Quantum gum

In Quantum physics, a quantum gum is a generic term to indicate a device making it possible to restore a quantum state of superposition whereas this one was deteriorated or removed. This device is used in a certain number of experiments aiming to illustrate and explore certain properties of quantum mechanics, in particular the quantum Intrication and the seemingly instantaneous influences EPR, even retro-temporal.

Indeed, an essential characteristic of a quantum gum (as any physical phenomenon bringing into play quantum intrication) is to be able to influence a quantum state in a way not-local, i.e. remote, even at very long distance, and without effect of the distance on the experiment. One can interpret the fact that the distance is without effect as being a testimony of an instantaneous influence, even coming from the future (but of other interpretations are possible). At all events, the influence is always done in an absolutely undetectable way without contribution of nonquantum information (typically of the correlations) coming from the remote device. This influence is thus done without violation of the Causalité, nor of impossibility for a signal of exceeding speed of light and thus respects the known principles resulting from the restricted Relativité and general.

Introduction

Before including/understanding the stakes and the operation of a quantum gum, it is necessary to briefly point out a fundamental experiment in quantum physics: the Slits of Young. In this experiment, individual Photon S are sent towards a double slit. With the crossing of the slits, the photon is in a quantum state of superposition of two states: 1) the photon passed by the slit of right-hand side 2) the photon passed by the slit of left. This state of superposition is highlighted by the appearance of a figure of interference on a photographic plate located behind the two slits.

This figure of interference appears only if one does not seek to know by which slit passed the individual photon. If a detector is placed on the slits, or if one uses a direct or indirect unspecified means allowing to know by which slit the photon (information " passed; which path" , or " which way" in English), the state of superposition disappears (the two possibilities are not more possible) and the figure of interference is destroyed.

An alternative of the experiment described above brings to quantum gum. If, instead of to detect the way on the level of the passage of the slits, one mark simply the photons with the passage of the slits (without making a real measurement at this point) to have the possibility of detecting, if it is wished, by which slit passed the photon: which is the consequence on the figure of interference? In this case also, simple the possibility of being able to determine the way of the photon is enough to destroy the figure of interference.

The point above is very important: a quantum measurement (causing a Décohérence and a Reduction of the package of wave) cannot be " gommée". There does not exist any " gum quantique" allowing to erase the consequences of a measurement. On the other hand a " marquage" can it be " gommé" ?

The quantum gum

In 1982, Marlan Scully and Kai Drühl wondered which would be the consequences if, before (and especially after, or elsewhere, in certain experiments!) to come to impress the photographic plate, information " which way" was physically scrambled so as to never be able again to determine by which slit the photon passed. The quantum state, faded a marking of information " which way" , would be restored so as to allow the interference again? This " brouillage" physics is what one calls a quantum gum.

The quantum gum seems, at first sight, commonplace and without private interest: a faded quantum state is positioned back by the action of a device. But it is not it any more, and it is what Scully and Drühl had provided, if the quantum gum is setting in motion after that the photon impressed the photographic plate, or if it is implemented in a nonlocal way, on an intricate photon with the photon of the photographic plate.

In 1992 an experiment was carried out to test this case of figure:

A transmitter of Photon has, emits individual photons towards a " converter bas" B. a " converter bas" two intricate photons starting from a photon in entry re-emits. One of the photons, called photon signal , is directed towards a device of Fentes of Young C. polarizing filters D and E are placed behind the slits in order to " marquer" the passage of the photon by one or the other slit. The photon is then detected on a photographic plate out of F.

In parallel, the other photon, called pilot photon , is emitted towards a detector of polarity in H. In G, is located the device of " gum quantique" , removable, about which we will speak further. Initially, we will admit that the gum is absent. Detection out of H then makes it possible to know by which slit the photon passed, by effect EPR. Indeed, the polarity taken by the photon signal in D or E influences the measured polarity of the pilot photon (see also Expérience of Aspect on this point). Naturally, this detection out of H scrambles the figure of interference out of F.

If, now, a polarizer is placed in G, the polarization measured out of H will be then always the same one, whatever the polarization taken by the photon signal in D or E. It thus becomes impossible to know by which slit the photon passed. This simple fact restores the figure of interference out of F . It is this polarizer which is described as quantum gum .

If distance BG is higher than distance BC, the quantum gum intervenes after that the photon signal impressed the photographic plate. How the photon signal " it knows " what a quantum gum was placed in G, in order to form a figure of interference or not? There is all the difficulty of the interpretation of this experiment.

A reproach (which had also been addressed to first experiments EPR, and which was solved by the Expérience of Aspect) can be addressed to this device: even if distance BG is higher than BC, if the polarizer G is there as of the emission of the photon in has, then it becomes possible to imagine that its presence influences on what occurs out of F. Indeed, G and the detectors H and F are connected physically within the same hardware device, because it is necessary to know to which photon out of F corresponds a photon detected in H. Donc of the artificial correlations, nonquantum (loops of mass.), could appear between H and F. to evacuate any possibility of nonquantum correlation between G/H and F, the ideal would be to place/remove G after the photon signal was detected out of F.

This is why a quantum experiment of gum to choice delayed was tried by Marlan Scully itself, where a quantum gum enters or not in action after the recording out of F: it is the Expérience of Marlan Scully.

See too

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