All Questions
80 questions
30
votes
5
answers
5k
views
Can quantum entanglement be used to coordinate actions at "FTL speeds" without breaking causality or actual faster-than-light communication?
I know there are a lot of similar question but I don't believe this to be a copy. I understand that if two people lived far away they could not transfer information through quantum entangled particles ...
- 493
9
votes
1
answer
2k
views
Entanglement and the Pauli exclusion priciple
Could you say that two electrons in the ground state of a helium atom experience quantum entanglement? They are both in the same energy level and cannot have the same quantum numbers. If one is spin ...
- 113
8
votes
0
answers
267
views
Free Will Theorem question
The Kochen-Specker Theorem says, if I understand it correctly, that the results of spin measurements cannot be predetermined independent of measurement. They get to this conclusion by describing 33 ...
- 81
6
votes
2
answers
846
views
Is entanglement *not* intrinsic to state, but dependent on division into subsystems? (Susskind QM)
I'm working through Susskind's "Quantum Mechanics" book (TTM series), which I quite like.
Background
In Lecture 7 (Chapter 7), he studies a 2-spin system. A single spin has eigenvectors:
$$|u\...
- 578
5
votes
3
answers
1k
views
Can 2-qubit systems be represented in the Bloch sphere representation?
I am studying a system with 2 qubits, so I need, for a given state, a Bloch representation for each qubit.
I am having difficulties because I get results that do not have sense at all. For example if ...
- 529
5
votes
2
answers
600
views
Quantum entanglement of spin along multiple orthogonal axes
Picture an entangled pair of spin 1/2-spin particles with total spin 0. In the diagram, particle 1 of the pair is moving to the left (-y), and particle 2 to the right (+y).
If a z-oriented SG$^*$ is ...
- 2,695
5
votes
2
answers
561
views
Entangled states and separable states
B
Two electrons in the same orbital is clearly an entangled quantum state since it is not a tensor product:
$$|\psi\rangle=\frac{1}{\sqrt{2}}(|\uparrow\rangle \otimes|\downarrow\rangle-|\downarrow\...
- 51
4
votes
3
answers
847
views
What happens to spin after measurement, does it evolve and randomise like position evolves after measurement?
When a particle's position is measured, if one considers the wavefunction to collapse then one can assume it collapses into a delta function peaked at the measured position, and then after some finite ...
- 2,654
4
votes
1
answer
2k
views
Bloch sphere for 2 and more qubits
if I represent my qubit in the Bloch sphere as
$|ψ⟩=a|u⟩+b|v⟩$
where
$|ψ⟩=\cos(θ_2)|u⟩+e^{iϕ}\sin(θ_2)|v⟩$,
I can see where my qubit is inside the sphere.
Now imagine I have 2 qubits or more and ...
- 79
4
votes
1
answer
523
views
Basic Quantum Entanglement
Quantum entanglement means that multiple particles are linked together in a way such that the measurement of one particle's quantum state determines the possible quantum states of the other particles. ...
- 1,923
4
votes
1
answer
279
views
Can different components of a wavefunction become entangled with different systems?
Imagine a $z$-basis spin Stern Gerlach experiment where a single "particle" is put through the apparatus.
Upon exiting the Stern Gerlach magnets, the result is the sum of two states. One state is ...
- 887
4
votes
1
answer
675
views
Connection between spin angular momentum of a photon and circular polarization of light
Do photons have spin angular momentum only if they are part of a circularly-polarized beam?
I suspect that every photon always has spin angular momentum, but in most cases they have a superposition ...
3
votes
1
answer
787
views
How to entangle two superconducting qubits?
I do understand qubits entanglement, and their spins but I do not understand that at circuit level how can we entangle 2 qubits? Is it related to 2 qubits gates?
- 151
3
votes
1
answer
2k
views
Evolution of Eigenstates when two spin systems are coupled
I would like to describe the following situation:
We have two spin systems: Spin 1 ($S_1$) and Spin 1/2 ($S_2$).
Now imagine you somehow change their interaction so that you can fine-tune
the ...
- 692
3
votes
1
answer
623
views
Bell inequality with triplet state
Is it possible to prove Bell inequality starting from a state formed from triplet states, i.e. $\frac{1}{\sqrt{2}}(|\uparrow>_A|\uparrow>_B+|\downarrow>_A|\downarrow>_B)$?
If not, why?
...
- 1,092
3
votes
1
answer
439
views
Entanglement and the uncertainty principle
Suppose you have two maximally entangled particles. You measure the spin about the x axis of the first and the spin about the y axis of the second. But both spins cannot be known simultaneously, so ...
- 172
3
votes
0
answers
93
views
Constructing wavefunction for a mixed state
This question is somehow the reverse of another question.
If a quantum system $S$ is in a pure state, then we can find a wavefunction that describes $S$. This wavefunction is unique up to a phase ...
- 1,502
3
votes
0
answers
75
views
Bounding the value of a function for separable spin states
Consider $N$ spin-1/2, for which we can define the collective spin operator $\vec{S}=\sum_i \frac{\vec{\sigma}^{(i)}}{2}$. My question is, what is the upper bound $U$ on
$$
f(\rho) = \text{Var}[ S_z ] ...
- 1,231
3
votes
0
answers
224
views
Spin polarization of decay products
A relativistic moving particle, e.g. muon $\mu^+$, described by its four-momentum vector $p_\mu$, charge $e$ and with a given spin polarization, ${\bf S}=(S_x,S_y,S_z)$, decays into three particles, e....
- 575
2
votes
3
answers
669
views
Is it possible to bind an entangled electron to the outer shell of an atom or just a proton?
Say we start with an entangled electron--positron pair and we separate them. I want to take the entangled electron and bind it to a proton or the outer shell of an atom.
Is it possible to do this ...
- 3,911
2
votes
2
answers
421
views
Is it only the spin of a particle that can be entangled with another particles spin?
Is it only the spin of a particle that can be entangled with another particles spin?
Also is there any good physical interpretation of the spin of a particle? because the rotational invariance of ...
- 21
2
votes
2
answers
112
views
Does it matter when we measure the spin of the other entangled particle?
Let's say, we have 2 entangled particles: A and B which are a light-year away from each other.
We know if we measure the spin of particle A, we can be certain the spin of particle B will be in the ...
2
votes
1
answer
1k
views
Quantum Entanglement - Measuring Twice
In the answer here and on the wiki article and many other articles it is mentioned that if one of 2 entangled particles is measured their state collapses according to the Copenhagen interpretation.
...
- 179
2
votes
1
answer
304
views
How do three electrons entangle with each other?
In a Quantum entity of two electrons entangled to each other, we know that when one electron's spin is known, then the other electron's spin is the opposite.
But, What's the case in a quantum entity ...
- 21
2
votes
1
answer
611
views
Fermionic (or Bosonic) state vs Entangled state
One can see that the wavefunction for a system of two electrons (not very far apart) is one that cannot be written as a tensor product of individual states.
The same is true for a bosonic state.
For ...
- 1,461
2
votes
1
answer
235
views
What is the state of a single electron in an entanglement?
If we consider a bipartie system that is entangled,
$$|\psi^{AB}\rangle=\frac{|{\uparrow_z\downarrow_z}\rangle-|{\downarrow_z\uparrow_z}\rangle}{\sqrt{2}} $$
And we want to know the probability ...
- 1,097
2
votes
2
answers
191
views
Why can't be the EPR experiment simplified?
Alice measures the spin of her electron on the x axis. She now knows the spin value of Bob's electron on the x axis at time T0. Bob measures the spin of his electron on the z axis. He now knows the ...
- 147
2
votes
0
answers
98
views
Entanglement measure for a continuous bipartite system
Consider a cavity-spin system and assume we want to quantify the correlation between them. The cavity system can be described using a bosonic mode whereas the spin system requires 2 bosons to describe ...
- 219
2
votes
1
answer
179
views
Does spin entanglement imply position entanglement?
My question is whether two electrons can be entangled only with respect to their spins but not with respect to some other observable, such as position.
I initially believed that spin-entanglement ...
- 1,073
2
votes
1
answer
299
views
Measurement of the spin of the EPR pair in two orthogonal directions and how did Einstein tackle this?
Let us consider two types of measurement in the EPR experiment. In Bohm's description of this experiment, the state of the electron-positron (called the EPR pair) is given by
$$
|{\rm EPR} \rangle =\...
- 12.3k
1
vote
1
answer
159
views
A question regarding quantum entanglement
According to wiki,
Quantum entanglement is the physical phenomenon that occurs when a group of particles are generated, interact, or share spatial proximity in a way such that the quantum state of ...
1
vote
1
answer
198
views
Can one measure two components of spin exactly by measuring two components of entangled, say, electrons?
Of a single electron, two different components of spin can't be have simultaneously well defined values. But what if we entangle two of them and we measure, say, $(S_1)_z$ and $(S_2)_x$ simultaneously....
1
vote
1
answer
75
views
Spin operator for an ensemble
Given that there are N number of particles and for each one of them $\hat{\sigma}_x$ is the usual Pauli matrix. How can we show that the Pauli matrix for the entire ensemble has the form $\frac{1}{N}\...
- 592
1
vote
2
answers
317
views
Spin Control and Entanglement
I have a thought-experiment sort of question and I don't know where to start. Suppose you have an entangled pair, e1 and e2, and you split them. Then BEFORE reading them, you spin control e1 to +, ...
- 111
1
vote
1
answer
60
views
When does multi-particle entanglement break? [closed]
Let's say we entangle four electrons based on their spin.
Now, let's say we measure the spin of electron A along the z-axis (up/down), and we observe a spin of 'up'. My understanding is that this ...
- 11
1
vote
1
answer
161
views
The Heisenberg model using the duality analysis
I would like to express the Heisenberg model using the duality analysis. It is shown here how to express the Ising model using Pauli matrices but I cannot get the relation $ \sigma _{i}^{z}= \prod_{...
- 11
1
vote
1
answer
116
views
Where, exactly, does the boundary lie between 'entangled' particles and merely 'interacting' or 'coupled' ones?
Have scientists discovered, yet, the precise amount of interaction needed to actually 'entangle' a pair or more of particles, beyond mere interaction or even coupling?
Is there a distinct difference, ...
- 4,709
1
vote
2
answers
736
views
Bell's theorem: why does "correlation = cosine" rules out hidden variables?
I was reading about entanglement and Bell's theorem and played around with the idea. Specifically the point that the correlation of the measurement of entangled electrons at arbitrary angles is given ...
- 29
1
vote
1
answer
270
views
Dirac Notation Tensor product
We can write a Singlet state of two $\frac{1}{2}$ spin particles like this:
$$|S\rangle = \frac{1}{\sqrt{2}}\left( |+ \rangle ⊗ |-\rangle - |-\rangle ⊗|+\rangle \right) $$
is this the same as ...
- 21
1
vote
3
answers
640
views
Quantum entanglement, how do we know there was no spin? [duplicate]
Im not a scientist, so go easy on the explanation!
As I understand it we can create two entangled particles. The entangled particles have a spin property which is opposing. When we measure one of the ...
1
vote
1
answer
146
views
Will a particle still be in a superposition state if unobserved from only the perspective of one observer?
In this scenario let us take a particle that can have either spin up or spin down. Let's say that observer 1 observed the particle's spin to be down. But observer 2 hasn't observed it yet. So, then ...
- 488
1
vote
1
answer
832
views
Quantum entanglement and information transfer
I know it was stated many times that information transfer using entanglement is not possible and I am most probably wrong but I would be glad if you can at least point me to where I am mistaken.
If ...
- 135
1
vote
2
answers
723
views
Why would hidden variables in Bell's Stern-Gerlach experiment related to entanglement require only 4/9ths of the measurements to be equal?
Bell's Stern-Gerlach experiment, consisting of 2 sets of detectors each with 3 randomly chosen detection directions, is one of the most important experiments cited as proving that entanglement has ...
- 798
1
vote
0
answers
68
views
Spin Correlation Analogy in Three Particle System
I have recently gone through the section on Bell's Inequality in "Modern Quantum Mechanics" by J.J. Sakurai and Jim Napolitano and I am curious as to weather or not there are analogous ...
- 11
1
vote
0
answers
71
views
Measurement of total angular momentum in quantum mechanics
Consider a system composed of two spin-1/2 particles. The total spin operator is defined as
$$
\mathbf{S} = \mathbf{S_1} + \mathbf{S_2}
$$
We can write a common eigenbasis of the operators $\mathbf{S^...
- 324
1
vote
0
answers
52
views
How is entanglement of non-spin variables incompatible with local realism?
It seems to me that spin/polarization entanglement is the only spooky one. That's because the entanglement of other observables would happen even in regular classical physics if we made a "...
- 2,587
1
vote
0
answers
115
views
Hamiltonian basis in multiple particle systems
I have a question about the choosing of basis that compose the Hamiltonian of multiple systems. For example if we have a system of N particles, that depend in our case of Pauli matrixes i will give ...
- 529
1
vote
0
answers
19
views
Do fullerenes have a better quantum yield than carbon quantum dot's of similar dimensions?
As a fullerene and a carbon quantum dot of similar dimensions would have the same surface area but different internal volume, as a Fullerene does not contain carbon within it's interior, but just the ...
- 471
1
vote
0
answers
134
views
Creation of entangled electrons
If one obtains two entangled in polarization photons by parametric down conversion and one of them is accepted by and electron 1 and other by electron 2 - are this electrons now entangled in spin?
- 679
1
vote
1
answer
98
views
Quantum entanglement continuous measurement
Is there a possibility (if not real than at least theoretical) that we could measure the spin of an electron continously over some (even very short) period of time, so that it does not change during ...
- 135