# Less than one photon, what does it mean?

• suma
In summary, "less than one photon" can refer to a situation where there is an average of less than one photon per pulse, or when the number of photons is used as a measure of energy and there is less than one photon in a device. It can also refer to the wavelength of the photon being larger than the length of a cavity. In general, it is a non-intuitive concept and can only be understood by considering the quantum nature of light.
suma
Hello,

What does less than one photon mean? 0.5 or 1/2 photon what does it mean?

Nothing without the context - where have you seen this?

There is nothing preventing you from having less than one photon on average. This is e.g. always the case for quantum key distribution experiments that use attentuated lasers as sources ; the distibution is Poissionian but by attenuating the source enough you can create a situation where you get pulses with either 0 or 1 photon most of the time (but you will of course also get som pulses with 2, 3,4 photons etc). However, if you do this you will find that each pulse has -on average- less than one photon in it.

It is also true for situation where "number of photons" is a measure of the energy/h*f; I frequently do experiment where we measure devices which have less than one photon in them, and when we plot our data we have <n> on the x-axis.

It's like the average family having 2.6 children ... what does 0.6 of a child mean?

... you can also have the situation where someone says "less than one photon is inside the cavity" and then they mean the wavelength of the photon is larger than the length of the cavity.

Then again it could be something else entirely.
i.e. 2.6 kids could mean two moderately fat ones or one obese one.

Ironically, the question "n photons, what does it mean?" is just as non-intuitive, because if we have an eigenstate of n, for n large, we get a zero electromagnetic field (in the limit). We can only get the classical limit of nonzero electromagnetic field by having a state that is not an eigenstate of number of photons.

1 person
hi,

thanks i understand now

## 1. What is a photon?

A photon is a fundamental particle of light that carries energy and travels at the speed of light.

## 2. Can there be less than one photon?

No, a photon is considered to be a discrete unit of light and cannot be divided into smaller parts.

## 3. How is a photon measured?

A photon is typically measured using a device called a photodetector, which counts the number of photons that hit the detector.

## 4. What does it mean to have less than one photon?

This means that there is not enough energy present to create a full photon. It may also refer to the probabilistic nature of photons, where there is a chance that no photons will be present in a given area.

## 5. Are there any practical applications for studying less than one photon?

Yes, studying the behavior of photons at very low energy levels can provide valuable insights into the quantum nature of light and the behavior of particles on a microscopic scale. This can have applications in fields such as quantum computing and cryptography.

• Quantum Physics
Replies
21
Views
1K
• Quantum Physics
Replies
16
Views
344
• Quantum Physics
Replies
4
Views
1K
• Quantum Physics
Replies
8
Views
1K
• Quantum Physics
Replies
18
Views
2K
• Quantum Physics
Replies
4
Views
501
• Quantum Physics
Replies
25
Views
635
• Quantum Physics
Replies
15
Views
2K
• Quantum Physics
Replies
5
Views
805
• Quantum Physics
Replies
15
Views
777