Last year, Anupam Mazumdar, a physicist at the University of Groningen, jointly proposed with British colleagues an experiment that could conclusively prove whether gravity is a quantum phenomenon. This experiment would focus on the observation of two relatively large and entangled quantum systems in free fall. In a new article, published on June 4 in Physical examination of research, scientists describe in more detail how two types of noise could be reduced. They suggest that quantum interference could be applied to produce a sensitive instrument capable of detecting the movement of objects ranging from butterflies to burglars and black holes.
Is gravity a quantum phenomenon? This is one of the big unanswered questions in physics. Last year, together with colleagues, assistant professor of theoretical physics at the University of Groningen, Anupam Mazumdar, jointly proposed an experiment that could resolve this question. At the heart of this experiment is a tiny diamond, only a few nanometers in size, in which one of the carbon atoms has been replaced by a nitrogen atom. According to quantum physics, the extra electron in this atom would or would not absorb photon energy from a laser.
Absorbing energy would change the electron’s spin value, a magnetic moment that can be either up or down. “Just like Schrödinger’s cat, which is dead and alive at the same time, this electronic spin absorbs and does not absorb photon energy, so its spin is both ascending and descending,” explains Mazumdar. This process results in a quantum superposition of the whole diamond. By applying a magnetic field, it is possible to separate the two quantum states. When these quantum states come together again by turning off the magnetic field, they create an interference pattern.
This diamond is small enough to support this layering, but it is also large enough to be affected by the pull of gravity. When two of these diamonds are placed next to each other in free fall conditions, they only interact through the force of gravity between them. The experiment was originally designed to test whether gravity itself is a quantum phenomenon. Simply put, since entanglement is a quantum phenomenon, the entanglement of two objects that only interact through gravity would serve as proof that gravity is a quantum phenomenon.
Any moving mass will have an effect on this very sensitive quantum system. In their latest article, Mazumdar and colleagues describe how these disruptions can be reduced. However, it also appears that this system could be used to detect moving masses. The first source of noise is the collision of gas with the free-falling experimental capsule. Even the impact of photons can create a disturbance. “Our calculations show that these effects are minimized by placing the experimental capsule inside a larger container, which creates a controlled environment,” explains Mazumdar. Inside such an outer container, this noise is negligible at a pressure of 10-6 Pascal, even at room temperature. The requirements for the conditions inside the experimental capsule are more stringent. Currently, scientists estimate a required pressure of 10-15 Pascal at about 1 Kelvin. Given the current state of technology, this is not yet feasible, but Mazumdar expects it to be possible within 20 years.
Moving objects, even as small as a butterfly, located near the experimental site are a second source of noise. Calculations show that this noise can also be mitigated relatively easily by limiting access to the experimental site. People must maintain a distance of at least 2 meters from the experimental site and cars must maintain a minimum distance of 10 meters from the site. The passage of planes more than 60 meters from the experimental site would not pose a problem. All of these requirements can be easily fulfilled.
Once the experiment is operational, its scope could be extended beyond an investigation into quantum gravity, according to Mazumdar. “You could put him in a spaceship, where he’s free-falling all the time. Then you can use it to detect incoming space debris. By using several systems, it would even be possible to obtain the trajectory of the debris ”. Another option is to place such a system in the Kuiper Belt, where it would detect the movement of our solar system in space. “And it could detect any black holes nearby,” Mazumdar adds.
Back on Earth, the quantum system would be able to detect tectonic movements and perhaps provide early warnings in the event of an earthquake. And, of course, the quantum system’s sensitivity to any movement occurring nearby would make it an ideal, if somewhat complex, motion sensor and burglar alarm. But for now, the goal for the next decades is to determine if gravity is a quantum phenomenon.
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