In 1994, a paper appeared in Scientific american¹. In this paper it was described how one could obtain which way information (namely through which slit the particle went through) without a remarkable transversal momentum transfer. In this experiments the particles that go through the slits are not electrons but whole atoms. Instead of using photons of visible light to detect the atom they used 2 microwave cavities each behind one of the slits.(See Figure 11)

Figure 11 -double slit experiment with 2 microwave cavities (one behind each slit)

The interaction between the atom and the microwave photon in one of the cavities reveals the wich way information. The result is the destruction of the interference pattern. If we stick to the Heisenbergs interpretation of uncertainity principle as described in previous chapter, we have a problem. The photon wave-length is of the order of millimeters and therefore the momentum of the photon and therefore the transversal momentum transferred to the atom is negligibly small, much smaller then the momentum transfer expected by the uncertainity relation to. That the interference pattern is destroyed despite this fact shows that the viewpoint “The reason for the destruction of the interference pattern is the randomly change in flight direction of the particle because of the inevitable transversal momentum transfer namely the transversal kick during position measurement” can not hold any more.

The authors claim that this leads to following conclusions.

1.That the uncertainity principle was not as universal as thought before, since one could get the which way information almost without disturbing the particle.

2. That the destruction of the interference was not due to the transversal momentum transfer but merely due to the obtained which way information. This shows that wave-particle duality is something more fundamental then the uncertainty

Concerning their conclusion-1 I would like to make following remarks:

The experiment doesn’t show that the uncertainty principle is violated but that Heisenbergs version of interpreting the uncertainity principle as discussed in chapter 9 is wrong.

This soberly interpreted form of the uncertainity principle is of course not violated by the quantum demolition experiments. As we said chapter 9 That the uncertainity of transversal momentum increased doesnt mean necessarily that a momentum transfer occured. A transversal momentum transfer would not change merely the uncertainity of momentum but the expectation value of transversal momentum namely its mean value.

Concerning their conclusion-2, I would like to say the following:

The destruction of the interference picture is only related to destruction of the phase coherence between the beams from both slits. It neither emphasizes the special role of information (namely the obtained which way information) nor does it support the concept of duality. On the contrary being a consequence of the relative phase shift, to me the experiment is the verification of the wave-function only viewpoint. The only mystery is why this phase shift (the detection of the particle) occurs rondomly each time in one or the other cavity. Pointlike particle, Duality , information are concepts that are completely inapropriate in a discussion to understand this experiment.

1 Englert 1994