I have read this question (no answer, just comments):

Light, including pulses of light, consists of many photons. A very short pulse has a wide range of frequencies. See The more general uncertainty principle, regarding Fourier transformsA single photon can also have a long or short duration, and a narrow or wide range of frequencies.

I have never heard of single photons with a wide rage of frequencies. Each photon is always supposed to have a single quantified energy/frequency, otherwise the quantum effects would not occur. Can you refer to studies that show single photons with a range of frequencies?

Relation between attosecond light pulses and photons?

And this:

This is very fast for a pulse of light, and it is actually so fast that the pulse of light is no longer a periodic electric-field oscillation, and instead it lasts only for a few cycles. But it is still not fast enough.

What is an "attosecond pulse", and what can you use it for?

Now we have had single photon emitters for a while and the photon is the smallest amount (quantum) of EM energy. But then the question remains, why are attosecond pulses better (for example to track electron orbitals) then single photon emissions? Very naively thinking, if you shoot single photons at the electron (atom), you are using the smallest quantum of energy to do so. Then why is attosecond pulse generation superior to single-photon emitters?


  1. How can an attosecond pulse be shorter than a single photon (quantum) emission?

1 Answer 1


"Single photon" means "anti-bunched" photons. It refers to the property where photons are emitted one at a time, with a reduced probability of emitting two or more photons simultaneously. It has nothing to do with the length of a pulse and it doesn't help you to confine when these anti-bunched photons interact with a probe. It's a property of the relation between photons.

Single-photon sources are designed to emit photons one at a time, but the emission process is governed by quantum mechanics, which inherently includes probabilistic behavior. This means that while you can control when the source is active and capable of emitting photons, the exact instant a photon is emitted within that period is not deterministically predictable. The activity-window can be long compared to the average time between emissions.

There is no classical representation of this emission in terms of a specific electromagnetic pulse propagating through space.

You don't know when a photon left the building (within the window of activity).

A laser pulse on the other hand is a burst of classical coherent light with a known length (e.g. by quickly switching the resonator on and off). You know when it left the building. You know when it hits the probe. So you can use it to probe the probe over time...

The only meaningful lengths that you can compare are given by the respective deterministic and measurable activity windows (and not the "photon length").

  • $\begingroup$ thank you so much! $\endgroup$ Nov 16, 2023 at 5:22

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