Interaction-free quantum measurement
In the macroscopic and “traditional” microscopic world, every measurement causes a disturbance of the observed state. However, quantum effects in the microscopic world of quanta allow objects to be recognized without having to expose them to a single light quantum. This does not change the object to be measured. This process is known as non-interacting quantum measurement.
It was first proposed in 1993 in a thought experiment by Avshalom Elitzur and Lev Vaidman and was first demonstrated experimentally in 1994 by a group led by Anton Zeilinger .
prehistory
Dennis Gábor , the discoverer of holography , made the claim in 1962 that an object can only be observed if this object is supported by at least one photon , i.e. H. a light quantum is hit.
In the classic notion of photons as particles, this is also absolutely correct and is still used today as the common explanatory model. In quantum physics, however, this problem can be partially avoided.
Thought experiments
A well-known thought experiment can be found in Greek mythology . Here Perseus was faced with the task of killing Medusa . The problem with that was that anyone who looked at Medusa turned to stone. With his eyes closed, it was hardly possible for an attacker to locate Medusa. Perseus solved this problem by holding his shiny shield up to Medusa so that Medusa saw herself and froze.
Another example is the shell game . Here a small object (e.g. a marble ) is hidden under one of several hats and cannot be recognized by an observer. In the thought experiment , this object should crumble into dust as soon as the cap is removed and the object comes into the light.
The interaction-free quantum measurement can solve these problems. With the interaction-free quantum measurement one could always win in the shell game. Perseus would have been able to locate Medusa without looking at her.
The double slit experiment
The principle of non-interacting quantum measurement uses the wave properties of light. If two such waves intersect, an interference occurs, as can be seen from the double slit experiment . In the particle model, a light stripe on the screen behind the slit is a point where a photon is very likely to occur; a dark, poorly exposed stripe on the screen, on the other hand, is a place where a photon is less likely to occur.
According to the principles of quantum mechanics , interference with a single light emitter, as in the double slit experiment, only occurs if there is more than one path a photon can take to get to a certain location.
In the double slit experiment, the light quanta can either fly through one or the other slit. If you can determine which path a light quantum takes, there is no stripe pattern, but lighting alternately from one side and then the other.
Elitzur-Vaidman scheme
The physicists AC Elitzur and L. Vaidman from Tel Aviv University in Israel imagined in their thought experiment a bomb that explodes when a quantum hits it. They succeeded in developing a method that is free of interaction in half of all measurements. The experimental set-up is also referred to as a crack test or bomb test.
In the method devised by Vaidman and Elitzur, the photon passes through a Mach-Zehnder interferometer . The photon beam from a laser is split into two beams with the help of a semi-transparent mirror. The two beams then meet again in another semi-transparent mirror via two deflection mirrors and can thus interfere. In this respect, the Mach-Zehnder interferometer is similar to the double-slit experiment.
Beam path without an object
The figure above shows the beam path without an object in the beam path. The part of the lower beam reflected in the second beam splitter combines with the part of the upper beam passing from the left and hits the right detector. The part of the lower beam that passes in the second beam splitter also combines with the upwardly reflected part of the upper beam and hits the upper detector. If the interferometer is suitably dimensioned, interference of the beams that meet it occurs . The rays are amplified in the direction of the detector on the right, up to about the intensity of the source, while in the direction of the upper detector they are canceled. This is caused by a different number of reflection-related phase shifts in the beam paths. The reflections on the half mirrors only shift the phase of the light by 180 ° if the light falls directly on the interface between the vacuum and the silver coating. If the light hits the silver coating only after it has passed through the glass substrate, there is no phase shift. With the appropriate alignment of the half mirrors, light that is phase-shifted by 360 ° meets light that is phase-shifted by 180 ° in the upper detector; the phase shift between the two beams is therefore 180 ° and they interfere destructively.
From a classic point of view, the wave character of light comes to the fore here. From a quantum mechanical point of view, it is objectively indeterminate which path a photon takes as long as this is not determined by a measurement.
Beam path with object
In the figure below, an opaque object is inserted into the upper light beam. This prevents the upper light beam from reaching the second semi-transparent mirror. This means that there is no interaction between the two light beams. Thus, only the lower light beam reaches the second beam splitter and is divided there in equal parts. Both detectors respond with the same intensity.
Alternatively, the photon can also be viewed as a particle. In the first beam splitter, the photon then takes either the lower path or the upper path with a probability of 50%. When the photon takes the upper path, it hits the target and is absorbed. The photon then does not reach either of the two detectors. If the photon takes the lower path, it can hit both detectors again, since the photon can take both paths from the upper semi-transparent mirror.
Hitting the right detector now corresponds to the result without an object in the upper beam path and therefore does not allow any conclusions to be drawn about the presence of the object. If, on the other hand, the upper detector hits, the photon has chosen the lower path without interaction, but was able to cross the second half mirror straight ahead. This possibility could not be realized before due to the interference of the beam paths.
The single photon did not interact with the object. It didn't even get near it. Nevertheless, the photon proves the existence of the object due to the changed behavior at the second beam splitter.
By adjusting the reflectivities of the half mirrors, the hit rate of 25% can be exceeded.
| object | interaction | Detector above | Detector on the right | probability |
|---|---|---|---|---|
| unavailable | No | dark | bright | 100% |
| available | Yes | dark | dark | 50% |
| No | bright | dark | 25% | |
| No | dark | bright | 25% |
literature
- Paul Kwiat, Harald Weinfurter , Anton Zeilinger : Interaction-free quantum measurement. In: Spectrum of Science. No. 1, 1997, pp. 42-49 ( preview ).
Individual evidence
- ↑ Zeilinger, Harald Weinfurter , Paul G. Kwiat , Thomas Herzog, Mark A. Kasevich Interaction-free Measurement , Physical Review Letters, Volume 74, 1995, p. 4763, abstract
- ↑ Elitzur, Vaidman Quantum mechanical interaction free measurement , Foundations of Physics, Volume 23, 1993, pp. 987-997
- ↑ Example: Dorn / Bader Physik 12/13 Gymnasium Sek II, 2000, p. 250f