# Thread: Coherence length of a single photon

1. What is the coherence length of a single photon? I suppose that is infinity, because the bandwidth (of a single photon) is zero. It is anyway very large, because there are lasers with very long coherence length. Because their interference in a double slit is the same as for single photons, also the coherence length of a single photon should be at least very last

2.

3. Originally Posted by DParlevliet
What is the coherence length of a single photon? I suppose that is infinity, because the bandwidth (of a single photon) is zero. It is anyway very large, because there are lasers with very long coherence length. Because their interference in a double slit is the same as for single photons, also the coherence length of a single photon should be at least very last
It's quite the opposite. Quantum states with precise photon numbers are incoherent.

4. But single photons in a double slit interfere (with itself).

5. Originally Posted by DParlevliet
But single photons in a double slit interfere (with itself).
The double-slit experiment uses a coherent source. The photon number is indefinite. Individual photons are only detected at the detector. What interferes is the coherent quantum state from both slits, not individual photons. And this remains true regardless of how low the intensity of the beam is.

6. So what is the coherence length of those quantum states? Or: what can be the different length of both paths while there is still interference on the detector (with single photon)?

7. Originally Posted by DParlevliet
So what is the coherence length of those quantum states? Or: what can be the different length of both paths while there is still interference on the detector (with single photon)?
I'm guessing that depends on the source.

8. That is what I did mean in my first post. If the wave of a photon is split, like in a double slit or a beam splitter, those waves have a infinite coherence length against each other. In other words, even with largely different path lengths, they will be abale to interefere. That is obvious, because the wave length of both waves is exactly the same.

9. Originally Posted by DParlevliet
That is what I did mean in my first post. If the wave of a photon is split, like in a double slit or a beam splitter, those waves have a infinite coherence length against each other. In other words, even with largely different path lengths, they will be abale to interefere. That is obvious, because the wave length of both waves is exactly the same.
But a coherent source, no matter how low the intensity, is not the same as a single photon. A single photon is not coherent. If you try to isolate a single photon from a coherent source, you destroy the coherence. The wavefunction of a coherent source describes the probability of finding a photon, but one can't say that there is or isn't a photon anywhere or how many photons there are. Counting them destroys the coherence. When it is said that the photons pass through the slits one at a time, this is to be taken as a statistical estimate, not a literal statement. The actual number of photons doesn't emerge until they are actually detected as part of the interference pattern.

10. Originally Posted by KJW
The wavefunction of a coherent source describes the probability of finding a photon
And what is the coherence length of this wave function against itself. If the wavefunction is split in parts, which each follow paths of different length, can those parts interfere with each other.

11. Originally Posted by DParlevliet
Originally Posted by KJW
The wavefunction of a coherent source describes the probability of finding a photon
And what is the coherence length of this wave function against itself. If the wavefunction is split in parts, which each follow paths of different length, can those parts interfere with each other.
It depends on the specific details of how the coherent source is produced. Perfect coherence implies infinite coherence length, but a real source of coherent light doesn't have perfect coherence.

12. In double slit the non-coherence is caused by different wave lengths. Now the source emitted a single photon. Will the (propability) wave interfere after the double slit at the detector?

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