# Has the collapse of wave function due to observation been recorded?

I've seen pictures like this one,

which depict the outcome of the Double-slit experiment with wave-like or particle features, depending how measurement has taken place.

The graphic showing multiple 'straight' lines on the detecting screen depicts the wave-like outcome, when the measurement of particles is taken at the target screen (on left).

The graphic showing only two lines on the detecting screen depicts the wave function collapse, when an additional measurement is taken on the slits, to detect through which slit the particle has traveled (on right).

The first effect has been recorded, as can bee seen here, but has the second one?

• Likely it is not possible to observe, since the wave function collapse picture is just one of many indistinguishable interpretations of quantum mechanics. – Nanite Apr 4 '14 at 12:01
• Note that the graphic is misleading in its rather sharp lines on the screen. You might get such lines for an diffraction grating, but for a double slit it's always much more "smeared", as shown in Anna v's answer. In particular, taking a measurement about which slit a particle went through will generally not make it suddenly "fly in a straight classical trajectory". The outcome will rather be a single indistinct blob on the screen. – leftaroundabout Apr 7 '14 at 10:22

The effect you are describing in your question is known as wave-particle duality and is a form of complementarity, it has been observed in various experiments. Realisations of Wheelers delayed choice thought experiment are what I find most interesting.

In a delayed choice experiment the particles are not measured before they go through the slits but labeled so which slit they go through is known. The only time a quantum system is not disturbed by a measurement is when no new information is gained from the measurement, labeling ensures which slit the particle went through can be known without disturbing the quantum interference$^1$ of the wavefunction. In this context the purpose of any measurement would be to tell which slit a particle went through anyway.

If a particle has a label when it is detected (at the screen) there is no interference and particle-like behavior is observed. If there are no labels there is interference or wave-like behavior, even if the labels are erased after the particles pass through the slits and it cannot be known which slit they passed through.

It appears that it is not possible to see interference and know which way the particles have gone simultaneously. If there is some which way information that enables a better than blind guess at which slit the particles went through the visibility of the interference is reduced.

However, observation of wave-particle duality does not really require wavefunction collapse. Has wavefunction collapse been observed? In my opinion no, but the publicists for this Nature paper disagree. Collapse is connected to interpretations of quantum mechanics.

Collapse of the wave function would imply that the wave function is real (ontic) as opposed to only representing what we know about a quantum system (epistemic). This is an open question, some physicists think one some the other. There is no experimental evidence either way yet, until there is some physicists might say "Collapse? Ontic? Epistemic? that's all about interpretation, shut up and calculate"

If the wavefunction is purely epistemic then there is nothing real to collapse, only the state of knowledge. If it's ontic then wave function collapse would be a possibility, but even then wavefunction collapse is not required to explain quantum measurement.

$^1$Interference is like the pattern on the left in your question, the wave-like behavior.

• Concerning the following experiment, the effect has been recorded, or just simulated? en.wikipedia.org/wiki/… – Kai Hartmann Apr 10 '14 at 11:32
• Yes that experiment was performed, and wave-particle duality was "recorded" or observed (not simulated). Its different to saying the wavefunction "collapsed" though, which is an interpretation of what happens in the experiment, and is not required to explain the experimental data. – baldrik Apr 11 '14 at 2:35

You will not easily find the second one because in a sense it is trivial.

Here is the single slit and double slit pattern from wikipedia

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If you try to detect which slit the particle went through you get two single slits except for some experiments that are very careful not to disturb too much the wave functions of the setup.

If you go to a lab that has the two slit experiment or set up one as described in this video you could check it with light.

• I assume it is not possible to record the second one with visible light? – Kai Hartmann Apr 10 '14 at 11:31
• We are talking visible light here so I do not understand what you mean – anna v Apr 10 '14 at 12:08
• Yes, but what I mean is a visible pattern as in the image you provided; an actual image of a double slit pattern which does not show interference. – Kai Hartmann Apr 10 '14 at 12:51
• yes, the single slit is such a pattern and what you get if you put detectors at the slit it is two such single slit patterns. – anna v Apr 10 '14 at 13:49
• But it has not been photographed, right? You cannot take a laser beam, detect each photon at the slits, and see an actual image on a surface which shows two such single slit patterns... – Kai Hartmann Apr 10 '14 at 14:15