Is gravity a quantum phenomenon? That has been one of the big outstanding questions in physics for decades. Physicists have proposed an experiment that could settle the issue. However, it requires studying two very large entangled quantum systems in freefall. Researchers now present a way to reduce background noise to make this experiment more manageable.

Three of the four fundamental forces in physics can be described in terms of quantum theory. This is not the case for the fourth force (gravity), which is described by Einstein’s theory of general relativity. The experiment that Mazumdar and his colleagues previously designed could prove or disprove the quantum nature of gravity.

Superposition

A well-known consequence of the quantum theory is the phenomenon called quantum superposition: in certain situations, quantum states can have two different values at the same time. Take an electron that is irradiated with laser light. Quantum theory says that it can either absorb or not absorb the photon energy from the light. Absorbing the energy would alter the electron’s spin, a magnetic moment that can be either up or down. The result of quantum superposition is that the spin is both up and down.

These quantum effects take place in tiny objects, such as electrons. By targeting an electron in a specially constructed miniature diamond, it is possible to create superposition in a much larger object. The diamond is small enough to sustain this superposition, but also large enough to feel the pull of gravity. This characteristic is what the experiment exploits: placing two of these diamonds next to each other in freefall and, therefore, cancelling out external gravity. This means that they only interact through the gravity between them.

Challenging

And that is where another quantum phenomenon comes in. Quantum entanglement means that when two or more particles are generated in close proximity, their quantum states are linked. In the case of the diamonds, if one is spin up, the other, entangled diamond should be spin down. So, the experiment is designed to determine whether quantum entanglement occurs in the pair during freefall, when the force of the gravity between the diamonds is the only way that they interact.

Story Source: Materials provided by University of Groningen. Note: Content may be edited for style and length.