## Top new questions this week:

### Eavesdropping in case of the BB84 Protocol

I try to understand eavesdropping in case of the BB84 protocol (lets assume we have single photons, no noise etc.). Alice and Bob generate a classical random bit $a_i'$ and $b_i'$. Alice generates an ...

bb84
 asked by choc1709 Score of 4
 answered by DaftWullie Score of 4

### What is a diference between solitons and anyons?

In the article Creation and annihilation of mobile fractional solitons in atomic chains the authors claim that they prepared 1D solitons which can be used in topological quantum computing. Based on ...

topological-quantum-computing anyons
 asked by Martin Vesely Score of 4

### Does the Lie closure of a set of Hamiltonians describe all unitaries you can generate with them?

Suppose we have $k$ Hermitian matrices $H_1, ..., H_k \in M_d(\mathbb{C})$. Define $\mathfrak{g} = \langle iH_1, ... iH_k \rangle_{\text{Lie}}$ to be the Lie algebra generated by these Hermitian ...

hamiltonian-simulation linear-algebra qaoa quantum-control
 asked by Alexey Uvarov Score of 3

### Can a polynomial-sized superposition of computational basis states be prepared with a polynomial-sized quantum circuit?

Suppose I am working with a class of states which consist of a superposition of $O(\text{poly}(N))$ computational basis states on $N$ qubits. Examples of this would be the subspace of states of fixed ...

quantum-gate quantum-state circuit-construction complexity-theory
 asked by Solarflare0 Score of 3
 answered by Sam Jaques Score of 0

### What are quantum computers actually doing today?

I am curious about what are quantum computers actually doing today. I seem to only be able to find information online about what they could be used for in the future (i.e. breaking encryption, quantum ...

 asked by Horus Score of 2
 answered by Cody Wang Score of 4

### Distinguishing $n$ pure states in an $n$ dimensional Hilbert space

Suppose we have $n$ pure states in an $n$ dimensional Hilbert space, and we would like to distinguish them using POVM or PVM. We get any one of the pure states with equal probability, and we may set ...

measurement probability projection-operator povm state-discrimination
 asked by Stan Score of 2

### How to implement subplots (several blochsphere plots) using qiskit?

Qiskit seems to use matplotlib for rendering bloch spheres under the hood. Therefore, it would be nice if we could also make use of matplotlib's subplot technique. I would like to implement subplots, ...

qiskit programming bloch-sphere matrix-representation
 asked by Eldar Sultanow Score of 2
 answered by Egretta.Thula Score of 4

## Greatest hits from previous weeks:

### What do the off-diagonal elements of a density matrix physically represent?

For simplicity, let's take a density matrix for a single qubit, written in the $\{|0\rangle,|1\rangle\}$ basis:  \rho = \begin{pmatrix} \rho_{00} & \rho_{01} \\ \rho_{10}^* & 1-\rho_{00} \...

quantum-state measurement density-matrix
 asked by KnightShuffler Score of 9
 answered by Carlos_San Score of 9

### Is quantum computing just pie in the sky?

I have a computer science degree. I work in IT, and have done so for many years. In that period "classical" computers have advanced by leaps and bounds. I now have a terabyte disk drive in my bedroom ...

classical-computing applications history
 asked by John Duffield Score of 174
 answered by user1271772 Score of 44

### Quantum states are unit vectors... with respect to which norm?

The most general definition of a quantum state I found is (rephrasing the definition from Wikipedia) Quantum states are represented by a ray in a finite- or infinite-dimensional Hilbert space over ...

quantum-state notation unitarity mathematics
 answered by user1271772 Score of 6

### Why can't quantum computation replace classical computation?

I am not a total novice of quantum computation (have read the first 6 chapters of Nielsen and Chuang, though not familiar with every part), but there are some fundamental questions that I don't know ...

classical-computing performance
 asked by Liren Lin Score of 8
 answered by epelaaez Score of 7

### 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 ...

qiskit programming
 asked by mavzolej Score of 12
 answered by Cryoris Score of 14

### Is there a layman's explanation for why Grover's algorithm works?

This blogpost by Scott Aaronson is a very useful and simple explanation of Shor's algorithm. I'm wondering if there is such an explanation for the second most famous quantum algorithm: Grover's ...

algorithm complexity-theory grovers-algorithm
 asked by Discrete lizard Score of 37
 answered by James Wootton Score of 30

### Why can't we simulate a Qubit using classical computer?

I am completely a noob in terms of quantum computing, have watched several videos to understand what Quantum computers are trying to achieve. I am a programmer of classical computers. We have a ...

quantum-state physical-qubit
 asked by Anurag Vohra Score of 4
 answered by totikom Score of 1

## Can you answer these questions?

### Why Quantum Computing can help in Portfolio Optimization?

Why is it said that Portfolio Optimization is a good case for quantum computing? Is it only speed? IF speed is the only benefit, why cant we build more powerful supercompuetrs?

algorithm quantum-computing-for-finance
 asked by user13034532 Score of 1
 answered by Martin Vesely Score of 0

### How to deterministically add two integers on a quantum computer

I am new to the Quantum Computer world. I understand the basics of Quantum mechanics and I have watched a few lectures on Quantum computers, Quantum gates and played a bit on IBM QC. As far as I ...

quantum-gate algorithm ibm-q-experience
 asked by Omar Elkhouly Score of 1
 answered by poig Score of 0

### The world's first quantum integrated circuit at the atomic scale - How does this advance quantum computing as a whole?

I have a hobbyist/enthusiast understanding of Quantum Computing, having read a few books on the subject. I came across this new article and referenced Nature paper. I know a goal of quantum computing ...

physical-realization architecture
 asked by user2521987 Score of 1
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