Questions tagged [qiskit]

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

Filter by
Sorted by
Tagged with
23
votes
3answers
6k views

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?
3
votes
2answers
632 views

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?
7
votes
1answer
4k views

How to create an arbitrary state in QISKit for a local_qasm_simulator?

I would like to play with a quantum circuit local_qasm_simulator in QISKit, but I do not want to implement a separate quantum circuit that would prepare an initial ...
3
votes
1answer
958 views

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 ...
1
vote
2answers
88 views

quantum random number generator implementation in quantum lab

How can I generate and print a 32 bit sequence of 0s and 1s in Qiskit or IBM Q experience quantum lab using quantum random number generator implementation?
8
votes
1answer
2k views

Phase-Shift Gate in Qiskit

How to implement the phase shift gate in qiskit or ibmq? Phase Shift Gate : $$\begin{pmatrix}e^{ia} && 0 \\ 0 && e^{ia}\end{pmatrix} = e^{ia}I$$
15
votes
6answers
5k views

How do I build a gate from a matrix on Qiskit?

I'm creating a gate for a project and need to test if it has the same results as the original circuit in a simulator, how do I build this gate on Qiskit? It's a 3 qubit gate, 8x8 matrix: $$ \frac{1}{...
7
votes
1answer
2k views

What are theta, phi and lambda in cu1(theta, ctl, tgt) and cu3(theta, phi, lam, ctl, tgt)? What are the rotation matrices being used?

I was reading the documentation for qiskit.QuantumCircuit and came across the functions cu1(theta, ctl, tgt) and ...
3
votes
2answers
602 views

Given a state $|\phi\rangle=\frac{1}{\sqrt{2}}(|0\rangle+e^{i\theta}|1\rangle)$, how do I know the angle $\theta$?

Question1. If there is a state $|\phi\rangle=\frac{1}{\sqrt{2}}(|0\rangle+e^{i\theta}|1\rangle)$, and I want to know the angle $\theta$. What kind of measurement should I do? Could somebody give me ...
0
votes
1answer
82 views

Error while trying to execute the Quantum Circuits in IBMQ

I have been trying to execute the quantum circuit in the backend of the IBM 16 qubit Melbourne machine. I end up getting ...
3
votes
2answers
234 views

In Grover's Algorithm, does the exact solution need to be given to the oracle?

From my understanding, the oracle function in Grover's algorithm is used as a way to check for the desired outcome. I have looked at this example which implements the Exactly-1 3-SAT problem and the ...
3
votes
2answers
736 views

How to decompose this two-qubit unitary matrix to the standard gate set?

I have read some of the other decomposition questions here however still don't understand how to do it manually. The matrix I wish to decompose is the following for 2 qubits: $M=\frac{1}{\sqrt2}\...
12
votes
3answers
1k views

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 ...
4
votes
1answer
248 views

What is the relationship between Choi and Chi matrix in Qiskit?

I'm struggling with the framework for quantum process tomography on Qiskit. The final step of such a framework is running fit method of ...
4
votes
1answer
78 views

Vary parameter between shots

Is it possible to tell Qiskit to execute a parameterized gate where the parameter varies between shots? Let's say, for whatever reason, I'd like to speed up the dephasing of a qubit during a run. I ...
4
votes
1answer
391 views

How to use external .csv data file in quantum support vector machine qiskit python code?

I am new to python programming and Qiskit. I would like to ask can we use sklearn for adding external .csv data file before Qiskit libraries, or without sklearn, we can do or is there any other Qiskit ...
4
votes
1answer
1k views

How to implement if statement based on measurement results in qiskit?

I tried to implement three qubit bit flip code in qiskit and need to get the result of measurements and then apply recovery quantum operations conditioned on the measurement results. The following is ...
7
votes
1answer
433 views

SWAP gate(s) in the $R(\lambda^{-1})$ step of the HHL circuit for $4\times 4$ systems

Context: On the 5th page of the paper Quantum circuit design for solving linear systems of equations (Cao et al, 2012) there's this circuit: Schematic: A brief schematic of what's actually ...
4
votes
1answer
518 views

How can noise on a device be simulated using measured noise parameters?

The real devices used as backends for Qiskit have calibration data that quantifies errors ($T_1$ and $T_2$, gate fidelities, etc). This can be accessed by, for example ...
2
votes
2answers
75 views

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. ...
2
votes
2answers
280 views

Mitigating the noise in a quantum circuit

I'm using Qiskit and I have a Quantum Circuit (say, circuit) that gives reasonable results when using the simulator, namely ...
1
vote
0answers
350 views

Decomposition of an arbitrary gate using given matrix in Qiskit

If I have an arbitrary non-unitary matrix of say $$ U = \begin{pmatrix} 1.5 & 0 & 0 & 0 \\ 0 & 0 & 0 & 1.6 \\ 0 & 0 & 1 & 0 \\ 0 & 1 & 0 & 0 \\ \end{...
11
votes
1answer
3k views

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 ...
8
votes
3answers
3k views

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^{...
6
votes
3answers
3k 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 ...
9
votes
3answers
532 views

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 ...
5
votes
2answers
377 views

Checking value of variable using quantum approach

A beginner question after watching few videos. Say, var=a; var can be either of two values, ...
4
votes
1answer
770 views

How to measure in another basis

I am new to qiskit and I have to simulate a quantum circuit. I read this documentation https://qiskit.org/textbook/ch-states/single-qubit-gates.html where it is left as an exercise to the reader to ...
4
votes
1answer
460 views

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. ...
10
votes
1answer
378 views

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. ...
9
votes
2answers
1k views

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, ...
8
votes
2answers
245 views

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 ...
8
votes
2answers
420 views

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 ...
6
votes
1answer
1k views

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 ...
5
votes
1answer
2k views

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....
5
votes
3answers
734 views

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 ...
5
votes
2answers
221 views

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?
5
votes
1answer
532 views

How to make circuit for n-control Z gate( i.e $C^3Z$ )?

I am trying to make circuit for $C^3Z$ gate I have seen a circuit for $C^2Z$ or $CCZ$ gate made by using $CCX$ gate so is there any way to make circuit for $C^3Z$ in this similar manner( i.e by using $...
4
votes
2answers
1k views

Comparing run times on IBM Quantum Experience

I have run this program - ...
4
votes
1answer
1k views

Can I remove gates from a QuantumCircuit?

Suppose I want to implement run several circuits one after another, but they are constructed in a similar fashion. I could reinstantiate a QuantumCircuit for each ...
4
votes
2answers
852 views

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 ...
4
votes
3answers
329 views

How to plot custom gate labels in Qiskit?

How can I create a custom gate in qiskit that is plotted using latex formatting (or another type of formatting)? For example the following code generates a circuit diagram ...
3
votes
3answers
489 views

How can I create an arbitrary superposition state of two qubits in Qiskit?

I am trying to produce any arbitrary superposition state of two qubits, for example $a|00\rangle + b|01\rangle + c|10\rangle + d|11\rangle$, such that $a^2 + b^2 + c^2 + d^2 = 1$ . How can I do that ...
3
votes
2answers
200 views

How to measure in the bell basis?

The question is quite straightforward, I have some two qubit state and I want to measure it in the Bell basis. This question answers me partly, but I still have some doubts because, I thought that ...
2
votes
1answer
57 views

Is it possible to implement c_if statement based on a measurement in a single specified classical bit in qiskit?

I'm trying to write a 1-bit teleportation error correction code and there is one part in it where I need to add a gate that's dependent on a measurement from earlier in the circuit. The measurement ...
2
votes
1answer
155 views

Different QFT results when using Simulator or Quantum Machine

I'm performing QFT using the following code: ...
2
votes
1answer
71 views

Gate corresponding to $-I$

I am implementing a quantum circuit in Qiskit. I create the equal superposition state $$ -|00\rangle + |01\rangle + |10\rangle +|11\rangle $$ but I want to obtain the quantum state $$ |00\rangle - |...
1
vote
1answer
272 views

Prepending initial state to a quantum circuit in Qiskit

I am trying to generate a Quantum Phase Estimation (QPE) circuit in QISKIT the following way. 1 - First, I generate a QPE circuit with the following code: ...
1
vote
1answer
407 views

How to run the Qiskit Aer simulator without noise?

I know that the Qasm Simulator execute quantum circuits with presence of noise, even if I do not specify the model of noise in options for that backend. But how can I execute the circuits without ...
1
vote
1answer
108 views

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 ...