Quantum Computing Stack Exchange Community Digest

Top new questions this week:

Preparing any superposition of fixed Hamming weight states

There exists a nice way of preparing any superposition (with real amplitudes — this is the case I'm interested in) of states $\{\ldots0001\rangle,\,|\ldots0010\rangle,\,|\ldots0100\rangle,\ldots\}$, ...

circuit-construction simulation  
asked by mavzolej 7 votes
answered by DaftWullie 2 votes

What to do when the amount of solutions is not known before applying Grovers Algorithm?

When running Grovers Algorithm one has to know how many solutions there are right? When the number of solutions are not known is then what do you do then?

algorithm grovers-algorithm  
asked by shxtxn 4 votes
answered by C. Kang 3 votes

Can anyone provide me with an example of the for Grover's algorithm on one qubit?

I'm having trouble understanding Grover's Algorithm, so I'd like to start with the case with 1 qubit. But I don't see anyone build it for 1 qubit, only 2 is the minimum. Is it because that's ...

algorithm circuit-construction grovers-algorithm  
asked by Kim Dong 4 votes
answered by Mariia Mykhailova 8 votes

What is the probability $\Pr(||U-I||_{op}<\varepsilon)$ of a Haar-random unitary being close to the identity?

If one generates an $n\times n$ Haar random unitary $U$, then clearly $\Pr(U=I)=0$. However, for every $\epsilon>0$, the probability $$\Pr(||U-I||_{op}<\varepsilon)$$ should be positive. How can ...

mathematics unitarity  
asked by Calvin Liu 4 votes

Defining a custom parametric gate in Qiskit in terms of standard gates?

Here it is explained how to define a custom non-parametric gate in Qiskit. How do I define a new parametric gate? For example, I want to have the $CCR_y$ gate defined as in order to use it as ...

qiskit programming circuit-construction  
asked by mavzolej 4 votes
answered by Matthew Stypulkoski 1 vote

Qiskit Portfolio Optimization Application

I recently got flung into the world of quantum computing and I'm a beginner at coding. I was assigned to do the Portfolio Optimization tutorial of the Qiskit Finance Tutorials and input real data. ...

qiskit programming optimization quantum-computing-for-finance  
asked by Lana 4 votes
answered by Lana 2 votes

What are the differences between the IBM machines?

I'm quite new to this field, and have started sending jobs to IBM's quantum computers. I have access to around 11 locations. I can see that these have different numbers of qubits within them, and then ...

ibm-q-experience machine-learning ibm pennylane  
asked by Andrew 3 votes
answered by Matthew Stypulkoski 2 votes

Greatest hits from previous weeks:

Toffoli gate as FANOUT

I was searching for examples of quantum circuits to exercise with Q# programming and I stumbled on this circuit: From: Examples of Quantum Circuit Diagrams - Michal Charemza During my ...

quantum-gate quirk cloning  
asked by D-Brc 16 votes
answered by kludg 10 votes

What countries are leading this "Global Quantum Computing Race"?

The terms Quantum Computing Race and Global Quantum Computing Race have been used in the press and research communities lately in an effort to describe countries making investments into a "battle" to ...

physical-realization quantum-advantage  
asked by Vikram Palakurthi 10 votes
answered by whurley 15 votes

Introductory material for quantum machine learning

In the past few days, I have been trying to collect material (mostly research papers) related to Quantum machine learning and its applications, for a summer project. Here are a few which I found ...

algorithm resource-request machine-learning  
asked by Sanchayan Dutta 27 votes
answered by user1271772 18 votes

What's the difference between a pure and mixed quantum state?

As per my limited understanding, a pure state is the quantum state where we have exact information about the quantum system. And the mixed state is the combination of probabilities of the information ...

asked by Koder101 14 votes
answered by Rob 13 votes

What does it mean for two qubits to be entangled?

I have done some sort of online research on qubits and the factors making them infamous i.e allowing qubits to hold 1 and 0 at the same time and another is that qubits can be entangled somehow so that ...

physical-qubit entanglement  
asked by Arshdeep Singh 15 votes
answered by snulty 17 votes

What is the opposite of measurement, in a quantum circuit?

My understanding is that at the level of quantum mechanics, almost all operations are reversible in time. Most gates in a quantum circuit clearly obey this rule; they can be reversed by applying some ...

quantum-gate measurement  
asked by lacker 11 votes
answered by Craig Gidney 12 votes

How to calculate circuit depth properly?

Is the circuit depth the longest sequence of gates applied on one of the qubits? Or is it something more complicated?

asked by C-Roux 8 votes
answered by Niel de Beaudrap 14 votes

Can you answer these questions?

Updating qiskit doesn't update qiskit version using jupyter notebook/terminal

I installed qiskit using qiskit documents from here https://qiskit.org/documentation/install.html. When I first installed qiskit the version was 0.12.xx on ubuntu with python installed. Everything was ...

qiskit programming  
asked by Hassan Afridi 1 vote
answered by e-eight 0 votes

Can a Qubit belong to 2 different registers?

I'm starting with Qiskit in Python. My question is regarding to have registers and sub-register. Something like this axreg=QuantumRegister(4,name='ax') ahreg=QuantumRegister(axreg[:2],name='ah') ...

qiskit programming  
asked by Sourcerer 2 votes
answered by Heriotic 0 votes

Qiskit QSVM - Alphas and Support Vectors

I just started using Qiskit and I have implemented the QSVM example. However, I having trouble with the return. The qsvm.predict() results are reasonable but when trying to calculate the margin ...

qiskit programming quantum-enhanced-machine-learning  
asked by Matthias 1 vote
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