# Questions tagged [vqe]

For questions about the Variational Quantum Eigensolver, a quantum/classical hybrid algorithm that can be used to find eigenvalues of a (often large) matrix H. When this algorithm is used in quantum simulations, H is typically the Hamiltonian of some system. In this hybrid algorithm, a quantum subroutine is run inside of a classical optimization loop.

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### How does the Active Space Transform work theoretically?

I have a question about the ActiveSpaceTransformer which is used in vqe calculations for a molecule of LiH in Qiskit. In the documentation the inactive Fock operator is defined.I don't understand why ...
1 vote
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### Calculating state fidelity and space complexity of Minimum Eigen Optimizers (VQE, QAOA and Grover Optimizer) in qiskit

I'm a beginner in using Qiskit and my Computer Science background is not extensive. But I understand the Quantum Physics aspects of it relatively well. I solved a QUBO problem in Qiskit using VQE (...
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### qiskit-nature accessing circuit

There are some very nice examples on how to handle circuits using qiskit. However, once we get to the qiskit-nature documentation, it feels like a complete new world for the untrained eye. I am ...
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### Best tutorial about the variational quantum eigensolver (VQE)

I want to find a tutorial on VQE with a good valance between the theoretical background of the method, and it's working implementation on QPU. Covering advanced topics such as quantum error ...
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1 vote
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### How do you add spin up and spin down fermions to a FermiHubbardModel class in Qiskit?

I've been programming a FermiHubbardModel class with the Qiskit library for a VQE, but the class only shows input options for the U (onsite) and t (hopping) parameters when I construct a LineLattice. ...
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1 vote
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### Qiskit: evaluating expectation value of S^2 operator

I am trying to obtain the expectation value of the S^2 operator with respect to VQE wave function in an older version of qiskit (0.26). To that end, I tried: ...
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### Why is HF Initial point seems to be better than MP2 initial point in qiskit?

I am trying to design a custom VQE algorithm using qiskit. The main customization of the algorithm is in the minimization of expectation value of the Hamiltonian. To accomplish this, I have used the ...
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### Difference between using CircuitSampler vs Execute methods on obtaining expectation value

For a hybrid, VQE-type optimization in qiskit, in order to compute the expectation value of a Hamiltonian, we can use CircuitSampler on some Pauli sum operators, as well as Execute method on a list of ...
1 vote
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### Interpreting VQE and QAOA based solutions in portfolio optimization

I am going through portfolio optimization using Qiskit. It really looks interesting. The module has both VQE and QAOA algorithms for optimizing the portfolios. In the VQE based solution the ...
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### SPSA optimizer picking up previous iterations evaluations

I would like to write a modified SPSA which takes up an optimization history of a previous VQE simulation. Is it something that can be done? The purpose of this is to cut down the amount of time and ...
1 vote
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### Clarification on the usage of class UCC of qiskit_nature

I am trying to develop a customized VQE algorithm in which I am making use of the class UCC of qiskit_nature. I am having trouble understanding the outputs of the methods of the class. I have ...
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1 vote
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### How to send another job to a device before finishing the last job?

Is there any way to create a for loop that would send a few jobs to run on a quantum computer simultaneously? I tried: ...
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### References for two-local forms in QISKIT

I am using 'Two local' form of QISKIT as a waveform ansatz in VQE to compute ground state. QISKIT documentation does not provide references to all the entangling options. Is there any other place ...
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### Help on PySCF library

I am a student and I am trying to use qiskit nature in order to calculate the ground state energy of a molecule. My probelm is that when I try to run the simulation an error message comes out: ...
1 vote
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### Why does Parity mapping allow 2 qubit reduction?

when mapping from fermionic to qubit hamiltonian, using Parity mapper allow us to reduce 2 qubit in the final hamiltonian. I would like to understand which kind of symmetries are being used and how it ...
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### For the same operator, when I use qiskit's VQE and QAOA solutions, the results are completely different

Specifically, I can't get the correct result using QAOA，A total of 6 qubits were used in this work，this is part of my code using VQE and QAOA localsim = BasicAer.get_backend('qasm_simulator') ...
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### Barren Plateau as a consequence of 2-design?

Initializing a parameterized quantum circuit with a unitary sampled from a uniform distribution (so sampling from the Haar distribution) or in a weaker form: a unitary which is a unitary 2-design, ...
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### Expected Minimum Clique Coverage?

I'm curious if there is literature on this and, if there is, where to find it. Here I'm using clique" to mean a set of observables which all commute with each other. I also include the identity ...
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### Performance metrics in QSVM

I have implemented QSVM on breast cancer dataset using link: "https://qiskit.org/documentation/stable/0.24/tutorials/machine_learning/01_qsvm_classification.html" but unable to compute other ...
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### Expectation value of Pauli string for VQE

One approach to get the expectation value $\langle\psi|P\psi\rangle$ of a pauli string $P\in \{I, X, Y, X\}^{\otimes n}$ is the following. Let $(a_i, |\lambda_i\rangle)$ be eigenvalue-eigenvector ...
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### How is this Variational Quantum Singular Value Decomposition paper efficient in any way?

Link to paper here. This algorithm seems neat but the unitary decomposition of the matrix M generally takes an exponential number of Pauli basis elements in the number of qubits $N$, therefore an ...
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1 vote
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### What is the original paper introducing the Variational Quantum Eigensolver?

My question is very simple. I am looking for original paper where Variational Quantum Eigensolver (VQE) has been introduced. After some Googling I was able to find many applications of VQE and its ...
1 vote
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### Expectation value of pauli strings for VQE

I am studying VQE and have boiled it down to a matter of determining the expectation value of pauli strings: $$\langle H \rangle = \sum_i \alpha_i \langle\psi|\hat{P_i}|\psi\rangle.$$ I have been ...
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### Hamiltonian & QubitMappingType cannot be imported from qiskit.chemistry.core

I'm trying to implement the same Jupyter notebook in The Variational Quantum Eigensolver — Programming on Quantum Computers — Coding with Qiskit S2E4;however,I found a problem: Error: ...
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### Convexity of cost function in quantum machine learning

I have a small confusion on convexity of the most typical cost function considered in many qml papers: $\text{Tr}(\rho O)$, where $O$ is a Hermitian operator and $\rho$ is a quantum state. This is ...
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### Qiskit: QAOAnsatz circuit with custom Hamiltonian

I am trying to implement the Quantum Approximate Optimization Ansatz by creating a parametrized subcircuit $$V (α) = e^{−iH_M α_1} e^{−iH_D b_1} ... e^{−iH_M α_n} e^{−iH_D b_n}$$ with the custom ...
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### Why does VQE with pennylane and scikit-optimize gp_minimize give 'func must be scalar' error?

I am trying to replace the SPSA optimizer in the VQE tutorial of pennylane by the bayesian optimizer of scikit-opimize. When running the code below I get the error "func should return a scalar&...
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### Find minimum eigenvalue of an Ising Hamiltonian combined with Error Mitigation

I am currently working on a variation of the Traveling Salesman Problem. For which I use the QUBO(Quadratic Unconstrained Binary Optimization). Within Qiskit there are potent functions to obtain the ...
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### How can I get the counts, as well as the solution bitstring from Qiskit VQE?

I just set up a Qiskit VQE algorithm and calculated the eigenstates. However, I'm searching now, how to show up the final, most likely solution-bit-string. And/or to print the counts. I actually ...
1 vote
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### VQE Execution error: The ansatz must be parameterized, but has 0 free parameters

I am experiencing some problems with qiskit implementation of VQE. First of all, in initialize an ansatz in one of the following ways: ...
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### Understanding QAOA from Basics/scratch

Recently after working on QAOA with finance and graph coloring problems. I have started exploring the QAOA from scratch. I would like to understand the QAOA derivation mathematically and have started ...
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### What are the important parameters when it comes to VQE algorithms in qiskit?

So I wrote an algorithm for a quadratic problem (qubo) and try to solve it. My question is why I don't get the correct results when running my algorithm on a quantum computer like IBM_manila. In the ...
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### Why am I able to simulate such high qubit numbers on my laptop?

Recently I've been working on a VQE-related project, for which I'm using Qiskit's TwoLocal variational form (qiskit version 0.34.2). I noticed that I was able to simulate this circuit up to ...
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1 vote
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### Mathematical Context of MPT formula in qiskit VQE

In the video at timestamp: 43:34, We input the mean-variance parameters. https://www.youtube.com/watch?v=UtMVoGXlz04&t=3327s Based on the formula as mentioned here in section Formulation https://...
1 vote
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### Qiskit best programming practices - how to speed up qiskit code?

I am currently doing some experiments using Variational Quantum Eigensolver in molecular dynamics using qiskit, and noticed that the time for execution on real backend is significantly higher than the ...
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### QVA with local Hadamard Overlap

I am trying to implement in Qiskit the quantum variational linear solver given here (https://arxiv.org/pdf/1909.05820.pdf) with a local Hadamard overlap test and failing substantially. The cost is not ...
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1 vote
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### Question about performing VQE with an embedded Hamiltonian

Say I have a physical Hamiltonian $\mathcal{H}$ which is $D$-dimensional and I encode it into a larger matrix $M$ which is $\Delta$-dimensional. In the cases I care about $\Delta$ is strictly greater ...
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### What exactly makes VQE faster than classical optimization?

I have been trying to understand Variational Quantum Eigensolver (VQE), particularly from the non-linear binary programming perspective. But after reading a few resources I'm still confused about ...
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### Measurement of a Hamiltonian with Junk States

TL;DR: If I encode a Hermitian matrix in a non-Hermitian matrix, does that matter for VQE? Is the entire $H$ required to be Hermitian or just the parts corresponding to physical states? Say I have a ...
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### How to implement the VQE in qiskit using UCCSD?

I am new to VQE and want to see the whole circuit of UCCSD in VQE implementation. I saw the code post here Visualize full UCC circuit in qiskit I tried to import all packages but have no idea with one/...
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### Hardware efficient ansatz versus Alternating Layered Ansatz

I'm reading a paper Cost function dependent barren plateaus in shallow parametrized quantum circuits. There they introduce what is called Alternating Layered Ansatz (ALA). It is said that: The ...
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1 vote
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### What are "unbounded loss functions" and "unbounded operators"?

I am reading this paper: Quantum Generative Training Using Rényi Divergences. In it, the authors mention the following multiple times: "...an unbounded loss function can circumvent the existing ...
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### How to optimise QAOA

I have a similarly naive question about the classical part of the optimisation in the Quantum Approximate Optimization Algorithm. Specifically, the cost function is given prescribed as \begin{align} ...
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### Variational quantum eigensolver in qiskit

I have implemented a hybrid quantum NN on a dataset to perform classification. Now I want to apply quantum PCA to my dataset to find the eigenvalues. After going through a few qiskit tutorials, I ...
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### VQE from scratch, what have I got wrong?

I've been working on a code to run VQE with a grouped measurement. For some reason, my expectation values are slightly off from those computed by pennylane, the software I decided to use for this ...
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### Qiskit - Simpler way to create a Hamiltonian

I am looking for an efficient way to create a Hamiltonian in Qiskit. Following are my desired input and outputs input: 'ZZI' output: Z^Z^I, where Z and I are operators. Here is how I am doing it ...
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### Cliffords to Transform into Common Eigenbasis

Say I have the following Hamiltonian (given in terms of Pauli operators): \begin{equation} H=aX_1Z_2+bZ_1X_2. \end{equation} Both Pauli terms commute with each other. I want to make a measurement of \$\...
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### Ground state compute issue by qiskit

I use the following code to calculate the ground state for the LiH molecule in an active space. I come across two problems. The first is I found that the Hartree Fock state gave energy that is far ...
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### Confusion about the objective function of VQEs and QAOAs

I am a bit puzzled on how the objective function of the VQEs and QAOAs. Of course, the parametrised state is constructed differently in these two algorithms but they do share a common objective to be ...
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