Experiment could test quantum nature of large masses for first time

An experiment presented by a team of British and Indian scientists led by UCL (University College London) could test whether relatively large masses have a quantum nature, thus resolving the question of whether quantum mechanical description works on a much larger scale. greater than that of particles. and atoms.

Quantum theory is generally considered to describe nature at the smallest scales, and no quantum effects have been observed in the laboratory for objects more massive than about a quintillionth of a gram, or more precisely 10.^-20g.

The new experience, described in an article published in Physical Examination Letters and involving researchers from UCL, the University of Southampton and the Bose Institute in Calcutta, India, could, in principle, test the quantum character of an object regardless of its mass or energy.

The proposed experiment exploits the principle of quantum mechanics according to which the act of measuring an object can change its nature. (The term measurement includes any interaction of the object with a probe, for example whether light shines on it or whether it emits light or heat).

The experiment focuses on a pendulum-like object swinging like a ball on a string. A light is projected onto half of the oscillation area, revealing information about the object’s location (i.e. if no scattered light is observed, then it can be concluded that the object is not in this half). A second light is on, indicating the location of the object further along its swing.

If the object is quantum, the first measurement (the first flash of light) will disrupt its trajectory (by measurement-induced collapse – an inherent property of quantum mechanics), thus changing the probability of knowing where it will be in the second flash of light, whereas if it is classical, then the act of observation will make no difference. Researchers can then compare scenarios in which they flash twice to those in which only the second flash of light occurs to see if there is a difference in the final distributions of the object.

Lead author Dr Debarshi Das (UCL Physics & Astronomy and Royal Society) said: “A crowd at a football match cannot affect the outcome of the match simply by staring. But with quantum mechanics, the act of observation or measurement itself changes. the system.”

“Our proposed experiment can test whether an object is classical or quantum by seeing whether an act of observation can lead to a change in its motion.”

According to the researchers, the proposal could be implemented with current technologies using nanocrystals or, in principle, even using mirrors from the LIGO (Laser Interferometer Gravitational-Wave Observatory) in the United States, which have an effective mass of 10 kg.

The four LIGO mirrors, which each weigh 40 kg but vibrate together as if they were a single 10 kg object, have already been cooled to the minimum energy state (a fraction above absolute zero ) which would be required in any experiment seeking to detect quantum behavior. .

Lead author Professor Sougato Bose (UCL Physics & Astronomy) said: “Our system has broad conceptual implications. It could test whether relatively large objects have definite properties, that is, whether their properties are real, even when we don’t measure them. the field of quantum mechanics and determine whether this fundamental theory of nature is only valid at certain scales or whether it is also true for larger masses.

“If we do not encounter a mass limit in quantum mechanics, this makes the problem of reconciling quantum theory with reality as we experience it even more acute.”

In quantum mechanics, objects do not have defined properties until they are observed or interact with their environment. Before observation, they do not exist in a defined location but can be in two places at once (superposition state). This led to Einstein’s remark: “Is the moon there when no one is looking at it?”

Quantum mechanics may seem at odds with our experience of reality, but its insights have contributed to the development of computers, smartphones, broadband, GPS and magnetic resonance imaging.

Most physicists believe that quantum mechanics is valid on larger scales, but is simply more difficult to observe because of the isolation required to preserve a quantum state. To detect the quantum behavior of an object, its temperature or vibrations must be reduced to their lowest possible level (its ground state) and it must be in a vacuum so that almost no atoms interact with it. This is because a quantum state will collapse, a process called decoherence if the object interacts with its environment.

The proposed new experiment is a development of an earlier quantum test designed by Professor Bose and colleagues in 2018. A project to conduct an experiment using this methodology, which will test the quantum nature of a nanocrystal with a billion atoms , is already underway, led by the University of Southampton.

This project is already aiming for a leap in terms of mass, with previous attempts to test the quantum nature of a macroscopic object limited to hundreds of thousands of atoms. The recently published project could be carried out with current technologies using a nanocrystal containing billions of atoms.