# Questions tagged [qiskit]

Qiskit is an open source SDK for working with quantum computers at the level of pulses, circuits and application modules. 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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### Evaluating expectation values of operators in Qiskit

I'm wondering how in Qiskit one can calculate the expectation value of an operator given as a WeightedPauli (or, at least, of a single ...
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### Big Endian vs. Little Endian in Qiskit

I've noticed that Q# favors Little Endian. Meaning that most operations are designed for this type of encoding. Is is it the same with Qiskit?
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### How is the Grover-Algorithm applied to a database?

Question I want to use the Grover-Algorithm to search an unsorted database for an element $x$. Now the question arises, how do I initialize index and value of the database with the qubits? Example ...
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### Can I find the axis of rotation for any single-qubit gate?

Suppose I have an arbitrary qiskit $U_3$ gate: $U_3(\theta,\phi,\lambda)$. Is there a way I can find which axis the gate is rotating around? In other words, given any real numbers $\theta,\phi,\lambda$...
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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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### Creating a Qiskit Circuit sending $|00\rangle$ to $|1,-\rangle$ and $|11\rangle$ to $|0,-\rangle$

I am trying to create a circuit in Qiskit that performs the following transformations: starting in state |00⟩ generates a √(2)/2 * (-|10⟩+|11⟩) state starting in state |11⟩ generates a √(2)/2 * (|00⟩-|...
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### Conjugate transpose of a U-gate

I want to be able to create the circuit depicted below, but running the code below results in an empty circuit when viewing the job afterwards. ...
1 vote
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### A problem with application of multi controlled rotation gates [closed]

This message pops up when I run an mcrx gate The mcrx gate needs a single qubit as target. Here is a part of code I run: ...
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### What does it mean to get 403 error when running a code on Qiskit? [closed]

I have been using Qiskit over the last few weeks to run different programs. But right now, whenever I try to run any code, it gives me a 403 error stating that I am out of credits. I am creating a ...
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### 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 ...
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### How to quickly calculate the custom U3 gate parameters $\theta, \phi$ and $\lambda$ for any unitary?

In IBM Qiskit and Quantum Experience, the custom U3 gate is defined as  U(\theta, \phi, \lambda) = \begin{pmatrix} \cos\left(\frac{\theta}{2}\right) & -e^{...
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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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### Qiskit - Z expectation value from counts?

For a given state $|\psi\rangle$, how would I work out $\langle\psi|Z|\psi\rangle$ ? If I run a quantum circuit and get the counts dictionary on qiskit, I get observables in the Z basis. For n=1 ...
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### Access results from an experiment without having to run the script again

I have ran an experiment in one of the IBM processors and the work has finished. However, I can not obtain the data as histograms or counts in the qiskit notebook because the experiment was concluded ...
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### How to create the logical $|0_L\rangle$ state for the Steane's 7-qubit code?

I don't know how to prepare using Qiskit the following state in order to implement a Steane's 7-qubit code circuit (I omit the normalization factor): \begin{align*} |0_L\rangle =&|0000000\rangle+|...
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### Question About How Qiskit Reset Gate Affects Other Entangled Qubits

I am trying to understand how the reset gate in Qiskit affects qubits its entangled with. Consider the following circuit with qubits $q_0$ and $q_1$: Where circuit240 takes $|0\rangle$ to \$a|0\rangle ...
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### How can I load a probability distribution using a quantum circuit in Qiskit?

Can someone suggest me a way to load a distribution (for example a discretized Gaussian distribution) into a quantum computer using a quantum circuit? I tried to implement the code using Qiskit.
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### What's meant by the depth of a quantum circuit?

I'm using Qiskit to create quantum circuits wherein I've this attribute called depth. The documentation states it is the length of the critical path here. What does ...
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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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### What are some current applications of Quantum Computing in drug discovery? Are there any test examples of this?

I am interested in applying the power of Quantum Computing to drug discovery. Although I realize that quantum computing is limited in regards to modeling drug-like compounds and their interactions ...
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