# The Naked Scientists Forum

### Author Topic: What is quantum entanglement?  (Read 6173 times)

#### Georgia

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##### What is quantum entanglement?
« on: 15/06/2015 16:12:12 »
Hello forum!

I've recently been enjoying hearing lots about quantum entanglement - I have a few questions about it.

1 - does it only work in twos - or can loads of particles be "entangled"?
2 - how often does entanglement happen outside of a lab?
3 - Could the big bang have caused entanglement?

Thanks!
« Last Edit: 15/06/2015 16:19:47 by chris »

#### PmbPhy

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##### Re: What is quantum entanglement?
« Reply #1 on: 15/06/2015 16:43:26 »
Quote from: Georgia
Hello forum!

I've recently been enjoying hearing lots about quantum entanglement - I have a few questions about it.

1 - does it only work in twos - or can loads of particles be "entangled"?
Great question. Let's review what entanglement is first. When a measurement is take on a system of two particles then the system collapses into one of its eigenstates. From that it's possible to create a system that knowing a property of one particle means you know the same property of the other one. For example;

Define "Electron 1" as the electron moving in the +x direction,  "Electron 2" as the electron moving in the -x direction.

Scatter these two electrons off each other. The initial state of electron 1 is spin up while the spin of the other electron is spin down. After the scattering the two particles are in a superposition of two states: defined as follows

Eigenstate 1: Electron 1 is has spin up, Electron 2 is spin down
Eigenstate 2: Electron 1 is has spin down, Electron 2 is spin up

When a measurement of the spin of one electron is measured then the other one is sure to have the opposite spin. I don't know if this can be extended to a system of more particles. I'll look into it and get back to you.

Quote from: Georgia
2 - how often does entanglement happen outside of a lab?
Great question. Since quantum entanglement has to do with taking measurements then it never happens without measurements being taken. Let's define a lab as having at least the property of a place where measurements are taken. As such it never happens outside the lab.

Quote from: Georgia
3 - Could the big bang have caused entanglement?
No.

#### Bill S

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##### Re: What is quantum entanglement?
« Reply #2 on: 15/06/2015 21:42:56 »
Quote from: Pete
Let's define a lab as having at least the property of a place where measurements are taken. As such it never happens outside the lab.

Is it that it doesn't happen, or that we don't know about it unless we take measurements?

#### sciconoclast

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##### Re: What is quantum entanglement?
« Reply #3 on: 16/06/2015 01:28:43 »
There are two types of quantum entanglement: initial, local entanglement and remote entanglement. Some events produce particles or photons which have opposite or mirror image properties such as the spins that were mentioned. The entanglement that you have probably been hearing about is most likely remote entanglement.

Remote entanglement is a requirement of prevailing theory. The theory requires that when one particle is tested for a property its entangled partner assumes the opposite property. That is because in the present interpretation the properties are not established until actually tested for.

Because of experimental results (example: E.R.P. Experiment) the only way remote entanglement can survive as a viable theory is if instantaneous communication between entangle members occurs. This idea has great appeal to the mystics and those who like the mysterious. Which is why you hear so much about it in popular media.

If it does exist it exist throughout the universe and not only in the lab. Yes it can occur with multiple particles following certain events.

However, the theory breaks down mathematically when certain numbers of particles are involved.
www.newscientist.com/article/mg21528834.70www.arxiv.org/pdf/1001.38300-new-math-triggers-a-call-to-ironout-quantum-world.html

I started a thread in this forum which was titled, "Is remote entanglement not proven". If you search for it in this forum you will find a long discussion with lots of references.

If you subscribe to the principle of remote entanglement then you must accept that two particles colliding in the early universe can generating entangled particles or photons, that, if they have not encounter any other particles, can communicate information instantaneously across the universe. There is an axiom that extreme claims require extreme proof.

By the way; "test for" , is shorthand for " an interaction that collapses the quantum probability field " and does not necessarily imply a measurement.
« Last Edit: 16/06/2015 01:59:51 by sciconoclast »

#### PmbPhy

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##### Re: What is quantum entanglement?
« Reply #4 on: 16/06/2015 05:16:51 »
Quote from: Pete
Let's define a lab as having at least the property of a place where measurements are taken. As such it never happens outside the lab.

Is it that it doesn't happen, or that we don't know about it unless we take measurements?
What is actually real and exists depends on whether we've measured it or not. E.g. a particle doesn't have a position until we measure it.

#### evan_au

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##### Re: What is quantum entanglement?
« Reply #5 on: 16/06/2015 11:45:06 »
Quote from: Georgia
can loads of particles be "entangled"?
The principle of quantum computing is that you can entangle multiple particles (or "qubits"). A system of n entangled particles acts as a superposition of 2n states, and could (in theory) solve some problems much faster than a sequential computer searching 2n states.

In 2011, researchers managed to factor the number 143=11*13 using 4 qubits. One can expect that the best results will not be published; some encryption mechanisms can be broken if you can factorise large numbers, and the US NSA is reportedly spending tens of millions of dollars on it - and they aren't in the habit of publishing their capabilities.

It is difficult to keep multiple qubits entangled, as any disturbance in the environment will break the entanglement. Some researchers have been looking for ways to provide error-correcting codes to recover from this decoherence.

#### evan_au

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##### Re: What is quantum entanglement?
« Reply #6 on: 16/06/2015 11:56:40 »
Quote from: PmbPhy
a particle doesn't have a position until we measure it...
Let's define a lab as having at least the property of a place where measurements are taken. As such it never happens outside the lab.
It has been hypothesised that some exquisitely sensitive mechanisms in nature utilise the quantum character of matter, including superposition of states and entanglement.

This is the infant (and somewhat contentious) field of Quantum Biology.

For example, it is hypothesised that the efficiency of photosynthesis is due to the ability of a photon to take many paths through the chlorophyll molecule in parallel, with the molecule capturing the energy of the photon in the most "efficient" path.

If this effect is proven, it shows that plants have been entangling photons in their leaves long before humans built anything remotely like a laboratory.

#### sciconoclast

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##### Re: What is quantum entanglement?
« Reply #7 on: 16/06/2015 19:30:58 »
Georgia: What is quantum entanglement? When an event occurs such as the collision between two particles which results in a shower of particles and or photons the sum of properties of the resulting entities must add up to the be the same as the sum of the original colliding particles. The resulting entities are said to be initially entangled.

This principle led to the search for some missing spin and the discovery of the neutrino.

The concept of remote entanglement assumes that the resulting particles or photons remain part of the same quantum probability field until it is collapsed by the actualization of one of the members.

Pmbphy. As for your comments that quantum entanglement cannot happen without measurement and that it never happens outside the lab; it has been going on in the universe long before humans arrived and will continue long after we are gone.

There where many early Gedankene experiments showing how ludicrous the logic of requirement for observation was. They were followed by physical experiments that also discredited the idea. Here is a recent one:  www.scientificexploration.org/journal/jse_06_4_jeffers.pdf.

All quantum experimental results can be explained by classical quantum mechanics without the more outrageous interpretations. There is a great lecture on the subject:  media.physics.harvard.edu/video/index.php?:id=SidneyColeman_QMIYF.flr   You need a background in quantum mechanics to follow most of this but there are some simplified examples in the lecture.

You may want to go to:  www.arxiv.org/pdf/1112.4522   It explains how all the weird speculative stuff is not in agreement with quantum theory.
« Last Edit: 16/06/2015 19:39:37 by sciconoclast »

#### PmbPhy

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##### Re: What is quantum entanglement?
« Reply #8 on: 17/06/2015 21:39:43 »
Quote from: Georgia
1 - does it only work in twos - or can loads of particles be "entangled"?
I looked into it as I said and the answer is that it can work in cases where there are more than two particles.

#### sciconoclast

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##### Re: What is quantum entanglement?
« Reply #9 on: 18/06/2015 14:43:42 »
I am glad to see agree with my statement that there can be a "shower" of multiple, entangle particles.

On the other hand if you interpreted my statement that the theory "breaks down" with certain multiple entanglements to mean that multiple entanglements do not occur, then you miss-interpreted my statement.
I was saying that there are mathematical problems when you try to explain this with the Bohr principle of remote entanglement.

The math is very complicated but there is a simplified down to earth article by L. Grossman in the September 2012 issue of New Scientist (issue 2883) " New Maths Triggers a Call to Ironout Quantum World" that you might want to look up.

#### lunar7

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##### Re: What is quantum entanglement?
« Reply #10 on: 20/06/2015 15:22:16 »
One idea I came across a few years ago is:
Consider two snooker balls and a pair of socks, one ball is  red and the other is blue. Show both balls to an observer and then place each ball into a sock without the observer knowing which ball went into which sock. Give one sock to the observer and you retain one. Now move apart several metres. Ask the observer to reveal their ball. Instantly the observer will know what colour is your ball.

Here is my thought on this experiment:
It is known that entanglement of say a pair of electrons share the same wavefunction. This I say is analogous to the observer knowing the colour of the two snooker balls.

#### PmbPhy

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##### Re: What is quantum entanglement?
« Reply #11 on: 20/06/2015 17:37:33 »
Quote from: lunar7
One idea I came across a few years ago is:
Consider two snooker balls and a pair of socks, one ball is  red and the other is blue. Show both balls to an observer and then place each ball into a sock without the observer knowing which ball went into which sock. Give one sock to the observer and you retain one. Now move apart several metres. Ask the observer to reveal their ball. Instantly the observer will know what colour is your ball.

Here is my thought on this experiment:
It is known that entanglement of say a pair of electrons share the same wavefunction. This I say is analogous to the observer knowing the colour of the two snooker balls.
That's a very common analogy to quantum entanglement. The difference being that in QM the electron doesn't even have a value of spin until its measured, unlike your experiment where the ball does have a color at all times.

#### jeffreyH

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##### Re: What is quantum entanglement?
« Reply #12 on: 21/06/2015 12:29:03 »
What happens if two spin up or two spin down electrons collide? Which one would change spin?

#### PmbPhy

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##### Re: What is quantum entanglement?
« Reply #13 on: 21/06/2015 12:56:58 »
What happens if two spin up or two spin down electrons collide? Which one would change spin?
Neither would change spin. There'd be no reason for it.

#### jeffreyH

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##### Re: What is quantum entanglement?
« Reply #14 on: 21/06/2015 13:07:12 »
What happens if two spin up or two spin down electrons collide? Which one would change spin?
Neither would change spin. There'd be no reason for it.

So does this mean there is no entanglement.

#### PmbPhy

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##### Re: What is quantum entanglement?
« Reply #15 on: 21/06/2015 13:21:53 »
Quote from: jeffreyH
So does this mean there is no entanglement.
Yes.

#### Georgia

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##### Re: What is quantum entanglement?
« Reply #16 on: 16/07/2015 11:14:40 »
Thanks everyone, really interesting reading :)

#### The Naked Scientists Forum

##### Re: What is quantum entanglement?
« Reply #16 on: 16/07/2015 11:14:40 »