Skip to main content
Share Your Experience: Take the 2024 Developer Survey
5 votes
Accepted

How to understand the idea of maximal entanglement intuitively?

Disclaimer: this is intuitive explanation, not necessary mathematically rigorous. But the question is about intuition behind. Yes, your ideda is right. For example $n$-qubit GHZ state, i.e. $\frac{1}{\...
Martin Vesely's user avatar
4 votes

How to mathematically describe the action of CNOT on the control qubit alone?

The action $\mathcal{E}$ of CNOT on just the control qubit is the composition of two functions: the action \begin{align} \mathcal{C}(\rho)=\text{CNOT}\,\rho\,\text{CNOT}\tag1 \end{align} of CNOT on ...
Adam Zalcman's user avatar
  • 22.9k
3 votes
Accepted

Can Bell states be reexpressed with respect to arbitrary local bases?

A maximally entangled state like $|00\rangle+|11\rangle$ can be generally written as $$\frac{|00\rangle+|11\rangle}{\sqrt2} = |u_0, \bar u_0\rangle+|u_1,\bar u_1\rangle$$ for any pair of orthonormal ...
glS's user avatar
  • 25.3k
2 votes
Accepted

Efficient Clifford simulation and entropy of reduced density matrices

TL;DR: Yes, we can efficiently compute von Neumann entropy $S(\rho)$ of a $k$-qubit mixed state $\rho$ obtained by tracing out $n-k$ qubits from a pure $n$-qubit stabilizer state $|\psi\rangle$ using \...
Adam Zalcman's user avatar
  • 22.9k
2 votes

Benefits of using quantum encrypted keys

A classical asymetric cipher is based on computationaly hard problems (assumed to be as so far we have not found an efficient algorithm). This means that classical computer is believed to be incapable ...
Martin Vesely's user avatar
2 votes
Accepted

doesn't a reset on an entangled qubit collapse a superposition?

In IBM quantum devices, reset instruction is implemented as a mid-circuit measurement followed by a conditional Pauli-X (see here). That means, your circuit is equivalent to the below circuit: This ...
Egretta.Thula's user avatar
2 votes
Accepted

Entanglement-based attack on E91 protocol

It doesn't matter which maximally entangled two-qubit state Alice and Bob share. They're all the same up to the action of a unitary on one of the qubits, which is just the same as changing the ...
DaftWullie's user avatar
  • 58.7k
2 votes

Can Bell states be reexpressed with respect to arbitrary local bases?

For your first question: If you pick a change of basis $U$ such that $|b_0\rangle = U|0\rangle$ and $|b_1\rangle = U|1\rangle$, then you are asking for conditions such that $$ (U\otimes U)|\Phi^+\...
forky40's user avatar
  • 7,103
2 votes

Do we consider the entangled-based protocol E91 to be a device-independent protocol?

Ekert's original protocol was never specified in a device independent way but, as you suggest, it's very close by virtue of the reliance on Bell inequalities. (He has commented to me that, with ...
DaftWullie's user avatar
  • 58.7k
1 vote

Do we consider the entangled-based protocol E91 to be a device-independent protocol?

I think you can safely say that E91 was the beginning of device-independent protocols. It has all the basic ideas in there and the intuition for why device-independent security holds is there too. ...
Rammus's user avatar
  • 5,853
1 vote

How to calculate the number of m-partitions of an N-partite quantum state?

This is a purely combinatorics problem. Let us index our registers by $1,\cdots,N$. We have $m$ "bags" $P_1,\cdots,P_m$ into which which we want to put our indexes. For the first index ($1$)...
Tristan Nemoz's user avatar
  • 6,462
1 vote

How to mathematically describe the action of CNOT on the control qubit alone?

In the ZX calculus, the CNOT gate factors into a Z type node for the control linked to an X type node for the target. The Z type node (the "control part of the operation") has three ports: $...
Craig Gidney's user avatar
  • 37.6k
1 vote

In quantum teleportation, why does measuring entanglement not destroy the whole state?

Good question, to answer it you can (and should) of course describe the system mathematically, but the problem is that the description never specifies what "classical transport" exactly does....
Jos Bergervoet's user avatar
1 vote

In quantum teleportation, why does measuring entanglement not destroy the whole state?

The measurement doesn't "destroy the whole quantum system", because you're not measuring the whole quantum system. You're only measuring two of the three systems. It's like saying: if you ...
glS's user avatar
  • 25.3k
1 vote

E91-Protocole the type of Bell state use in this protocol

In the Ekert-91 quantum key distribution (QKD) protocol, Bell states are used to create shared entanglement between Alice and Bob. There are four Bell states, two of which are correlated, and two are ...
Shravan Patel's user avatar
1 vote

How to analyse a quantum circuit with an uncertain gate F?

Disclaimer: analysis below works only for gates $F$ changing phase only. Once absolute values of amplitudes are changed, other results than predicted below are returned. Lets analyze your circuit step ...
Martin Vesely's user avatar

Only top scored, non community-wiki answers of a minimum length are eligible