The quantum eraser in the thermal interpretation

(For the background to the experiment see http://theory.gsi.de/~vanhees/faq/qradierer/qradierer.html where the experimental setup and its statistical interpretation are discussed in detail.)

An electromagnetic quantum field is produced by the source. Up to the slit the (in principle) measurable field strength <E(x)> is concentrated in a small neighbourhood of a beam.

After passing through the filter, the field strength (in the cross-section parallel to the slits) is two spherical waves. Behind the quarter-wave plates the field strength is still that of a section of the two spherical waves, but in the two halves the spherical waves are differently polarised (there are six cases altogether: each spherical wave can be left- right- or un-polarised).

All this can be objectively verified, in that one can place a detector at the position that one wishes to test for long enough to collect sufficient statistical data. The detectors measure the corresponding intensities, since these are proportional to the statistical mean of the discharges. The direction of polarisation can also be measured by putting another polarising filter in front of the detector.

If one uses coupled photon pairs (from parametric down conversion) the situation is a bit more complicated, since the system is no longer sufficiently well described by local variables. But the thermal interpretation knows about non-local variables: pair-correlations of electromagnetic fields. These play an important role even at the classical level, where they are necessary to describe polarisation phenomena classically. See for example

L. Mandel and E. Wolf,

Optical Coherence and Quantum Optics,

Cambridge University Press, 1995.

To measure pair-correlations one needs detectors placed at two different points. (This is where the non-locality comes from.)

In the thermal interpretation particles do not come into the description of experiments at all. The usual assumption that single clicks result from the arrival of single photons, in purely metaphysical, and can neither be confirmed nor refuted within quantum mechanics. In the thermal interpretation they are cast aside.

Instead of them we simply have local concentrations of fields (along a small neighbourhood of worldlines) which behave phenomenologically like particles. This is much like the way in which printed letters are local concentrations of the field of a printer's ink.

This allows all experimental discoveries to be described without difficulties by quantum theory as interpreted by the thermal interpretation.

Arnold Neumaier (Arnold.Neumaier@univie.ac.at) A theoretical physics FAQ