# One-photon-at-a-time effect on double slit experiment with detectors?

I do not have a access to a physics lab, nor have I come across this type of a double slit experiment, hence I am curious if someone has tried this.

Given a double slit experiment setup with a detector. What happens if we have a detector at one of the slits, but we do not send one photon at a time (in other words, we send a bunch of photons without concerning ourselves about the count)?

My assumption is that we will see a particle (bullet) pattern like we would if we sent one photon at a time, is this correct?

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There is a simple demonstration for the interference pattern of one photon at a time, though it needs sophisticated lab equipment, in this fragment of a talk : teachspin.com/instruments/two_slit/index.shtml . When one slit is closed ( detector) the count on the screen doubles, which means that the photons passing through the other slit are not interfering destructively and are counting in excess at the screen. – anna v Sep 30 '12 at 3:56

If you put a detector at one of the two slits, you obtain no interference. But this doesn't depend on whether you send a bunch of photons, or one photon at a time. If both slits are open and without detectors, you will obtain interference, no matter if you send one photon at a time or a bunch of photons. The "bullet" pattern is not due to sending one photon at a time, but to sending then through only one slit.

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I wish I could accept two answers. Please see Ondřej Černotík's elaboration as well. – Tristan Sep 30 '12 at 21:20
@Tristan: I'll "share the prize" with Ondřej Černotík and I'll give him a +1 :) – Cristi Stoica Sep 30 '12 at 21:35

Theoretically speaking, Cristi Stoica is right. But we can also consider more realistic scenarios or different kinds of detectors.

If we start with a realistic detector able of detecting single photons coming through the slit, it will necessarily have limited time resolution. This means that it can detect only one photon in a certain time interval. But if there are more photons coming to the slit during this interval, they will go through undetected, thus interfering behind the slits. Therefore, the interference will still occur, but the contrast will be decreased.

Similar situation is when the detector can only distinguish between nothing coming through and something coming through. This corresponds to standard single photon detectors that cannot resolve the number of photons but detect only their presence or absence. This is equivalent to a detector with single photon resolution and small time resolution, thus leading to the same result as above.

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