July 28, 2000
Justin Mullin reported (29 July, p12) on the latest quantum magic from Geneva: Wolfgang Tittel and his team’s estimate of the speed of travel of quantum information. What he did not report, though – and I can see why, as it is not mentioned in the e-print he quotes – is that the team have yet to establish that any quantum information is involved at all!
None of the recent Geneva experiments has been accompanied by satisfactory checks, so that none has ruled out the possibility that the observed correlations are more than just an interesting consequence of perfectly ordinary shared values carried from the source.
What we are talking about here is “quantum entanglement”, and this means that we are concerned with our old friends, the EPR (Einstein-Podolsky-Rosen) or Bell test experiments that have been with us now since about 1970. They have all had loopholes, and I have now corresponded with many of the experimenters concerned, including several members of the Geneva team. I pointed out that their 1997 experiment (http://arxiv.org/abs/quant-ph/9707042) used an invalid test (they subtracted “accidentals”, which is perfectly OK in some contexts but ruins your Bell test). They agreed, and in their next experiment were careful to make sure that they did not depend on the subtraction. However, there were various other possible pitfalls, and they have not been able to convince me that they have not fallen foul of at least one of them.
The experiment that Justin’s report relies on is quant-ph/007009 (referenced from quant-ph/007008). In this it seems clear that they made no attempt to block one potentially fatal loophole! They kept one detector fixed and altered the setting of the other, which is only allowable if you have first checked that your source is “rotationally invariant”. They do not mention this.
For more on the Bell test loopholes, do visit my site (http://www.aber.ac.uk/~cat) or look at my own contributions to the quant-ph archive, notably 9611037, 9903066 and 9912082.
Sorry, folks, but quantum entanglement is a house of cards that would collapse the instant the loopholes were properly investigated. If there is no entanglement, then of course an experiment that measures its speed is pure fantasy!
Caroline H Thompson