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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
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