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There are a lot of questions on this site about photons and wavepackets.

Relation between radio waves and photons generated by a classical current

This one lists a lot of them as reference. None of them in detail specifically answers my question.

What happens when a photon hits a beamsplitter?

I have read this:

https://arxiv.org/ftp/arxiv/papers/1410/1410.3416.pdf

A wave.packet enters the beam splitter and breaks into two smaller wave.packets. The incident particle, shown as a dot in picture, emerges along path a accompanied by a wave packet. The wave packet in path b is empty.

I am not asking about the photon traveling as a wave, or a particle, or why the photon is delocalized when traveling and why it is taking all possible paths. I am not asking what happens when a photon hits a beam splitter.

I am asking if it is really possible what the paper suggests, that there can exist a wavepacket, without a photon, that is, where the photon number is 0.

In physics, a wave packet (or wave train) is a short "burst" or "envelope" of localized wave action that travels as a unit.

https://en.wikipedia.org/wiki/Wave_packet

The best definition of a wavepacket I found on this site is saying that a wavepacket is basically a localized excitation of the EM field. This is a contradiction, because the photon itself is usually defined as the excitation of the EM field. How could there be a wavepacket (a localized excitation), without a photon (excitation of the EM field)?

Question:

  1. What is a wavepacket without a photon?
Árpád Szendrei
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2 Answers2

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Wave packet is a notion that applies to any type of waves, not necessarily light waves and not necessarily quantized waves (while photons are quanta of electromagnetic waves). There is no rigorous definition of a wave packet, but usually it is a combination of waves that can be treated as rather localized in time or space or both. This necessarily means that its spectrum is continuous and is usually a broadened peak around a central frequency.

The arXiv paper cited deals with a specific interpretation of quantum theory: in this interpretation the particle (photon) and wave properties are supposedly separate. With such a definition one may have concievably a wavepacket without a photon... but this falls out of the mainstream understanding of photons.

Roger V.
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A wave packet is just a localized superposition of waves. In the context of QM it describes the probability of detecting a particle like any other quantum wave function.

my2cts
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