Quantum eraser experiment

Under a quantum eraser is meant an experimental arrangement that can change in interference experiments with quantum objects are the result of fundamental and illuminating look at some peculiarities of quantum physics. While the availability of "Which Way" information excludes the occurrence of an interference pattern, a quantum eraser can delete this information again, so then but an interference pattern can be observed. The term eraser to describe the fact that a seemingly change took place at the quantum object is completely reversed by subsequent destruction of information.

Striking about the quantum eraser, for example, the fact that its presence does not affect locally, that is, to all involved measurements, regardless of their spatial arrangement and temporal sequence. From the perspective of the Copenhagen interpretation, this shows that it is meaningless to speak without measuring a physical reality.

Since the quantum eraser is relatively simple, it is increasingly being addressed in physics textbooks and curricula.

Wave -particle duality and "Which Way" information

Quantum objects have always not only wave, but also has particle ( wave -particle duality ). Interested In an attempt similar to the double-slit experiment the pathways of individual quantum differ, the light behaves teilchenartig ( no interference ); are the ways indistinguishable, it reveals its wave character ( interference fringes). A "What - way " detector, with which one can determine the path of a quantum object, this sets the particle aspect of the quantum object, that is observed due to the presence of "Which Way" information instead of interference fringes an accumulation distribution around a single point.

Double-slit interference pattern without

Frequency distribution of the interference pattern

Frequency distribution without the interference pattern

This phenomenon is explained in the Copenhagen interpretation of quantum mechanics by the so-called collapse of the wave function. While the machine when the interference pattern occurrence is in a state where the overlap of the two possible paths, performs a measurement of the actual path ( just the "Which Way" information) to the fact that even more of these "used" is (thus no interference pattern occurs).

So if photons or electrons pass through an appropriate arrangement, such as a double-slit arrangement, the interference pattern depends on whether you can figure out which way ( through slit 1 or slit 2) took the quantum object. This is true even if the path of the quantum object is not detected even when passing through the column, but only later ( delayed measurement process ). Only if the path is objectively indeterminate in the sense that a recovery of the "Which Way" never took place - information arises behind the double slit, an interference image.

A typical example for the production of "Which Way" information are different ( orthogonal ) oriented polarizers in the two columns. A measurement of the polarization of the coming quantum particle provides the "Which - way " information, so destroy such polarizers, the interference - even if this is not due to low intensities (as in the experiment of Taylor) according to classical physics course. As quantum eraser used in this example in addition mounted in front of the screen further polarizer again nullifies this information (which will happen if its polarization direction is aligned at 45 ° intervals about the gap polarizers ).

Earlier interpretation, which is why the interference disappears

In the pursuit of the proper interpretation of quantum mechanics, Niels Bohr and Werner Heisenberg were agreed that such a disappearance of interference fringes due to the interference of the quantum object when measuring the way done. If you measure, for example, in the thought experiment of Niels Bohr the pulse of the screen, which contains the gap holes, with sufficient accuracy, and (for particle image ) to take conclusions on the path of the photon, thus creating the Bohr about the Heisenberg uncertainty principle, a position uncertainty, the interference pattern that " smeared " and thus destroyed. The coherence of the two waves is irreparably damaged as a result of measurement in order to prevent their interference. Until the 1990s, some theorists, such as Pippa Storey represented (then at the University of Auckland, New Zealand), this interpretation of the measurement process as an irreversible disorder of the object. This therefore insisted on the impossibility of Quantenradierern.

Experimental realization

Leonard Mandel et al varied the 1991 double-slit structure in such a way that they can determine the information by means of suitable crystals which path the photon took. Once you can collect this information, the interference pattern disappears. The interference pattern is produced again, if one ( eg with a semi-transparent mirror ) deletes the "Which -way information " before the photon reaches the shield.

A research group led by Anton Zeilinger demonstrated in 1994 in an apparatus in which a birefringent crystal was excited by laser radiation to emit vertically polarized photon pairs, an analyzer transmitting light at a 45 degrees inclined plane of polarization as a quantum eraser.

Based on an experiment of Marlan Scully, BG Englert, and Herbert Walther ( 1991) also presented a delayed quantum eraser in 2000 by Stephen Patrick Walborn.

Consequences of the existence of Quantenradierern

The quantum eraser shows that it is possible in quantum physics completely eliminate the traces of an observation process. You just have to make sure that even the information learned within the system - that is, before it comes to an external observer - will be deleted without a trace. This seems paradoxical then, when ( the everyday experience with macroscopic objects analog) believes that the collection of "Which -way information " the quantum object changes straight away about him a certain polarization " mitgibt " in the above example. The absence of interference then appears as a causal result of this change, one picked this change is inconceivable. According to the Copenhagen interpretation you have made ​​experiments like the quantum eraser, however, necessarily conclude that the physical reality does not exist without measurement, that is, the polarization manifests itself only when it is measured, not by the polarizer.

Applied to the everyday level would be analogous to a colorant, which is green, the hair of a person, but only acts when the first observer has actually had looked - before can be done without consequences undo the effect of the color.

Swell

• Michael Springer: wave or a particle - a test with the quantum eraser. In: Scientific American. 1/1996, Scientific American Academic Publishers, 1996.