It will be shown although the results from quantum eraser experiments may seem to be mysterious they are no more mysterious than the results from classical two slit experiments. ........................................................................................................................

Part 1. Classical Observations

In the classical two slit experiment we can make various observations including the following:

1. If light has access to the screen through one slit only a diffraction pattern will be observed

2. The diffraction patterns due to each slit separately are centred at different locations.

3. If light has unhindered access through both slits equally an interference pattern will be observed. The pattern will contain alternate bright and dark fringes modulated by a diffraction envelope.

4. If an interference pattern is observed and changes are made such that the access through one slit is disrupted and reduced we would observe a reduction in the visibility of the interference pattern, for example the regions of destructive interference will become brighter and the regions of constructive interference will become less bright. If the disruption is total the only thing remaining would be the diffraction pattern due to the undisrupted slit.

4. If the intensity of the incident light is reduced a corresponding reduction in the intensity of the observed pattern will be reduced.

Similar results to those above will be observed for very low intensity illuminations including those which can be described as “one photon at a time” illuminations. In some low illumination

 Introduction to Quantum Erasers

 We shall consider two different quantum eraser experiments. Both experiments used coincidence counting methods and entangled photons generated from a BBO crystal illuminated by a laser. The resulting illumination was very low.

The experiments also used “which way detectors”. With these detectors it’s possible to obtain “which way information” in other words to observe through which slit each photon passes. It’s assumed that if it’s possible to obtain which way information it’s impossible to observe interference. That might seem to be weird.

But are quantum eraser experiments really weird? In this paper it will be shown they are no weirder than classical experiments. In fact the results obtained from quantum eraser experiments can be explained in terms of classical physics.

The first experiment was published in 2000 and the second experiment published in 2002. Readers are advised to familiarise themselves with the details of these experiments before reading the notes that follow.

 The 2002 experiment

In this experiment one from each pair of entangled photons, the signal photon, was directed towards a double slit. A detector behind the double slit was used to make observations of the pattern produced by the arrival of the signal photons. The second photon of each entangled pair, the idler photon, was sent to a second detector. The two detectors were linked by means of a coincidence counter.  

 The observations made can be explained in terms of classical physics:

Observation 1. Photons were allowed undisrupted access through the two slits and an interference pattern was revealed. This should be expected since two parallel coherent linear polarised waves can interfere.

The patterns produced by the horizontally polarised waves and the vertically polarised waves were in phase and they combined to give the resultant pattern.

 Observation 2. Quarter wave plates were used one in front of each slit. The plates were arranged so that the two sets of waves emerged from the slits in orthogonal states of circular polarisation. Because of the arrangement used the ordinary waves from the first slit had the same plane of polarisation as the extraordinary waves from the second slit and therefore the two sets of waves interfered. In a similar way the ordinary waves from the second slit interfered with the extraordinary waves from the first slit.

Because of the phase differences between the two sets of waves the resultant interference patterns were 180 degrees out of phase and there was overlap between the regions of constructive and destructive interference. The resultant pattern which was observed resembled a single slit diffraction pattern.

 Observation 3. When a polariser was placed between the idler and the second detector and with its axis parallel to an axis of one of the plates, one of the interference patterns was clearly observed. When the polariser was rotated through ninety degrees so that its axis became parallel to the corresponding axis on the second plate, the second interference pattern was clearly observed.

This result should be expected because the plane of polarisation of the signal photons was correlated with the plane of polarisation of the polariser. When the polariser axis was parallel to an axis of one of the quarter wave plates, one of the sets of o and e signal waves passed through the slits with minimum reduction of amplitude whilst the other set had maximum reduction of amplitude. When the polariser was rotated the pattern changed from one to the other due to the changes of amplitude of both sets of o and e waves. For a rotation of 90 degrees the set of waves that previously had minimum reduction of amplitude now had maximum reduction of amplitude and vice versa.

Quantum theory gives different explanations for observations 2 and 3:

It was assumed that the placement of the quarter wave plates enabled the possibility of gaining which way information and therefore interference was not observed. The subsequent placement of the polariser erased the possibility of gaining which way information and therefore interference was observed.

The 2002 experiment can be considered as a variation of earlier experiments and in particular an experiment carried out by Piano and Pescetti in the 1970s. The earlier experiment was more classical in construction. It used a screen and the illumination was high. The polariser was placed between the double slit and the screen.

The observations made in the 1970s experiment were explained in terms of the generalised laws of interference according to Fresnel and Arago. The relevant laws can be summarised as follows:

Two linear waves in orthogonal states of elliptical (and therefore circular) polarisation cannot interfere unless bought into the same plane.

The experimenters seemed to confirm these laws because when they used the quarter wave plates they did not observe any two slit interference patterns but when they used the polariser they did. All relevant observations made were basically the same in both experiments.