Quantum Computing Stack Exchange Community Digest

Top new questions this week:

Why do I get this extra factor when working out the dynamics of an adiabatic quantum computation?

I was trying to revise my understanding of adiabatic quantum computation via a simple example. I'm familiar with the overall concept -- that you have an overall Hamiltonian $$ H(s)=(1-s)H_0+s H_f $$ ...

asked by DaftWullie 8 votes
answered by Jonathan Trousdale 1 vote

Characteristics of the IBM quantum computer

On the IBM Quantum Composer website, there are characteristics of qubit computers. For example, ibmq_16_melbourne. But there is no description anywhere of what: ...

quantum-gate ibm-q-experience error-correction ibm  
asked by alexhak 6 votes
answered by JSdJ 9 votes

Optical quantum computers VS Superconducting quantum computers VS Electron-spin quantum computers

One of the main problems of superconducting quantum computers is that it is necessary to maintain a very low temperature (near absolute zero). Such cooling is very expensive and complex. Why, then, ...

ibm superconducting-quantum-computing photonics spintronics  
asked by alexhak 5 votes

States used in lossless quantum compression?

I was reading about quantum compression in this article and have some doubts regarding an example mentioned. Specifically, I have two questions: In example they represented $|a\rangle = \dfrac{1}{\...

quantum-state information-theory communication  
asked by John Jones 4 votes

Degree of $N$-bit Majority function is larger or equal to $N/2$

I am looking to prove that the $N$-bit Majority function $f$, which is 1 if its input $x \in \{0, 1\}^N$ has Hamming weight $> N/2$, and 0 if its input has Hamming weight $\leq N/2$ has degree $\...

asked by Christiaan van Asperen 4 votes
answered by DaftWullie 2 votes

What is the actual precision of the IBM quantum computer?

In working with the IBM quantum computer, one can use for example the composer in order to build simple quantum algorithms. Essentially, one usually feeds a phase in order to affect one of the qubits. ...

asked by user2820579 3 votes

What is the difference between "Shot-Noise-Limit" and "Standard Quantum Limit"?

It seems that in a lot of papers in the field of quantum metrology, there are two terms Shot-Noise-Limit and Standard Quantum Limit which are frequently referred to. What's the difference between them,...

terminology quantum-fisher-information quantum-metrology  
asked by narip 3 votes

Greatest hits from previous weeks:

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 ...

programming qiskit measurement textbook-and-exercises vqe  
asked by C-Roux 6 votes
answered by Josh Izaac 5 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 16 votes
answered by snulty 17 votes

How do you write a simple program for a D-Wave device?

I would like to know how a job for a D-Wave device is written in code and submitted to the device. In the answer it would be best to see a specific example of this for a simple problem. I guess that ...

d-wave programming  
asked by James Wootton 32 votes
answered by Mark Fingerhuth 26 votes

Do there exist problems known to be computationally intractable for quantum computer, but tractable for classical computer?

Or alternatively phrased, is it believed that the complexity class P is a complete subset of BQP? Consider the following diagram à la MIT OpenCourseWare, which seems to explicitly state as much.

complexity-theory bqp  
asked by Eoin Dowling 3 votes
answered by Adam Zalcman 9 votes

Does Moore's law apply to quantum computing?

Plain and simple. Does Moore's law apply to quantum computing, or is it similar but with the numbers adjusted (ex. triples every 2 years). Also, if Moore's law doesn't apply, why do qubits change it?

classical-computing history scalability  
asked by Alex Jone 26 votes
answered by agaitaarino 23 votes

How do I show that a two-qubit state is an entangled state?

The Bell state $|\Phi^{+}\rangle = \frac{1}{\sqrt{2}}(|00\rangle + |11\rangle )$ is an entangled state. But why is that the case? How do I mathematically prove that?

entanglement tensor-product  
asked by nbro 29 votes
answered by nbro 30 votes

Is qsphere an actual term representing 5 qubits?

I'm a total beginner, I've been brought here by the featured stackoverflow blog post so I started studying. Watching this youtube video (A Beginner’s Guide To Quantum Computing (3:58), I saw this ...

asked by Adelin 10 votes

Can you answer these questions?

When writing data into qRAM, can I do it in a superposition state?

In general, quantum algorithms are said to be hybrid algorithms. Especially when storing data in qRAM, it seems to be done through classical calculations. Is it possible to write this part directly to ...

quantum-gate superposition quantum-memory  
asked by PioneerBaek 2 votes

How do you visualize multi-qubit interactions?

I am trying to understand single qubit operations from Bloch sphere, but I was told that the limitation of Bloch sphere is that it can only visualize or simulate 1 qubit. What are some instances do I ...

asked by Jazz Ang 1 vote
answered by user2820579 0 votes

What happens in the Cramer-Rao bound if the quantum Fisher information is zero?

The famous Cramer-Rao bound is $$\Delta\theta\ge\frac{1}{\sqrt {F[\rho,H]}}$$ But what happens if the denominator vanishes, i.e., $F[\rho,H]=0$ ($F[\rho,H]$ here stands for the quantum fisher ...

mathematics quantum-fisher-information quantum-metrology  
asked by narip 2 votes
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