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.
1,158
questions
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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?
21
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6answers
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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}{...
16
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1answer
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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 ...
13
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3answers
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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
...
11
votes
1answer
431 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. ...
11
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2answers
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Composing multiple quantum circuits in single quantum program in QISKit
I was wondering if there is a way to compose a program with multiple quantum circuits without having the register reinitialized at $0$ for each circuit.
Specifically, I would like run a second ...
9
votes
5answers
953 views
If quantum computing always return random measurement (or uncertain measurement), why do we still need it?
I am very new to quantum computing and currently studying quantum computing on my own through various resources (Youtube Qiskit, Qiskit website, book).
As my mindset is still "locked" with ...
9
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1answer
2k views
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 ...
9
votes
2answers
606 views
How to program a controlled Hadamard-Hadamard gate?
I'm trying to program a controlled gate as the figure below in Qiskit. Should it be sufficient to separate and control individually the Hadamard gates?
9
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1answer
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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$$
9
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1answer
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Evaluating expectation values of operators in Qiskit
I'm wondering how in Qiskit one can calculate the expectation value of an operator given as a WeightedPauli (or, at least, of a single ...
9
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3answers
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How to quickly calculate the custom U3 gate parameters $\theta, \phi$ and $\lambda$ for any unitary?
In IBM Qiskit and Quantum Experience, the custom U3 gate is defined as
$$
U(\theta, \phi, \lambda) =
\begin{pmatrix}
\cos\left(\frac{\theta}{2}\right) & -e^{...
9
votes
3answers
575 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 ...
9
votes
2answers
215 views
Quantum implementation of arcsin
I am looking to implement a quantum version of the arcsinus function. Such a problem is motivated by the HHL algorithm where $x\mapsto 1/x$ and $\arcsin$ can be used to get $1/x$ from the ...
9
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2answers
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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, ...
8
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2answers
685 views
Qubits specification on IBMQ devices
As it is shown here, CNOT gates between different qubits have different error rates. I have the following questions:
1) While defining a circuit on QISkit, does ...
8
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3answers
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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 ...
8
votes
2answers
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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 ...
8
votes
1answer
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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 ...
8
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2answers
691 views
There was an error in the circuit!. Error = 'QISkit Time Out'
I've got the following quantum code using QISKit (based on hello_quantum.py):
...
8
votes
1answer
657 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 ...
8
votes
2answers
468 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 ...
8
votes
1answer
855 views
How to delete pending jobs on IBM Quantum Computer to retrieve units?
I am trying to run some code using qiskit, but I get the error message, that I have run out of necessary Experiment Units. I ...
8
votes
2answers
281 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
1answer
602 views
Changing indices of qubits in QISKit
I was wondering if something like this is possible in QISKit: let's say we have two registers containing target and ancilla qubits:
$a_0$ -------------------
$a_1$--------------------
$\vdots$
$...
8
votes
1answer
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Is it possible to run a general implementation Shor's algorithm on a real IBM quantum computer at least for N = 15?
I need to make a general implementation of Shor's algorithm that factors, at least, N = 15. I have been able to perform an implementation that works in simulators, with ProjectQ, but when running it ...
8
votes
1answer
425 views
How does measurement calibration work?
One of the possible ways to improve the results of an experiment on the IBM machine using the Qiskit language is to use the measurement calibration methods. This is the link to the documentation.
I ...
8
votes
1answer
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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 ...
8
votes
0answers
94 views
Can boson sampling be simulated in Qiskit?
In December 2020, there was this claim of quantum advantage/supremacy by a team of USTC using Gaussian boson sampling. Here is the paper and here is an explanatory news article in Nature. To get an ...
7
votes
3answers
800 views
Quantum circuit to implement matrix exponential
I want to build a circuit which will implement $e^{iAt}$, where $ A=
\begin{pmatrix}
1.5 & 0.5\\
0.5 & 1.5\\
\end{pmatrix}
$ and $t= \pi/2 $.
We see that $A$ can be written as, $A=1.5I+0.5X$. ...
7
votes
3answers
604 views
Qubit Connectivity of IBM Quantum Computer
When simulating a quantum circuit using the qiskit qasm_simulator, the qubits that you assign to the quantum circuit may be ...
7
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3answers
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How to run a qasm file on IBMQ device?
I can find many qasm examples.
How can I run them on different IBMQ devices?
7
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1answer
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Would IBM's "compiler" turn my identity circuit into nothing?
If I were to create a circuit with the following gate:
$$\tag{1}R_\phi = \begin{bmatrix} 1 & 0 \\ 0 & e^{i \phi} \end{bmatrix},$$
with $\phi$ specified to be equal to 0, then the gate that I ...
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 ...
7
votes
1answer
793 views
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.
7
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3answers
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7
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1answer
228 views
Status of software packages for quantum compiling
By "quantum compiling", what I mean is classical algorithms to solve the following problem: given a $SU(D)$ matrix $U$ (the goal) and a set of $SU(D)$ unitary matrices $V_1 \cdots V_N$ (the gates), ...
7
votes
1answer
2k 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 ...
7
votes
1answer
123 views
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 ...
7
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1answer
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How to calculate the fidelity of a certain gate of a IBMQ device in Qiskit using randomized benchmarking/tomography?
For example, I want to calculate the fidelity of a 1-qubit and 2-qubit gates (similar to the result shown in figure 2 in this paper). Is there any way to do that in Qiskit? I've gone through the ...
7
votes
1answer
140 views
Why does job.running in QISKit output False, even if the job is still running?
I submitted a job in the 0.5.0 version of QISKit using
job = execute(qc, 'ibmqx5', shots=shots)
This just submits the job, and does not wait for a result.
I ...
7
votes
1answer
469 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 ...
7
votes
1answer
438 views
One-qubit gate results in QISKit
I found it odd that the result of the action of identity gate (namely a $2\times2$ identity matrix) on a pure state $|0\rangle$ (namely the vector corresponding to the $2\times1$ matrix $\begin{...
6
votes
2answers
691 views
Is Qiskit more popular than cirq? Why?
It seems like most people are using Qiskit, what's the difference?
6
votes
3answers
670 views
XOR gate for n control qubits in qiskit
I am currently creating a blackbox for a grover search algorithm to solve the graph colouring problem with qiskit. One of my search criteria is that one node can only have one color. Therefore, with ...
6
votes
3answers
4k views
What does fidelity mean?
I am learning qiskit software and this term keeps popping up and I am unable to get a grasp on the technical definition given by wikipedia. For example, the functions state fidelity and process ...
6
votes
3answers
250 views
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 $\theta,\phi,\lambda$...
6
votes
3answers
832 views
What is the location of the Qiskit config file?
I'm trying to follow the instructions here https://qiskit.org/documentation/getting_started.html. It says
The default backend for qiskit.visualization.circuit_drawer() or
QuantumCircuit.draw() is ...
6
votes
2answers
273 views
Apply readout error mitigation to mid-circuit measurement
I'm trying to construct a quantum circuit with 3 mid-circuit measurements, here's an example:
...
6
votes
2answers
633 views
How to draw Qiskit's HHL algorithm as a circuit?
Qiskit Aqua has a module that implements the HHL algorithm:
https://qiskit.org/documentation/_modules/qiskit/aqua/algorithms/single_sample/hhl/hhl.html
How does one draw the implemented circuit?
* ...
