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ECS 189A Lecture 17: Quantum Circuit
Identities
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1
Recap
• Familiarize yourself with circuit identities! (They are very useful.)
• Single-qubit gates: Switching between X , Y , and Z
• Controlled-gates: Think of it as a little computer program, and then
simplify.
Next lecture: More on controlled gates, and then measurements
2
Controlled-gates
H H
H H
=?
3
Controlled-gates: Propagation
X
=
U1
U2
U1,U2 =?
4
Controlled-gates: Propagation
X
=
U1
U2
U1,U2 =?
5
Controlled-gates: Propagation
Z
=
U1
U2
U1,U2 =?
6
Controlled-gates: Propagation
Z
=
U1
U2
U1,U2 =?
7
Composing two controlled-gates
=?
8
Measurement
9
Measurement
X
=
Z
H
.
What does this mean? And why is this true?
10
Measurement outcome
Throughout this course, we assumed that the measurement is performed
in the Z -basis.
Z =
(
1 0
0 −1
)
.
Depending on the measurement outcome, we get |0⟩ or |1⟩, the
eigenstates of Z .
11
Eigenstate vs. Eigenvalue
For each eigenstate, there is an associated eigenvalue:
Z |0⟩ = (+1)︸︷︷︸
Eigenvalue
|0⟩
Z |1⟩ = (−1)︸︷︷︸
Eigenvalue
|1⟩
Since the eigenvalues are different, we could have specified the eigenstate
by their eigenvalues.
12
Measurement in terms of eigenvalues
Z
The measurement device is a machine that reports one of the
eigenvalues, +1 or −1.
• +1 case: |0⟩ (+1 eigenstate)
• −1 case: |1⟩ (−1 eigenstate)
Let |ψ⟩ be the state.
Prob[Measured + 1] = |⟨0|ψ⟩|2
Prob[Measured − 1] = |⟨1|ψ⟩|2
13
Measurement in terms of eigenvalues
Z
The measurement device is a machine that reports one of the
eigenvalues, +1 or −1.
• +1 case: |0⟩ (+1 eigenstate)
• −1 case: |1⟩ (−1 eigenstate)
Let |ψ⟩ be the state.
Prob[Measured + 1] = |⟨0|ψ⟩|2
Prob[Measured − 1] = |⟨1|ψ⟩|2
13
Measurement in terms of eigenvalues
X
The measurement device is a machine that reports one of the
eigenvalues, +1 or −1.
• +1 case: |+⟩ (+1 eigenstate)
• −1 case: |−⟩ (−1 eigenstate)
Let |ψ⟩ be the state.
Prob[Measured + 1] = |⟨+|ψ⟩|2
Prob[Measured − 1] = |⟨−|ψ⟩|2
14
Measurement in terms of eigenvalues
X
The measurement device is a machine that reports one of the
eigenvalues, +1 or −1.
• +1 case: |+⟩ (+1 eigenstate)
• −1 case: |−⟩ (−1 eigenstate)
Let |ψ⟩ be the state.
Prob[Measured + 1] = |⟨+|ψ⟩|2
Prob[Measured − 1] = |⟨−|ψ⟩|2
14
Proving the identity
X
=
Z
H
.
15
Exercise
Y
=
X
U
.
What is U?
16
Measurement and feedback
17
Recap
• Controlled-unitaries
• Measurements in different bases
Congrats, now you learned everything about quantum circuits!
Next week: Quantum computation, in the language of quantum circuit
(ex. Deutsch-Jozsa algorithm)