# Questions tagged [quantum-entanglement]

Quantum entanglement is the mechanism by which quantum correlations between two sub-systems survive even after being physically separated from an interaction region. The correlations could in principle survive without neither time nor space constraint.

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### Can a camera affect the Quantum Entanglement? [duplicate]

'Spooky action at a distance' is how Albert Einstein famously derided the concept of quantum entanglement, wherein objects can become linked and instantaneously influence one another. Entanglement is ...
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### How do I compute the partial trace of $\sqrt{p}|0\rangle_a|0\rangle_b+\sqrt{1-p}|1\rangle_a |1\rangle_b$?

Here I have a two pure state system composed from systems A and B: $$\Psi_{ab} = \sqrt{p}\, |0\rangle_a |0\rangle_b + \sqrt{1-p}\, |1\rangle_a |1\rangle_b$$ How do I extract system A in matrix form ...
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### How does this quantum circuit work?

I am getting a bit confused about how to trace through a quantum circuit. So, for example, if you have the circuit below: I know that the first gate would split an arbitrary state alpha|0> + beta|...
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### Integration with respect to Haar measure and reduced density matrix

Consider a bipartite system $\mathcal{H}_A \otimes \mathcal{H}_B$, with $|A|,|B|>>1$ and not necceserly $|A|=|B|$. Following Jerusalem Lectures on Black Holes and Quantum Information (eq. 5.8) ...
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### Entangling the atoms in a molecule

I came across several papers that use entangled states of 2 ions in a trap to perform measurements with a much higher accuracy than classical (non-entanglement) methods. Here is an example of such a ...
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### If I make decision based on quantum random number generator does that make my decision not predetermined or non deterministic?

If or assume the quantum random number generator generates true random numbers and the result is not predetermined by anything due to quantum mechanics. I make a decision such as choose between A and ...
2answers
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### How does which-slit information destroy interference?

How exactly does which-slit information destroy interference? I've seen in so many places online claiming: "which-slit information destroys interference pattern"..."the act of ...
1answer
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### Delayed choice quantum eraser and changing the past?

I've been trying to get my head around the delayed choice quantum eraser experiments (see Kim et al. (1999) and Wiki for details). Many sources online claim that the experiment indicates ...
1answer
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### Entanglement Hamiltonian and correlation matrix of a system

I am new to quantum entanglement and condensed matter physics. I encounter a difficulty in understanding the relation between entanglement Hamiltonian and correlation function. Currently, I am reading ...
0answers
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### How are the particles on the boundary connected to the particles inside (in the AdS/CFT correspondence)?

To put it very simplistically, in the AdS/CFT correspondence a conformal quantum chromodynamics with a high number of colors is defined on the 4-dimensional boundary of a 5-dimensional anti-de Sitter ...
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### Entanglement Hamiltonian of a subsystem and the reduced density matrix

I encounter a problem in finding the entanglement Hamiltonian of a subsystem. Suppose my system consists of two sites and two fermions, and the Hamiltonian of the full system is the following: \begin{...
2answers
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### Does measuring an entangled pure state imply the result on the other subsystem?

From the definition of entanglement of pure states: Let the first system be in state $|\psi\rangle_A$ and the second in $|\psi\rangle_B$ with eigenstates $a_n|a_n\rangle$ and $b_m|b_m\rangle$ ...
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### Black Holes and Entangled Particles

What if you created 2 entangled particles and sent one into a black hole. Could you measure the one you have and get information out of a black hole?
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### Cooper pairs and superconductivity

This might be a dumb series of questions, because I'm not a theoretical or condensed matter physicist. If there was a method of pairing all electrons before entering a semiconductor, would you ...
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### Why real quantum mechanics fails in multipartite systems?

I was reading this paper "Quantum physics needs complex numbers" by Renou et al. In the paper, they propose the standard entanglement swapping scenario and use CHSH3 inequality to show that ...
1answer
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### Quantum Entanglement and corresponding probability of 2 Dices System

I am new to quantum entanglement and I want to have a simple example that can relate entangled states with probability. Therefore, I try to use the 2 dices system to examine whether am I really ...
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### Topological order and volume-law entanglement

Topological order is a property traditionally most associated with ground states of gapped Hamiltonians. However, using the notion that topological order is fundamentally about a form of "long-...
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### Entangled spin anti-correlations observed relative to what?

Alice and Bob work in a large room. In the middle of the room, they have a device that generates spin-entangled particle pairs; one member of each pair goes to Alice’s bench, the other one, to Bob’s. ...
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### Hong-Ou-Mandel in Fock Space for Arbitrary Polarization and Number Inputs

I would like to input an state: |\psi\rangle_{in}=\frac{1}{\sqrt{n_H!n_V!m_H!m_V!}}(a_{H}^{\dagger})^{n_H}(a_{V}^{\dagger})^{n_V}(b_{H}^{\dagger})^{m_H}(b_{V}^{\dagger})^{m_V}|0000\rangle_{...
1answer
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### Can two photons with correlated polarizations pass into a black hole and be emitted as Hawking Radiation with the correlation intact?

In this scenario photons would be emitted before they reach the centre of the black hole and before colliding with matter from a collapsed star.
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### How to determine if a state is entangled or pure in general?

Given a state like $a|00\rangle+b|01\rangle+c|10\rangle+d|11\rangle$ where $a,b,c,d$ satisfy normalization, how can we know if it's pure?
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### Many Worlds locality and EPR experiment

I've read in Sean Carroll's book (Something Deeply Hidden) that the Many-Worlds interpretation of quantum mechanics might be the only way to avoid instantaneous effects of entanglement, by having the ...
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### How does QFT explain QM phenomena?

I've been studying QFT for a couple of years now, and until today I haven't encountered any of the phenomena that I've studied in my QM course: tunneling, entanglement, probability measurements and ...
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### What gate combinations create entangled two-qubit states?

I know that in order to make a two qubit entangled state, this quantum circuit is used: But I was wondering if there are any other gate combinations which also create entangled two quit states. If ...
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### Can quantum entanglement contribute to entangled particle's decay?

Does a quantum entanglement have an influence on the entangled particle's decay?
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### Where does the concept of quantum entanglement originate?

A video I watched from PBS spacetime talked about how the pilot wave theory provides an explanation for why quantum entanglement occurs, where it is a result of having global hidden variables. I just ...
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### What does different “basis” mean

I am reading about quantum cryptography BB84 method at the moment. It talks about two basis: V/H and ${V_{45}/H_{45}}$ where the light is sent through a polariser at 45 degrees. These are then the two ...
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### If quantum entanglement causes the violations of Bell's inequality why don't we see perfect correlation?

From wikipedia: For example, if a pair of entangled particles is generated such that their total spin is known to be zero, and one particle is found to have clockwise spin on a first axis, then the ...
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### Is entanglement time-symmetric?

It is common to describe an experiment as "causing" entanglement. For example, two quantum particles that interact become entangled as a result of their interaction, so we are likely to say ...
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### How to extend a quantum operation to an auxiliary Hilbert space?

In Breuer and Petruccione's book 'The theory of open quantum systems', section 2.4.3, 'representation theorem of quantum operations', a quantum operation $\phi_m$ corresponding to a measurement ...
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### How does the many-worlds interpretation look like in bra ket notation?

If I understand correctly the many worlds interpretation says the universe is continously splitting into multiple branches and quantum measurements occur when decoherence causes a quantum state to ...
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### Faster than light communication protocol via GHZ-state

I had a chat with a guy recently who claimed that multipartite entanglement might actually be used for superluminal communication, and his argument is very simple. Suppose that 2 observers (Alice and ...
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### Any theory or explanation on the relation between temperature and causality?

Any theory or explanation on the relation between temperature and causality? I guess causality is correlated to information and temperature.
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### Entangle masers?

If I point a collection of masers at an antenna, I can broadcast a signal to phase-lock them to, so that their output will interfere constructively at my antenna. Is it possible, in principle, to not ...
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### What are the differences between quantum random number generator methods, and are there consequences to using them for decision-making?

I have two related questions. The first is that there are multiple ways to get quantum random numbers. But is there a difference between a method that extracts the prevalent quantum fluctuations from ...