Physicists experimentally registered the Casimir effect

Physicists experimentally registered the Casimir effect

Physicists from Sweden, USA and Japan described the first case of observing the dynamic Casimir effect.

Recall: in quantum field theory, the vacuum is considered not as an absolutely empty space, but as a region in which virtual particles are constantly being born and disappeared. Scientists quickly realized that these vacuum fluctuations could produce measurable results – for example, a Lamb shift, a shift in the energy levels of bound states of an electron in an external field.

The creation and disappearance of virtual particles is also associated with the “usual” Casimir effect, which is most often explained by the example of two mirrors (conducting uncharged plates) placed in a vacuum. When approaching, such plates, as Hendrik Casimir found out in 1948, should experience a noticeable mutual attraction. We can say that the creation of virtual photons is suppressed in a narrow gap between uncharged surfaces, and in the rest of the space – is not limited by anything. Since the photons exert pressure on the mirrors, the gravitational force is recorded in the experiments.

The dynamic Casimir effect, predicted about forty years ago, has the same physical basis and is described as the creation of real photons from a vacuum in a nonstationary cavity. Nonstationarity can be ensured, say, by the movement of the wall bounding the cavity, and it must move at a speed, the ratio of which to the speed of light is large enough.

It is extremely difficult to implement such an experiment using some kind of material mirror that performs mechanical motion: the ratio of the velocities will be low, the energy consumption will be huge, and the frequency of the appearance of photons will be scanty. The authors, of course, knew about this and developed their own method, devoid of the indicated drawbacks.

The SQUID used in the experiment under a scanning electron microscope (illustrated by the authors of the work).

The main elements of the proposed experimental scheme were the transmission line and the simplest superconducting quantum interference device (SQUID) located at its end. It is a ring with two Josephson contacts – superconductors, separated from each other by a thin dielectric layer. In our case, this superconducting structure is used as a variable inductance, for which the value of the external magnetic flux through the SQUID loop is varied in the experiment.

Such changes can be thought of as fluctuations in the electrical length of the transmission line. Variation in electrical length is mathematically analogous to mechanical movement.

As practice shows, the effective speed of the “mirror movement” in this scheme (the rate of change in the electrical length) can be made large enough, which increases the frequency of appearance of particles by orders of magnitude. Having performed the necessary calculations and cooled the installation to 50 mK, physicists tried to register the emerging quanta of electromagnetic radiation – and found the desired microwave photons.


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