17 votes

Why does the "Phase Kickback" mechanism work in the Quantum phase estimation algorithm?

A first remark This same phenomenon of 'control' qubits changing states in some circumstances also occurs with controlled-NOT gates; in fact, this is the entire basis of eigenvalue estimation. So not ...
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12 votes
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Who discovered the phase kickback trick?

The phase kickback trick appears in this paper: Richard Cleve, Artur Ekert, Chiara Macchiavello, Michele Mosca. Quantum Algorithms Revisited. Proceedings of the Royal Society of London A, 454(1969):...
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11 votes
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Why does the "Phase Kickback" mechanism work in the Quantum phase estimation algorithm?

Imagine you have an eigenvector $|u\rangle$ of $U$. If you have a state such as $|1\rangle|u\rangle$ and you apply controlled-$U$ to it, you get out $e^{i\phi}|1\rangle|u\rangle$. The phase isn't ...
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5 votes
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Can we use Hadamard test to estimate phases?

So QPE using $\mathcal{O}(1/\epsilon)$ queries to $U$ outputs an estimate of the eigenphase $\theta$ given a corresponding eigenvector with additive error and $\Omega(1)$ probability. The method using ...
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  • 294
4 votes

What is Quantum Phase Estimation in Shor's Algorithm?

Phase estimation is the process by which you are given a controlled-$U$ unitary, and a state that you are promised is an eigenvector of $U$ with eigenvalue $e^{2\pi ix/2^t}$, then you can use a $t$-...
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4 votes

Question about the phase kickback in the phase estimation algorithm

The phase is applied to the overall wave function $|\phi\rangle$, therefore you can factor the phase to any individual qubit. For example if we have a wave function as a result of a controlled ...
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3 votes
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In the Bernstein-Vazirani circuit, if the secret is all 0s, would the oracle just be nothing?

Yes, this is because the initial state will be starting as $|\psi_{init} \rangle = |000\cdots0\rangle = |0\rangle^{\otimes 8} $. Then you apply a layer of Hadamard gates, follow by the Oracle ...
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3 votes

Unitary Operator impact on both - the Control Qubit and the Target Register in Shor's Algorithm

The key idea in understanding phase kickback is that phase factors do not belong to one register or the other, but instead belong to terms in superposition and are shared by the registers. For example,...
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3 votes

How to understand intuitively the quantum gate phase kickback?

Part of the problem people usually have here is a sort-of-classical intuition. Because you're trying to describe the action of a gate such as controlled-$U$, we divide it up as "if the control ...
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3 votes

Is the $|-\rangle$ state the only one that can do the trick for Grover's algorithm?

Remember that the states $|+\rangle$ and $|-\rangle$ form a basis. That means that any state $|\psi\rangle$ that you use can be written as $$ |\psi\rangle=\alpha|+\rangle+\beta|-\rangle. $$ By ...
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3 votes
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Is the $|-\rangle$ state the only one that can do the trick for Grover's algorithm?

It would still work if you rotated the $|−\rangle$ state, you refer to, by an angle with cosine less than $\frac{1}{L}$ where $L$ is the square root of the size of the search space (which is a power ...
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  • 306
2 votes

Show that these two expressions for the oracle transformation are equivalent

You are given a quantum circuit for $U$ compiled into the H/CNOT/T gateset. Derive a controlled version of $U$ by adding a control qubit $q$, replacing every H with a controlled H, every CNOT with a ...
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2 votes
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How to decompose a multi-target controlled gate?

An example of constructing (with help of Qiskit) a controlled version of some simple 4x4 unitary matrix: $$ U = \begin{pmatrix} \mathrm{e}^{i g_1} & 0 & 0 & 0 \\ 0 & \mathrm{e}^{i g_2}...
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2 votes

What is Quantum Phase Estimation in Shor's Algorithm?

If you want to learn more generally about phase estimation, the book I am helping write has a whole chapter on it, Learn Quantum Computing with Python and Q# (chapter 8). Ping me here or via email ...
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2 votes
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In Shor's algorithm, how can we guarantee that each controlled-U will kickback to the same eigenvalue?

I understand, that cU has multiple eigenvalues with a factor s. How can be guaranteed that each of the controlled-Us will kickback the same eigenvalue? Or, why it is not important? All the $U$s in ...
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2 votes
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Resources and references about phase kickback trick

The IBM Qiskit text has a section on this that you may find useful. For something more "academic" Mermin Sections 4.2-4.5, Schuld and Petruccione Sections 3.5.1-3.5.3, or Nielson and Chuang ...
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2 votes

Who discovered the phase kickback trick?

As to "who discovered/invented the quantum phase estimation algorithm," in his 2011 lecture at Keio University describing the linear equations algorithm, at about the 18 minute mark Lloyd claims that ...
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1 vote
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Understanding phase kickback caused by the CNOT gate

There are two ways to see this. simply factor the sum you have: $$\frac{1}{2}(|00\rangle - |01\rangle - |10\rangle + |11\rangle) \\ = \frac{1}{2}(|0\rangle - |1\rangle)(0\rangle - |1\rangle) \\ =|-\...
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1 vote

In the Bernstein-Vazirani circuit, if the secret is all 0s, would the oracle just be nothing?

With the Quantum Oracle $|x \rangle \xrightarrow{f_s} (-1)^{s\cdot x} |x \rangle$ (between the Hadamard sandwich) for the BV algorithm, with ($n$-bit) input secret bits $s$, here is what it does: $|00\...
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1 vote
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Why does the phase of the eigenstate get kicked up to the ancilla qubit?

Here is a basic example of a two system that might help you to see this better. Suppose I have these two circuits: Circuit 1: Which put the "Controlled qubit" in the state $|1\rangle$ and ...
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1 vote

In Shor's algorithm, how can we guarantee that each controlled-U will kickback to the same eigenvalue?

Have you seen this document? https://qiskit.org/textbook/ch-algorithms/shor.html Note that in Shor's algorithm, we use the quantum computer as a subroutine to essentially find the period of the ...
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1 vote

controlled-Z rotation gates in symmetrical fashion

For the mathematical explanation, check here: Why is the action of controlled-Z unaltered by exchanging target control qubits? Maybe it would help you to see CZ in a different (symmetric) notation, ...
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1 vote
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Phase Kickback and Controlled Operations

Without a bit more context of how they use the notation in the rest of the book, I'm not certain, but the way I would interpret that is saying "if the control qubit is 0, apply unitary $\hat U_{f(0)}$ ...
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1 vote
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How does Inverse QFT work in Quantum Phase Estimation?

The expression you obtain after applying the QFT contains sums of the unit square, $e^{2\pi i/2^n}$, which sum up to 0 if you sum over the full range of $2^n$: $$ \sum_{k=0}^{2^n - 1} e^{\frac{2\pi i}...
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  • 2,203
1 vote

Quantum Phase Estimation implementation

3 things I see from your implementation of inverse QFT: SWAP gates are missing prior to applying Hadamard gates and cu1 gates. The Hadamard gate should come first before cu1 gates. The angles of cu1 ...
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1 vote

Obtaining phases of all basis states

So, are you asking whether it's possible (in the qubit case) to perform an arbitrary map of the form $$ ae^{i\theta}|0\rangle+be^{i\phi}|1\rangle\rightarrow \frac{1}{\sqrt{2}}(e^{i\theta}|0\rangle+e^{...
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1 vote
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Show that these two expressions for the oracle transformation are equivalent

If we express the action of $O_x$ on the basis $\mid i, \pm \rangle$ instead of $\mid i , b \rangle$ \begin{eqnarray*} O_x \mid i , + \rangle = \mid i , + \rangle\\ O_x \mid i , - \rangle = (-1)^{x_i}...
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