect a difference for conditionally phase $pi$ and phase $0$ (but does detect for phases $pi/2$, $pi/4$, $pi/8$ if $P$ is $S$, $T$, $U1(pi/8)$, respectively).
Is it possible to detect the conditionally phase $P$ $pi$ or 0 for this circuit to prepare state?
I think unlike the relative phase in the answer you reference, it is a global phase in your case:
Your XHP-circuit where P=ID, prepares the state: [0.707+0j,-0.707+0j], where P=X, prepares the state: [-0.707+0j, 0.707+0j]. These states are differ by a global phase ${e}^{ipi}=-1$.
But the global phase is undetectable $|ψ⟩:={e}^{iδ}|ψ⟩$, also see the answer.
Correct answer by Aleksey Zhuravlev on August 20, 2021
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