Abstract
What could be more amazing than teleportation? An object disappears here, only to reappear over there without ever passing through any intermediate points! The communication technologies have sometimes given consideration to this. An email leaves my computer and a few seconds later appears on the computer screen of a friend on the other side of the world. But for an email, we know very well that a whole network of Wi-Fi signals, electrons in copper cables, and photons in optical fibres must have carried my email continuously from one point to the next through space until it reached its destination. Teleportation is quite different, because the object ‘jumps’ directly from here to there without anything passing through any intermediate point. It smacks of magic, or science fiction. Unless there is some way of exploiting quantum nonlocality, this uncanny link between distant places.
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Notes
- 1.
Here is a little story to illustrate the ground that has been covered since the beginning of the second quantum revolution in the 1990s. In 1983, when I was a young Postdoc in theUnited States, an important professor came up to me with a broad smile on his face and informed me that he had saved my life. He admitted having been the referee for one of my first scientific publications, in which I had committed the unforgivable blasphemy of writing that, in quantum physics, it seemed possible “that a system might disappear here and reappear somewhere else”. Today that is reminiscent of teleportation, but in reality I was not thinking of that at all. It was just an intuition.My ‘saviour’ had accepted my paper for publication solely on the condition that the above-mentioned blasphemy be removed. At the time, my assertion would have earned me universal disapproval! One may wonder how many opportunities have been lost thanks to worthy professors endlessly insisting that Bohr had sorted everything out. How many talented young minds may have left physics as a result? And how many great professors still insist that Bohr really did sort everything out?
- 2.
Bennett, C.H., Brassard, G., Crepeau, C., Jozsa, R., Peres, A. and Woottters, W.K.: Teleporting an unknown quantum state via dual classical and Einstein-Podolsky-Rosen channels, Phys. Rev. Lett. 70, 1895–1899 (1993).
- 3.
This was how we began our proposed publication on the first long range teleportation experiment. However, the editors of the famous journal Nature rejected any citation going back as far as Aristotle! I strongly urged my students to give up the idea of publishing in Nature, but the pressure was too great and in the end we gave in to the editors’ demands. I. Marcikic, H. de Riedmatten, W. Tittel, H. Zbinden, N. Gisin: Long-distance teleportation of qubits at telecommunication wavelengths. I, Nature 421, 509–513 (2003) (submitted paper arXiv:quant-ph/0301178).
- 4.
For a photon with well-defined polarisation, there is a polariser through which the photon is sure to pass. On the other hand, a completely unpolarised photon always has a one in two chance of passing through any polariser, whatever the orientation of this polariser. In the first case, the photon carries a structure that a polariser can confirm, while in the second, the response ‘goes through or doesn’t go through’ is always 50–50, whatever the polariser, whence the photon has no such structure.
- 5.
As we saw in Chap. 6, the energy of a photon can be indeterminate. In fact, the same goes for the mass, e.g., of a Bose–Einstein condensate. The important thing is that the substance, mass or energy, should already be present, at least potentially, at the destination.
- 6.
For physicist readers, this is true if we teleport all the photon’s characteristics. If we only teleport its polarisation, the photons will only be indistinguishable if their other characteristics, such as their spectra, are already indistinguishable at the outset.
- 7.
Here I must admit to the reader that there are many entangled states. Up to now, to simplify, I have always spoken of entanglement in which the same result is invariably produced for the same measurements. However, there are other entangled states. For example, there are some in which different results are always produced for the same measurements. And in fact, there are many others too, but we shall have no need for them in the present account. For physicist readers, there are four orthogonal states of maximal entanglement for the polarisation of two photons. For each of these, Bob can apply a rotation (a unitary transformation) to the polarisation of his photon in such a way that it ends up in precisely the initial state of Alice’s photon, but still without actually knowing what that state was.
- 8.
To do this, he must rotate the photon state. For example, if the qubit is polarisation encoded, he must reverse the polarisation state using a birefringent crystal.
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Gisin, N. (2014). Quantum Teleportation. In: Quantum Chance. Copernicus, Cham. https://doi.org/10.1007/978-3-319-05473-5_8
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DOI: https://doi.org/10.1007/978-3-319-05473-5_8
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