Questions tagged [vqe]

For questions about the Variational Quantum Eigensolver, an algorithm which can find eigenvalues of large Hermitian operators.

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44 views

In which situations is it hard to find an ansatz?

Looking at the single-qubit, toy example of VQE it's pretty much trivial that arbitrary X and Y rotations are sufficient to cover all of state space for our toy system. Unfortunately, the toy example ...
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1answer
57 views

VQE: Can I build a non-hermitian Hamiltonian with just Pauli matrices?

From the VQE paper they claim that a Hamiltonian can be expressed as a polynomial series of pauli operators (equation 1). While coding up VQE from scratch I made a function which would allow me to ...
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Resource recommendation on quantum simulations

I would like to know more about quantum simulations, so as to start on a few standard physical models (maybe particle in a box, harmonic oscillator, etc.) and then build up on more complex things. But ...
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1answer
84 views

What's the point of VQE if classical computers can solve for eigenvalues easily?

From a few VQE tutorials online I see that they normally start with something like: VQE is a way of getting a good estimate for the upper bound of the ground state of a quantum system's Hamiltonian....
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73 views

Creating a custom VariationalForm in Qiskit

I would like to use the built-in Qiskit VQE() function, with my own variational form. What is the proper way to define it? I was expecting ...
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1answer
96 views

VQE restart from a previous computation

I'm running VQE algorithm on ibmq-qasm-simulator. I'm trying to implement a restart mechanism in order to be able to start a new computation from the result of a ...
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1answer
35 views

SPSA max_trials

I'm using SPSA as an optimizer in VQE algorithm. The code runs on ibmq-qasm-simulator. I've set SPSA max_trials parameter to 500, but, when I run the code, it makes ...
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1answer
39 views

How do I get intermediate values of VQE algorithm using callback?

I'm new to using Qiskit Aqua, and I'm trying to get the intermediate values in the VQE algorithm to study the convergence. The documentation mentions an internal callback function, but I can't figure ...
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25 views

Is there a limit to the size of problems that your simulators can run on Qiskit Aqua?

I'm trying to solve QUBO problems using Qiskit QAOA and VQE solvers. However, I have the experience that I can only solve small problems. I tried with both QAOA and VQE and both experience the same ...
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37 views

How to convert a qubit hamiltonian to QUBO and vice versa?

This is my hamiltonian. Solving this by hand, Numpy Python package and VQE algorithm gives the minimum energy eigenvalue -2. If we want to find the minimum energy of this hamiltonian with Quantum ...
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65 views

In which type of Hamiltonian does the ansatz need an entangler in VQE?

I would like to prepare an ansatz for VQE. I know UCC ansatz is one of the most useful ansatz, but I would like to reduce the number of CNOT. From the matrix of a given Hamiltonian, can we know the ...
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124 views

VQE for Beginners : Using Tutorial and Cirq

I have been following this tutorial: https://dkopczyk.quantee.co.uk/vqe/ I am using Cirq to try to teach myself VQE, replicate their results, and also try to understand more about ansatz for ...
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1answer
44 views

QAOA Belongs into VQE or the other way around?

I have been reading a couple of papers in the arxiv and wanted to get a clarification regarding the relation between these two methods; is one a subset of the other?
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Ansatz state for finding the lowest eigenvalue of a $2^n\times 2^n$ real matrix using VQE

I would like to find the lowest eigenvalue of a $2^n\times 2^n$ real matrix $H$ using the VQE procedure. The measurement part is simple — I just expand $H$ in a sum of all possible $n$-qubit Pauli ...
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62 views

Decomposing Hamiltonian into qubit model representation

One of the main application of VQE is its application to find the approximation to the ground state energy (smallest eigenvalue) for a particular molecule through an iterative method. To be able to ...
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1answer
123 views

Cirq: easiest way to compute qubit Hamiltonian expectation value with number of shots

I was wondering what was the easiest way to take a qubit Hamiltonian, and get an expectation value estimated with a certain number of shots over a parameterized quantum circuit (or ansatz) with Cirq.
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95 views

What is the risk factor on IBM's portfolio optimization notebook?

In the notebook "portfolio optimization" on IBM's platform the goal is to calculate the optimal stock selection using a classical and a quantum algorithm (VQE). A random portfolio is generated and ...
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1answer
167 views

VQE Cirq example

Is my understanding correct that in this example the Hamiltonian measurement is not performed through measuring individual Pauli operators because all its terms are mutually commuting? So, for each ...
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1answer
64 views

Finding optimal values of depth and max_trials parameters in vqe algorithm

I am running vqe algorithm in qasm_simulator and cannot get the optimal solution. Are there any methods for finding the optimal value of depth for variational form and max_iter for optimizer ...
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1answer
92 views

Modifying Qiskit Hamiltonian

I'm fairly new to the Qiskit API. I was wondering if I could get some assistance with trying to implement our technique of projecting out a new Hamiltonian from the ground-state Hamiltonian. The ...
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1answer
80 views

Qiskit statevector_simulator returns different results

I'm running VQE algorithm from qiskit aqua on statevector_simulator and each time I run it I get different results which are sometimes correct and sometimes not. I'...
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1answer
48 views

VQE with error dim. mismatch using qiskit.aqua.vqe

when i try to call the VQE from qiskit I get an error of dimmismatch. The input matrix is of a 16x16 shape and I could not find out what where I introduce the error. In the picture The input matrix ...
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2answers
90 views

Circuit of a very trivial thing

I am writing to double check that if have a hamiltonian of the form $H = I_1 \otimes I_2$, when I seek to find the unitary, $e^{-i\gamma I_1 \otimes I_2}$, there really is no need to convert this into ...
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1answer
104 views

What is a better ansatz for VQE?

I am using Qiskit. I made two quantum circuits to generate $\cos\theta|00\rangle + \sin\theta|00\rangle$ for VQE calculations as follows. Apparently, the first one which has fewer CNOT gates is ...
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1answer
61 views

How to explain that I get a value lower than the smallest possible through minimization procedure in VQE?

As far as I know after minimization I have to obtain a value $E_{0}\le \frac{\langle \psi (\theta)|H|\psi (\theta)\rangle}{\langle \psi (\theta)|\psi (\theta)\rangle}$, where $E_{0}$ - eigenvalue of ...
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165 views

How to prepare Unitary Coupled Cluster ansatz for VQE in a circuit?

I have read the following paper by Dumitrescu et al. To make a Unitary Coupled Cluster (UCC) ansatz, one prepares with the following equation. $$ | \psi_{\rm{UCC}} \rangle = U(\theta)|\mathrm{HF}\...
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39 views

Graph coloring to reduce the number of qubits in VQE

I am reading through the following article: https://arxiv.org/abs/1312.2579 and I really struggle to understand anything of the section "D. The standard graph-coloring method" on page 7 and "E. The ...
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53 views

Active areas of research for NISQ algorithms

What areas of research in NISQ algorithms have heavy focus? I'm interested in quantum chemistry algorithms because of previous work (e.g. VQEs), and I'd love to learn more about other near-term ...
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60 views

Many jobs hanging after creating

I am running a VQE calculation running from a cluster using nohup. However, some of my jobs keep hanging with 'creating' and therefore my VQE is not converging/not working. However, at the same time,...
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282 views

Is there a mistake in the VQE Ansatz in Cirq's tutorial?

I have been going through Cirq's VQE background tutorial and after examining the Ansatz it seems to me that the only layer that actually affects the final measurement is the rot_x_layer. The other ...
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148 views

Problem with measurement error mitigation using VQE

I am attempting to mitigate measurement error while running on one of IBM's real devices and am doing so by passing CompleteMeasFitter into ...
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1answer
816 views

Is VQE a class of algorithms or a specific algorithm?

Is VQE a class of algorithms or a specific algorithm? For example, is QAOA a VQE or is VQE an algorithm distinct from QAOA that solves the same class of problems? If VQE is a specific algorithm, what ...
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2answers
2k views

How to calculate an Expected Value of some operator acting on qubits?

I'm trying to implement the Variational Quantum Eigensolver in Qiskit. Suppose, I have an operator $A = \sigma_1^z\sigma_2^z$ acting on some two-qubit state $|\psi\rangle$. After a measurement I get ...