All Questions
10,586
questions
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Quantum optimization and correlation function
If one imagines the nodes of the classical weighted graph as atoms, and paths between them as coupling strength, then to find the shortest path in the weighted graph, can one also look at the minima/...
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0
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24
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is the main purpose of using Schmidt decompositon to visualize entanglement?
Is it correct to say that an entanglement state is entangled in any basis, but this fact may not be evident (like in the state $ \frac{1}{ 2} | 00 \rangle + \frac{1}{ 2}| 01 \rangle + \frac{1}{ 2}| 10 ...
- 11
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14
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Product for block-encoding of two density operators
I know for density operator like $\rho$, we have a block-encoding matrix with purification operator and SWAP gate: $BE_{\rho} = \begin{bmatrix} \rho & \cdot \\ \cdot & \cdot \end{bmatrix}$. ...
- 41
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24
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Error on expectation value even with zero noise
If I use V2 fake backends, I can set a certain noise equal to 0 with
backend.target['measure'][(0,)].error = 0
and I can do that for each kind of instruction ('id',...
- 373
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53
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How to determine the single-qubit phase?
In some works it is suggested to find the qubit phase by the following method:
Apply $X/2$ (or $Y/2$) gate - preparing the qubit in superposition.
Detune the qubit by flux pulse, for example here ...
- 249
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6
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Self referencing algorithms for multiplication of binary polynomials
Currently, I am going through the paper "Space-efficient quantum multiplication of polynomials for binary finite fields with sub-quadratic Toffoli gate count" by Iggy van Hoof.
Two ...
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8
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-3
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23
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The Quantum Mobility Quest - Airbus & BMW Group [closed]
Came across this article online and thought I'd share in case anyone was interested in registering, seems like a good opportunity but not quite for me right now.
Airbus and BMW Group are launching a ...
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1
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1
answer
25
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Post selection during hook injection
I was reading the paper about "Cleaner magic states with hook injection"1, which is really cool, and I get technically, but it is unclear in which stages do you perform the post-selection.
...
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2
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1
answer
126
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Questions on multi-controlled Toffolis and their implementation in Qiskit
Is it possible to decompose an $n$-controlled Toffoli into $O(n)$ CNOTs without extra working qubits?
If so, is such an decomposition currently available in Qiskit or any other (preferably Python and ...
- 1,852
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How do I calculate the expectation of the rational function, in the sense of the Haar measure?
I want to know the analytical solution of $\mathbb{E}_{\psi}\frac{\langle \psi |A|\psi\rangle}{\langle \psi |A^2|\psi\rangle}$. I see similar questions before approximate average, but it does not ...
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2
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48
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Does the $\text{CNOT}$ gate activate if the control qubit is $-|1⟩?$
I understand how the $\text{CNOT}$ gate works intuitively:
However, say we have a circuit where the $Z$ gate is applied to $|1⟩$, which turns $|1⟩ \to -|1⟩$. Then, there is a $\text{CNOT}$ gate with ...
1
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20
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How to actually solve for the eigenstates in the adiabatic version of Grover's algorithm
The adiabatic version of Grover's algorithm says that the eigenstates of the Hamiltonian $H(t)=(1-\frac{t}{T})H_0+\frac{t}{T}H_m$ (equation 10 in the linked paper) can be used to solve the eigenvalue ...
- 173
1
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1
answer
20
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References for mapping qudit gates to qubits equivalent
Let's assume that I'm modelling a d-levels qudit as a set of n qubits, where $d = 2^n$. I would need to map the application of quantum gates on the qudit to a set of operations of the relative qubits ...
- 11
1
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15
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Can I get valid solution with HHL algorithm even if the QPE is not completely correct?
Can I get the valid solution of linear problem using HHL algorithm even if the QPE is not completely correct?
My example is $A=diag(0.5, 0.2, 0.3, 0.6)$, so the solution is $[0.5, 0.2, 0.3, 0.6]$ ...
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25
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Deriving a equation in "Data re-uploading for a universal quantum classifier" paper: U(phi1,phi2,phi3) U(x1,x2,x3) = U(theta + w *x)
In "Data re-uploading for a universal quantum classifier" paper the U(phi1,phi2,phi3) U(x1,x2,x3) is derived as U(theta + w *x) in compact state.
How to derive the above equation ?
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19
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How can I initialize a classic register in qiskit?
is there a way to initialize a classic register in qiskit? I will go quickly through my problem and then suggest one solution but I am not satisfied with that and ask if there is another solution.
...
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2
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2
answers
33
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How to estimate the negative amplitude of multiple qubits?
The probability of measurement is the square of amplitude.
After measurement, how to guess the original amplitude of state??
For example, in linear problem, we would like to know the exact solution, ...
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1
answer
34
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general way to decompose a CZ gate on $n$-qubit system
Suppose I have a CZ(i,j) gate (or CNOT) acting on qubit $i$ and $j$, on a $n$-qubit system. Is there a general way to decompose this gate into a set of gates that only involve single-qubit gates and ...
1
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1
answer
32
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For arbitrary single-qubit unitary gate, does there exist a code such that the given gate is transversal?
Background
Transversal gates are considered to be fault-tolerant logical operation as they won't lead to the propagation of local errors. Here are some examples:
CSS code has transversal Pauli and ...
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36
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Parallelize estimation of expectation values in Qiskit
Edit
I tried using python multiprocessing library. The result with 8 cores isn't always faster than the original script that uses only one core. How can i improve the algorithm?
Suppose i've a matrix ...
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55
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Can you push a CZ gate through a Toffoli gate?
Is it possible to push a CZ gate through a Toffoli gate?
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1
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29
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In Qiskit, no state vector after measure and reset
The code below doesn't work. I am looking for a workaround.
OK, I can save the state vector as numpy array, recreate the whole circuit and initialize it thanks to this array. It works, but any simpler ...
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1
answer
17
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Passing a book-keeping parameter to Qiskit Ibm Runtime Estimator
I have a Qiskit.QuantumCircuit object circ which prepares the state $| \Psi \rangle$, and a ...
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1
answer
56
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Quantum Error Correction on a hexagonal lattice
I have an exercise in QEC, where I have given a hexagonal lattice with periodic boundary conditions (wrapped around a torus), with a qubit at each vertex. I have also given the Stabilizer generators $...
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19
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Implementing a Quantum Recurrent Neural Network
I want to implement a Quantum Recurrent Neural Network and use it for time series forecasting.
As a starting point, I chose to implement it using the architecture given in this paper: https://arxiv....
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1
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16
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Specifying the image of a set of states under the action of a channel
I have a generic channel $\mathcal{N}$ acting on a subspace of states defined on a $d$-dimensional Hilbert space $\mathcal{H}$. I am trying to make a statement about the dimension of the image of that ...
- 6,143
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1
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64
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If all terms of a local Hamiltonian commute, how hard is it to learn the ground state (energy)?
Suppose we have a $k$-local Hamiltonian with each of $m$ terms acting on $k$ of $n$ qudits of constant dimension $d$:
$$H=H_1+H_2+\cdots+H_m.$$
If at least some of the terms don't commute, e.g., if $[...
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24
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0
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23
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1
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1
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810
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Why can't Grover's algorithm apparently solve this simple situation?
I'm utilizing Grover's algorithm to solve a straightforward problem. It presents eight options, but only four are correct (the 1's in the S column)
$$\begin{array}{c|c|c|c}
q_0& q_1\ & q_2\ &...
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1
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2
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38
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Access individual qubits of a quantum circuit in qiskit by qubit name
I am very new to quantum computing and only recently started learning Qiskit, so the question may sound a bit silly.
Essentially, when I create a QuantumCircuit ...
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1
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1
answer
33
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Measuring one qubit in an entangled pair in another basis?
Qubits are usually measured in the computational basis, but we can change the basis by a unitary $U$ to measure in the basis formed by the columns of $U$.
For example, if $| \psi \rangle = | 0 \rangle$...
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6
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QISKIT Metal Saves Error Generating Python Code
Background
I'm testing out the ability of the Metal GUI to save a simple circuit but it keeps generating code that gives rise to various errors. In the below code for example generates an error show ...
- 175
1
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1
answer
19
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Is qiskit.pulse.SamplePulse deprecated?
I'm currently following along the book "Learn Quantum Computing with Python and IBM Quantum Experience" by Rodert Loredo and I am at chapter 8 - Generating pulse schedules on hardware. The ...
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0
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11
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Accuracy of QPE using histogram fit
In regular QPE, accuracy is $1/2^n$, where $n$ is estimation register size, and it is done with few shots and depth as $O(2^n)$ (total cost of $O(2^n*NumShots))=2^n=1/resolution$).
I thought of the ...
- 1,184
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31
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What is the difference between shot and iteration of the quantum circuit?
Hi this question is stem from What is meant with "shot" in quantum computation?.
If we just look for the distribution of the result, is there any theoretical difference between method 1 and ...
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27
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ibmq belem noise model backend shows best result. why? [closed]
i run one circuit in presence of three basic backend noise model from this link
https://qiskit.org/ecosystem/aer/apidocs/aer_noise.html
ibmq belem, lima, quite, and fake provider noise model we used.....
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0
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49
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A measure of entanglement created by a unitary operation
Let $U$ be a unitary matrix acting on a 3-qubit system. If there is no correlation among any pairs of the three qubits, the unitary operation can be represented as $U = U_1 \otimes U_2 \otimes U_3$, ...
1
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0
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27
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Get confused about the implementation of Amplitude Amplifier
I have a circuit $A$, which produces
$$A|0^n\rangle _a |00\rangle _b = \phi_0 |00\rangle _b + \phi_1 |01\rangle _b+ \phi_2 |10\rangle _b + \phi_3 |11\rangle _b$$
And only $\phi_0 |00\rangle _b$ is ...
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3
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1
answer
77
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What is the difference between classical-quantum and completely classical states?
States that are completly classical :
$$
\begin{aligned}
\tilde\rho_{A B} & =\sum_{x \in \mathcal{X}} \sum_{y \in \mathcal{Y}} p_{X, Y}(x, y)(|x\rangle \otimes|y\rangle)(\langle x| \otimes\langle ...
- 433
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15
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How to deal with unused qubits in application specific quantum chips?
Background
I am a complete novice to quantum hardware but am trying to give myself an introduction to Qiskit Metal. I have set myself the challenge of designing a custom quantum chip for the Bernstein-...
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2
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1
answer
124
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Finding the "dual" basis of an overcomplete basis for Quantum State Tomography
This question is related to this stack exchange post: What does the POVM corresponding to single-qubit state tomography look like?
From what I understand, when we are interested in reconstructing a ...
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43
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Generalization of representing SU(2) as quaternions
I am familiar with the isomorphism between $SU(2)$ and the unit quaternions, and the group homomorphism from them to $SO(3)$. I am interested in knowing if there is a generalization for $SU(2^n)$. My ...
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2
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Why $\sqrt{\rho} = P \sqrt{\rho}$ in the proof of quantum error correction conditions in Nielsen & Chuang?
I have trouble understanding this proof in Nielsen & Chuang, specifically the identity in $(10.20)$, which reads $$ U_k^\dagger P_k F_l \sqrt{\rho} = U_k^\dagger P_k^\dagger P_k^\dagger F_l P \...
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votes
1
answer
53
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Custom gate labels in PennyLane
I have a circuit with Rx gate, and I want to draw it with changed gate label. For example, instead of "RX" I want to have "custom_gate" written in the gate box.
I have tried ...
0
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0
answers
9
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Quantum Annealing on IBMQ using Qiskit-Pulse
Qiskit-Pulse is the analog level control SDK of IBMQ hardware. Is it possible to implement an adiabatic annealing protocol/algorithm on IBMQ hardware using Qiskit-Pulse?
Not asking about QAOA, which I ...
1
vote
1
answer
91
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Problem with the mathematical definition of the eigenvalue algorithm on a specific exercise
I think I understand well how the eigenvalue algorithm works but when I try to define it mathematically I have problems.
Specifically I have the matrix U:
$$
U =
\begin{pmatrix}
0 & i \\
i & 0
...
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2
votes
1
answer
54
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Why can't we do pure state tomography using interference with a known reference state?
I am new to quantum tomography, and right now I am trying to understand tomography with pure states. It's my understanding if we have the amplitude magnitudes as well as relative phases, we can ...
2
votes
1
answer
59
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What does "lift" mean in the Lifted Product (LP) Code?
I am watching this talk about Lifted Product Code (LP). For me, it's quite straightforward to see how to construct a HyperGraph Product Code (HGP) using two separate classical codes. However, I failed ...
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