# Questions tagged [qiskit]

Quantum Software Development Kit for writing quantum computing experiments, programs, and applications. If appropriate, also use the [programming] tag.

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### How can I implement an n-bit Toffoli gate?

I want to create a Toffoli gate controlled by n qubits, and implement it in QISKit. Can this be done? If so, how?
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### What is a “barrier” in Qiskit circuits?

I just started studying IBM Qiskit, and cannot find details about the barrier method on the QuantumCircuit class. It is shown in the circuit drawing, but I never heard about it after reading quantum ...
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### Does conditional gate collapse controller's superposition?

I've created a simple circuit in Q-Kit to understand conditional gates and outputted states on each step: In the beginning there is clear 00 state, which is the input The first qubit is passed ...
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### Checking value of variable using quantum approach

A beginner question after watching few videos. Say, var=a; var can be either of two values, ...
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### How good is basic_device_noise_model() simulating the noise in the quantum computer?

Is there any paper or article about the performance of the noise model using basic_device_noise_model()? For example, like the noise model in the code below. ...
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### Submitting jobs to IBM devices without Python

Qiskit can be used to submit jobs to IBM devices using Python (or Swift and JavaScript). But what if I want to use a different language? For example, suppose I have a specific circuit I want to run. ...
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### Quantum algorithm for linear systems of equations (HHL09): Step 2 - Preparation of the initial states $|\Psi_0\rangle$ and $|b\rangle$

This is a continuation of Quantum algorithm for linear systems of equations (HHL09): Step 2 - What is $|\Psi_0\rangle$? In the paper: Quantum algorithm for linear systems of equations (Harrow, ...
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### Construct Controlled-$G^{\dagger}$ from known Controlled-$G$

Let there be a known a scheme (quantum circuit) of Controlled-G, where unitary gate G has G$^†$ such that G≠G$^†$ and GG$^†$=I (for example S and S$^†$, T and T$^†$, V and V$^†$, but not Pauli and H ...
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### Why do we need a Classical Register for carrying out Quantum Computations?

I've just started to mess about with QISKit on Python and one thing is confusing me a fair bit. Given that we are building Quantum Circuits what is the need for a classical register ? Is it because ...
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### How would one implement a quantum equivalent of a while loop in IBM QISkit?

I'm writing a simple multiplication algorithm that uses the Quantum Fourier Transform to repetitively add a number (the multiplicand) to itself and decrements another number (the multiplier). The ...
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### How to implement the CCH gate in quantum computers available in clouds?

How to implement CCH gate in quantum computers available in clouds? If there is not any gate directly available for it, what are the possible ways to represent CCH?
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### Get result measurement into a circuit with Qiskit

I'm trying to implement the three qubit bit flip code in Qiskit. Therefore, I need to get the result of a measure of two ancilla qubits, to deduce which gate I need to use do recover my logical qubit....
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### Are $\mathrm{cRz}$ with angle $\pi$ and $\mathrm{cZ}$ equal on the IBM Quantum Experience?

I was trying to implement the 2-qubit Grover algorithm without the auxiliary bit. Before finding the gate symbol for the controlled $\mathrm{Z}$, I worked with the controlled $\mathrm{Rz}$. The gate ...
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### How is it possible to perform a c-not on 4 qubits using the same control qubits on IBM Q computers?

I was wondering after seeing ibmq_16_melbourne layout how it is possible to perform this circuit : It seems when looking at ibm q computers that a qubits can at max target 3 qubits because the arrow ...
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### $\lambda$ parameter for U3 gate in qiskit Bloch sphere visualisation

It is easy to see how $\theta$ (rotation from the positive z-axis) and $\varphi$ (rotation from the positive z-axis) affect the initial state of the qubit when looking at the Bloch sphere but I have ...
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### What is the convention of indices for the one and two-body integrals in qiskit

I was able to calculate the one and two-body integrals for the H2 with the following code ...
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### Visualization of Quantum Circuits when using IBM QISKit

I have followed the installation steps, regarding QISKit working environment. For circuit visualization, I've installed latex as in addition to poppler to convert from PDF to images. Afterwards, I ...
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### What is the purpose of the TDG gate in QISKit?

The QISKIT documentation doesn't explain what a TDG gate does and I can't find it anywhere else online.
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### Getting a layout for qubits in Qiskit

I'm trying to use Qiskit's built in transpiler passes to do the following. I would like to take a quantum circuit and allocate the logical qubits to physical qubits. The tutorial doesn't really ...
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### Making custom gate in Qiskit?

I have been trying to make a gate in qiskit in terms of the basic gates, but I keep on getting an error when I apply it to a circuit. This is my gate: ...
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### Qiskit in Julia language?

Is it possible to use Qiskit in Julia language? Or is there any wrapper that can be used to call the package from Julia language?
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### Why won't Qiskit ccx gate accept registers?

In Qiskit, they have the concept of a Toffoli gate (ccx). However, the following code throws an error: ...
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### How can we construct a square root of NOT gate in Qiskit and IBMQ circuit composer using universal gates?

I have tried it with decomposing controlled S then conjugating with H gate. But I want to construct it using a minimum number of gates.
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### Implementing a complex circuit for a Szegedy quantum walk in qiskit

Problem definition I'm implementing a quantum circuit in qiskit for a Szegedy quantum walk, (reference, Fig 21.). It uses two registers of dimension $N$ ($N=3$) each one. The challenges I'm facing are:...