Questions tagged [entanglement]

For questions about the principle and application of quantum entanglement. It is a physical phenomenon which occurs when pairs or groups of particles are generated, interact, or share spatial proximity in ways such that the quantum state of each particle cannot be described independently of the state of the other(s), even when the particles are separated by a large distance—instead, a quantum state must be described for the system as a whole. (Wikipedia)

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Why a basis in super dense quantum coding?

In the classical super dense quantum coding scheme of Bennet and Wiesner (1992), the authors start from an element in the Bell base (which consists of $4 = 2^2 = 2 \times 2$ states, in dimension $4$), ...
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Tests of entanglement between one and many qubits

I have a 5-qubit state $|\psi \rangle$, which has a physical interpretation that the middle qubit is the "impurity" with spin $s_{imp} = 1/2$. The rest 4 qubits highlight the presence of ...
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CHSH game sharing more general states

In the CHSH game, when the players share EPR states they can achieve best the winning rate of approximately 85% which can be proved through the Tsirelson's bound. I wonder what the best winning rate ...
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Is purity convex in time?

Is the purity $\mathrm{Tr}[\rho(t)^2]$ of a quantum state $\rho(t)$ that evolves according to a time-independent Lindbladian $\partial_t \rho = \mathcal{L}[\rho]$ convex in time $t$? I suspect the the ...
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What is the stabilizer rank of the W state?

The $n$ qubit $W$ state is defined here https://en.wikipedia.org/wiki/W_state The stabilizer rank of a quantum state $|\psi\rangle$ is the minimal $r$ such that |{\psi}\rangle = \...
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Why do we need to put the control qubit in superposition in Bell State?

I'm new to quantum computing, so I hope you don't find this question tedious. However, after watching many hours of lectures and reading online, I still don't 100% understand why we need to initialize ...
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Do we consider the entangled-based protocol E91 to be a device-independent protocol?

Since both rely on Bell inequalities to maintain security, does the Ekert protocol rely on trustworthiness of the devices?
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How to calculate the number of m-partitions of an N-partite quantum state?

I am reading the structure of multipartite entanglement from section 3.3. It is stated there that the number of possible partitions of $N$ parties into $m$ parts is given by $\frac{m^N}{m!}$. I could ...
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How do two-way protocols allow to extract entanglement from states like Werner's?

As discussed in [Lo and Popescu 1999], it is always possible to distill entanglement from pure bipartite states using a one-way protocol. The gist of the idea, as I understand it, is Proposition 1 in ...
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Can Bell states be reexpressed with respect to arbitrary local bases?

Suppose I take the Bell state $\Phi^+ = \vert 00\rangle + \vert 11\rangle$. Is it true that I can choose any basis with basis vectors $\vert b_0\rangle$ and $\vert b_1\rangle$ and reexpress the same ...
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Synthesizing quantum circuits from unitary matrices vs quantum state vectors, and determining qubit entanglement

I'm studying how to build quantum circuits and understand qubit entanglement. The following are my two confusing questions. I would greatly appreciate any assistance or insights regarding these ...
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Does entanglement vanishes when an electron runs from the s orbital?

In the s orbital of a helium atom there are two entangled electrons. They are entangled because they have different spins. Suppose we give energy to one electron and let it runs from the atom. Does ...
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On difference in the number of two-qubit stabilizer states that are separable (36) vs those that are maximally entangled (24) and partial entanglement

We have a set of two-qubit stabilizer states. There are 60 of them: 36 separable and 24 maximally entangled (MES). I was wondering whether we can somehow compare the size of the set of partially ...
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Is there a proof that any pure two-qubit Partially entangled state lies somewhere in between a separable and maximally entangled state?

Intuitively this seems true, but is there a proof of the following: We have two-qubit pure state. Given a Partially entangled state (PES) $|P\rangle$ we can always find a separable state $|S\rangle$, ...
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What is the relation between the purity of a bipartite state and its subsystems?

I know for a separable state $\rho_{AB}$ we have $$P_{AB}= P_{A}=P_{B}=1$$ Where P is purity. What if the relation between the purities when the bipartite state is entangled? Is there any inequality ...
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What does one-tangle says in an n-qubit system?

One-tangle is negativity with respect to qubit x. I know in a 3 qubit system if one-tangle with respect to qubit x is 0, then qubit x is separatable. Is this statement generalized for n-qubit systems? ...
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