How can one explain the quantum beat?



The quantum beat is a phenomenon of quantum theory that proves that particle states can be superimposed. While a particle in classical physics can only have either a lot of energy or little energy, in quantum physics it can bothTake states at the same time. This superimposition of states is clearly described in the thought experiment of Schrödinger's cat.

Bohr's model of the atom says that electrons can only move in very specific paths in the atom, which correspond to different energies. This is a simple first model that explains a lot. But basically it's wrong. Just as the electron goes through both gaps in the double slit experiment, it can also be located in the atom on both the upper and the lower path. So it can have a lot and little energy. To do this, you first take a gas of atoms and briefly irradiate it with photons of low energy, which the electrons of the atom can lift from the ground state, i.e. from the innermost electron path, to the second innermost. Shortly afterwards, the electrons return to their ground state and emit a photon of the same energy as the one that was recorded. The radiation decreases evenly (exponentially), similar to the radioactive decay of atoms. We can also carry out the same experiment with photons of higher energy, so that the electrons are lifted from the ground state to the third electron orbit. The result remains the same, only that naturally more energetic photons are emitted.

Now you can also irradiate the gas with both types of photons at the same time. You then do not know which path the electrons will take. In fact, it is lifted on both orbits, and each of the emitted photons has both energies. This seemingly absurd remark can be illustrated as follows: If two waves with different wavelengths are superimposed, beats occur as we know them from tuning instruments: If the tone of the instrument is close to the reference tone, the overall tone sounds periodically louder and quieter. The closer the two tones are together, the longer the period between loud and quiet.

Exactly the same effect can also be observed with the photons of the above experiment. Photons of different energies have different wavelengths. If the atoms are excited with both types of energy at the same time, the different waves overlap and the photons no longer appear uniformly, decreasing logarithmically, but periodically alternating, sometimes heaped, sometimes almost not. The superposition of two energy states thus leads to beats, i.e. at certain times suddenly no more photons are emitted at all.

One might wonder why every single one Atom have both energies and both frequencies. Couldn't half of the photons have a low frequency and the other half a higher frequency? In fact, you can measure the energy of individual atoms. If you do this, you will always find the electron either on the outer or on the inner orbit. Only then the beat disappears at the same time, just as in the double slit experiment the interference disappears when you measure the slit through which the electron passes. The beat occurs only when it is uncertain what energy the individual atoms have. If there were a mixture of some atoms in the high-energy state and some in the lower-energy state, then there would be no beating. Such a mixture looks completely different from a set of atoms that are all in both states at the same time.

Incidentally, today a single atom can be put into two different excited states at the same time in this way. This effect plays an important role in quantum computers. The quantum beat described above is an example of a phenomenon that is called Schrödinger's cat in many physics books.

Category: quantum physics