# Questions tagged [quantum-algorithms]

For questions about quantum algorithms, that is, sequences of quantum gates, operations, and measurements, whose purpose which achieve some goal. Standard examples are Shor's and Grover's algorithms.

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### Can quantum algorithm still success if some gates fail and cannot be measured?

I know the power of quantum computation is to simultaneously compute all the qubits if they are entangled. I wonder if the quantum algorithm is still functional if one of the quantum gates fails (...
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### Problem with SwitchCase in qiskit while using general unitary gate

I am trying to run a circuit using SwithCase Op on one of the 127 qubits backends. My code is the following: ...
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### Difference between PQCs and VQCs

I'm going through this TensorFlow tutorial on quantum machine learning. The code implements reinforcement learning algorithms based on parameterized/variational quantum circuits (PQCs or VQCs). What ...
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### Is there an "in place" Toffoli gate?

I am trying to implement the multiplication of a scalar in $\mathbb{Z}_2$ and a polynomial in $GF(2^n)$ in Qiskit. One naive method of implementation that I can think of is: ...
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1 vote
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### Need help regarding building a custom VQE

I want to write code for a custom Variational Quantum Eigensolver (VQE) capable of computing eigenvalue(s) for non-Hermitian systems, based on this paper. I have formulated a cost function (provided ...
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### How is controlled constant addition implemented for binary polynomials?

So, currently I am going through the paper Concrete Quantum Cryptanalysis of Binary Elliptic Curves. The section on point addition mentions that for adding two points $P_1$ and $P_2$, they assume that ...
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### Why does the definition of the oracle in Deutsch's algorithm fail to specify its action on superpositions?

I’m trying to understand the Deutsch algorithm. I can see that the math shows the algorithm to be correct, but I don’t understand how the math represents the given conditions. The oracle is supposed ...
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### Open Source Quantum Algorithm Development projects

Is there a platform where people contribute together to solving tasks in the area of quantum algorithm development?
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1 vote
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### How write a marking oracle if greater than with Q#?

I'm trying to write a code to find a minimum in a list. I follow this paper : A quantum algorithm for finding the minimum. So I need to write an operator to mark the case $T [j] < T[y]$. My idea is ...
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### Why is "reducing Hamiltonian energy" also optimizing a Quantum Machine Learning model?

From what I observed, most hybrid qml architectures surround the ideas of Hamiltonian states, and it seems like our goal to optimize a circuit is to keep energy states as low as possible. But why is ...
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### quantum algorithm for multilevel/hierarchical dataset

The Radon dataset is a well-known hierarchical/multilevel dataset. It contains Radon samples from houses in counties across the United States. The goal of the model is to estimate the (log) Radon ...
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### Confusion about simulation of projective measurements

The authors present an algorithm to obtain a state that is proportional to the solution of a system of linear equations. The algorithm works by evolving with a hamiltonian $H(s+ \delta)$ on the ground ...
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### What is the intuition behind uniform Hadamard superposition?

I noticed that in many quantum algorithms, you apply a layer of uniform Hadamard transformation before and after the operations in between. For example, Deutsch-Josza, Simon's, and Grover's search. ...
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1 vote
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### Preparation of Grover’s algorithm: How to implement the oracle $U_f$?

In order to understand Grover’s algorithm, we need to be able to implement $U_f$ from a black box implementing $f$, which we know how to do. Suppose you have this black box as we can find in some ...
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### Does qiskit make any rounding errors or anything similar, or is it a perfect (but slow) simulation of a quantum computer?

I ran the following circuit to calculate the real part of the Bargmann Invariant of a system of quantum states. The invariant is defined as ...
1 vote
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### qiskit statevector ordering issue

I am currently studying Qiskit. I don't understand the two pictures below while studying. When I apply circuit.x(1), the X gate should be applied to the second qubit, resulting in the state |01>. ...
117 views

### Imaginary bases lead to complex entropy. Is complex entropy useful in quantum computing? [closed]

I'm working through the Programming Drill 10.2.1 from Quantum Computing for Computer Scientists. I am not a student; this is not homework. I'm just doing this for my own betterment. I notice that ...
26 views

### 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: $BE_{\rho} = \begin{bmatrix} \rho & \cdot \\ \cdot & \cdot \end{bmatrix}$. ...
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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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1 vote
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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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### 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....
108 views

### 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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1 vote
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### Intuition for failure of strong Fourier sampling for the symmetric group

I am trying to read and understand the following two papers: The Symmetric Group Defies Strong Fourier Sampling: Part I The Symmetric Group Defies Strong Fourier Sampling: Part II I have a pretty good ...
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### Is it possible to simulate a quantum algorithm on a classical computer? [duplicate]

I am designing a quantum algorithm that I would like to simulate and test, but I need to be able to simulate it on a classical computer.
1 vote
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### A Decision Algorithm That Maybe Related to Quantum Search

As is well known Grover's algorithm can invert a function on a domain of 2^n possible values in other words it can find an x such that f(x)=1 if such an x exists for the oracle f(x). Suppose we are ...
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### gate actions in classical (not in the physics sense) superdense coding narrative

I am currently reading through superdense coding and the setup is as follows: Both Alice and Bob shares a fixed entangled state $|\psi\rangle = \frac{1}{\sqrt{2}}[|00\rangle + |11\rangle]$ with the ...
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### Is there a quantum algorithm for this?

While a well-established quantum algorithm exists for finding an estimate of the phase of the eigenvalue of a unitary matrix, the following question has a bit of a different requirement. Suppose we ...
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