A proposed experiment to test quantum gravity

[mfb]

Gravity is weak - while we can study the other interactions with individual particles, for gravity we need macroscopic objects to get measurable forces. This makes it easy to measure quantum effects for the other interactions, but hard to do so with gravity. Gravitational forces are always measured with a source mass and a test mass. Quantum objects as test masses have been demonstrated quite some time ago, most notably neutrons bouncing above a surface with quantized energy levels (e.g. V. V. Nesvizhevsky et al, hep-ph/0306198).

What about a quantum object as source mass? How does the gravitational field of an object look like that is in a superposition of two places? From quantum mechanics we have a pretty solid expectation how - it should be a superposition as well, and objects in this field will go into a superposition over time, getting entangled with the source mass. But in the framework of general relativity this would mean a superposition of spacetime geometries, and it is unclear how meaningful such a concept is. Testing this experimentally is very challenging. A massive object will lead to quick entanglement, but it will also quickly lose a superposition of states. A less massive object is easier to keep in superposition, but its tiny gravitational force means a longer measurement time and more sensitivity to other influences. So far all experiments were far away from the required parameters for such a test.

A group of scientists proposed a new experiment that could achieve the required combination of source mass, coherence time, and distances involved. For this experiment, two microscopic diamonds with a special defect (nitrogen atom and a vacancy) are prepared in a trap. Their defects are put in a superposition of two spin orientations. As next step the diamonds are put into a magnetic field, where they move based on its spin. Then the trap is released and the diamonds can fall down freely for three seconds. The gravitational force on the diamonds then depends on the position of the other diamond, and the states get entangled. Afterwards another magnetic field removes the superposition in space. If gravity acts like quantum mechanics predicts, the spins should now be entangled, and in the proposed setup opposite spins are more likely than aligned spins.

What can we learn from this experiment?
If the result is as expected, it will be a first experimental confirmation that gravity can be quantized - but it won't tell us how. It is an experiment at low energies, where effective theories of quantized gravity work without any issues already. It is probably still worth a Nobel prize, but it won't solve the old puzzle how to unify general relativity and quantum field theory.
If the result is unexpected, then we might learn a lot. Gravity could be special somehow, and this experiment could tell us in which direction we have to look.

It will take years to prepare such an ambitious experiment, but it is certainly an interesting project.The original publications:
Spin Entanglement Witness for Quantum Gravity
Gravitationally Induced Entanglement between Two Massive Particles is Sufficient Evidence of Quantum Effects in Gravity

A blog article:
Physicists Find a Way to See the ‘Grin’ of Quantum Gravity

 

[Vanadium 50]

I don't quite understand what is new about this.

We know that there are quantum-mechanical effects produced by gravity: Colella, Overhauser and Werner showed this in 1975. We know that ultracold neutrons in the Earth's gravity obey the QM prediction, both in energy eigenstates and their time evolution. And we know that neutrons are engangled composite particles. So what do we learn from this measurement?

 

[mfb]

We know that ultracold neutrons in the Earth's gravity obey the QM prediction, both in energy eigenstates and their time evolution.

The neutrons are the test particles in these experiments. The gravitational potential they see could be a classical one.
That leads to concepts like gravity-induced decoherence. Which I see under the "unexpected" results.

 

[Vanadium 50]

I'm still not getting it. You can still get entanglement - what it seems they are talking about - in classical potentials. If I can go to A-level for a second, I see the difference between classical and quantum potentials is that you need second quantization to accurately describe the behavior of the latter, and I don't see how this happens.

 

[mfb]

You can still get entanglement - what it seems they are talking about - in classical potentials.

If you want entanglement with the source, you'll need an experiment as the one proposed here.

The neutrons in the other experiments are not entangled with the position of Earth. They are quantum particles in a purely classical potential that does not show any superposition.

 

[Auto-Didact]

Wonderful experiment, definitely belongs on this list.

We know that ultracold neutrons in the Earth's gravity obey the QM prediction, both in energy eigenstates and their time evolution. And we know that neutrons are engangled composite particles. So what do we learn from this measurement?

There are theoretical motivations for questioning that massive particles will actually continue to obey QM, based of course on our obvious macroscopic empirical situation. One of the more popular theoretical proposals is Diosi-Penrose quantum state reduction, which is a prediction of intrinsic mass-dependent wave-function collapse as an actual phenomenon in an as yet to be discovered non-linear extension of QM. This undiscovered theory has QM as its limiting case with quantum state reduction mass $m_r \rightarrow 0$ in QM, with the actual value for $m_r$ being the Planck mass.

Since Colella et al. there have been improvements with more massive quantum systems measured than neutrons, particularly large organic molecules (of $60 Å$, up to $7000 \rm{amu}$) were shown by Gerlach et al. in 2011 to not deviate from QM. However as mfb says in the second paragraph:

A massive object will lead to quick entanglement, but it will also quickly lose a superposition of states. A less massive object is easier to keep in superposition, but its tiny gravitational force means a longer measurement time and more sensitivity to other influences. So far all experiments were far away from the required parameters for such a test.

This means that we in fact do not know that anything more massive than about $10^{-23} \rm{kg}$, and certainly not that anything close to $10^{-8} \rm{kg}$, will strictly obey QM under the influence of a gravitational field like Earth's. There is a large theory space left to be excluded and a large parameter space left to be explored here, which is exactly where experiments using mesoscopic nano-crystals and the like come in.

I'm still not getting it. You can still get entanglement - what it seems they are talking about - in classical potentials. If I can go to A-level for a second, I see the difference between classical and quantum potentials is that you need second quantization to accurately describe the behavior of the latter, and I don't see how this happens.

Just as you say, this can't be done since making superpositions of different gravitational fields i.e. of different spacetimes is prohibited within the rules of standard QFT. This is due to the fact that these fields explicitly refer to different vacuum states. A non-linear extension of QFT may resolve this issue if spacetime bifurcation during superposition only becomes relevant for large masses, i.e. above some particular bifurcation parameter, a natural candidate being the Planck mass.

 

[the_pulp]

Would the negative result falsify string theory? (given that as far as I know string theory depends on qm)

Correct me if I'm wrong

Regards

 

[rootone]

Would the negative result falsify string theory?

No, but it could mean we need to look more at other theories.

 

[atyy]

Free archive versions

https://arxiv.org/abs/1707.06050
A Spin Entanglement Witness for Quantum Gravity
Sougato Bose, Anupam Mazumdar, Gavin W. Morley, Hendrik Ulbricht, Marko Toroš, Mauro Paternostro, Andrew Geraci, Peter Barker, M. S. Kim, Gerard Milburn

https://arxiv.org/abs/1707.06036
Gravitationally-induced entanglement between two massive particles is sufficient evidence of quantum effects in gravity
Chiara Marletto, Vlatko Vedral

 

[atyy]

https://arxiv.org/abs/1707.07974
On two recent proposals for witnessing nonclassical gravity
Michael J. W. Hall, Marcel Reginatto
(Submitted on 25 Jul 2017 (v1), last revised 5 Jan 2018 (this version, v3))
Two very similar proposals have been made recently for witnessing nonclassical features of gravity, by Bose et al. and by Marletto and Vedral. However, while these proposals are asserted to be very general, they are in fact based on a very strong claim: that quantum systems cannot become entangled via a classical intermediary. We point out that the support provided for this claim is only applicable to a very limited class of quantum-classical interaction models, corresponding to Koopman-type dynamics. We show that the claim is also valid for mean-field models, but that it is contradicted by explicit counterexamples based on the configuration-ensemble model. Thus, neither proposal provides a definitive test of nonclassical gravity.

 

[the_pulp]

No, but it could mean we need to look more at other theories.

Why wouldn't these experiments falsify st? Additionally, given that they wouldn't falsify st, why it would mean that we need to look more other theories?

Thanks in advance!

 

[rootone]

Why wouldn't these experiments falsify st? Additionally, given that they wouldn't falsify st, why it would mean that we need to look more other theories?

Thanks in advance!

I meant that I can't see how this experiment would either falsify or give weight to string theories.
Thus the situation would remain; that there is as yet no experimental evidence for string theory.
That doesn't mean string theory is wrong. only that there is still scope for other theories which could describe nature at it's most fundimental

 

[rrogers]

Wonderful experiment, definitely belongs on this list.There are theoretical motivations for questioning that massive particles will actually continue to obey QM, based of course on our obvious macroscopic empirical situation. One of the more popular theoretical proposals is Diosi-Penrose quantum state reduction, which is a prediction of intrinsic mass-dependent wave-function collapse as an actual phenomenon in an as yet to be discovered non-linear extension of QM. This undiscovered theory has QM as its limiting case with quantum state reduction mass $m_r \rightarrow 0$ in QM, with the actual value for $m_r$ being the Planck mass.

Since Colella et al. there have been improvements with more massive quantum systems measured than neutrons, particularly large organic molecules (of $60 Å$, up to $7000 \rm{amu}$) were shown by Gerlach et al. in 2011 to not deviate from QM. However as mfb says in the second paragraph:This means that we in fact do not know that anything more massive than about $10^{-23} \rm{kg}$, and certainly not that anything close to $10^{-8} \rm{kg}$, will strictly obey QM under the influence of a gravitational field like Earth's. There is a large theory space left to be excluded and a large parameter space left to be explored here, which is exactly where experiments using mesoscopic nano-crystals and the like come in.
Just as you say, this can't be done since making superpositions of different gravitational fields i.e. of different spacetimes is prohibited within the rules of standard QFT. This is due to the fact that these fields explicitly refer to different vacuum states. A non-linear extension of QFT may resolve this issue if spacetime bifurcation during superposition only becomes relevant for large masses, i.e. above some particular bifurcation parameter, a natural candidate being the Planck mass.

What about?
"
Professor Mika Sillanpää of the Department of Applied Physics and O.V. Lounasmaa laboratory at Aalto University is carrying out basic research on micromechanical resonators measured at ultralow temperatures.

Read more at: https://phys.org/news/2015-02-quantum-mechanical-behaviour-macroscale.html#jCp
"
To me they are larger than mentioned.