# Gravity and entanglement

1. Dec 11, 2013

### Jilang

https://www.simonsfoundation.org/quanta/20131107-physicists-eye-quantum-gravity-interface/
I believe experiments have maintained quantum superpositions across great distances over the surface of the earth. But does anyone know if they have done so over significant vertical distances? The article seems to suggest that gravity would destroy the entanglement. I can see why this must be the case, but I just wondered if there was evidence for it.

2. Dec 11, 2013

### serp777

It's not logical that gravity would cause entanglement to decohere unless you mean a black hole or neutron star. Gravity affects space in a way that causes an acceleration. If acceleration broke entanglement, then you could never move the entangled atoms apart in the first place (I.E. you could never start moving them because that implies an acceleration).

3. Dec 12, 2013

### Jilang

I think the reasoning must be that if the gravitational term in the Hamiltonians of the two particles were different it would break the phase relationship between the entangled particles. If the same force is applied to both I don't see that there would be an issue.

4. Dec 12, 2013

### kith

Simply adding external Hamiltonians to the particle Hamiltonians doesn't break entanglement. Let's say we produce an entangled particle pair with state |1>⊗|2> + |2>⊗|1> and have two different time evolution operators UA and UB which depend on whether the particle moves from the interaction region to point A or B. Then your final state will be UA|1>⊗UB|2> + UA|2>⊗UB|1> which is still entangled in general.

5. Dec 12, 2013

### Jilang

Thanks for this, I think the article is discussing an intrinsic type of gravitational decoherence (self-decoherence?) after looking at the link to the paper. I can see that the energy of each particle would still be conserved so there could be no change in the Hamiltonian anyway!

But if there was I am still unclear as to how they could remain entangled if their speeds were different. I guess you are saying that indistinguishability has nothing to do with entanglement, so I will hold that thought!

6. Dec 12, 2013

### kith

What is your definition of entanglement and why do you think it couldn't apply to particles with different speeds?

7. Dec 13, 2013

### Jilang

Is it something like that the state of one depends on the state of the other and their properties are correlated?

8. Dec 13, 2013

### Jilang

http://arxiv.org/pdf/1306.1933v2.pdf
Which indicates that gravity, motion and space-time dynamics can create and degrade entanglement.
(Lansing and Fuentes Classical and Quantum Gravity 29,224001 2012). I'm going see if I can find this.

9. Dec 13, 2013

### Zarqon

There's no reason why gravity would negatively affect entanglement of degrees of freedom that is not connected to gravity in a direct way (and most aren't). Experiment-wise there are concrete plans to entangle a satellite in space with ground stations, by the year 2016. Plans to do this is described in this paper by Pan, where they also report on experiments already performed using hot air balloons at some gravitational altitude as entanglement relayers.

10. Dec 13, 2013

### kith

That's a bit handwavy. The actual definition of an entangled state is that it is not seperable.

As my previous post implies, for certain special conditions the state may evolve coherently from an entangled state to a seperable state |α>⊗|β>. But this has nothing to do with decoherence and your paper doesn't mention decoherence either.

If you look at fig. 2, you see that the degree of entanglement (which is measured by the so-called negativity here) shows a periodic behaviour wrt the difference in the gravitational field strength. So entanglement may also increase.

I haven't read the paper and can't comment on it in more detail. But I don't think you will benefit much from such papers unless you know the basics of entanglement.

Last edited: Dec 13, 2013
11. Dec 13, 2013

### Jilang

Yes Kith you are right, it doesn't talk about decoherence, just degradation of entanglement. It's really interesting that it would come back again at the right orbital separations. I hope they get to do this experiment for real one day!

12. Dec 13, 2013

### Jilang

Thanks Zarqon that's really interesting. Great name by the way.

13. Dec 13, 2013

### audioloop

gravity induced decoherence.
superposition broken by gravity.
penrose idea (karolyhazy and diosi have similar models too).

non linear quantum mechanics unlike standard quantum mechanics (a linear one).

Last edited: Dec 13, 2013