Quantum Zeno effect

The quantum Zeno effect is an effect of quantum mechanics, in which the transition of a quantum system can be stopped by a state to another, eg by light emission of an excited atom, by repeated measurements performed. Thus, the effect is reminiscent of the Arrow paradox of the Greek philosopher Zeno of Elea.

Pictorial reasoning

When an atom decays spontaneously, this happens according to the laws of quantum mechanics, not at a predetermined time, but randomly, ie by purely statistical regularities. This event may be the easiest as a superposition ( superposition ) of the states A ( not decay ) and B ( decay ) are considered. From this superposition state, there is then a certain probability with which the decomposition occurs. In a spontaneously decaying system, this probability increases with the progression of time. Now, if looked at a certain time, if the atom is already decayed or not, one finds either condition A ( atom does not decay ) or B (atomic decay ) before. This corresponds to a quantum mechanical measurement with the basic property that only eigenstates of the measuring operator ( that is, A or B), and no overlapping conditions can be detected. The probability of encountering of state of A and B is given by the weight fraction of each measured state in the superposition, which shifts with time more and more from A to B. Due to the measurement process itself, the wave function is reduced to state A or B, then one speaks of a collapse of the wave function.

Now, if at the beginning of the single atom in state A ( not decay ), then the proportion of the state B ( decay ) is after a short time is extremely low. During a measurement, it is therefore very likely not be disintegrated. By observing it goes in this case back to the eigenstate of A ( 100% not decay ) over, and the decay process starts anew.

Overall, we thus get a decay rate that is significantly below the unobserved decay rate with frequent observation. Leaving the distances of the observations fall to zero, corresponding to a long-term observation is tantamount, as well as the decay probability approaches zero, that is, the atom continuously observed should no longer fall apart due to this observation.

The quantum Zeno effect has been confirmed by several groups around the world using methods of laser technology and nuclear physics experiments.

A German -language popular science work-up was released in 1994 according to measurements at the Ludwig- Maximilians- University of Munich: The motion of a quantum system has been proven there alone brought to a standstill by a sequence dense measurements, which underpinned the theoretical modeling of the quantum Zeno effect.

The general requirement

Preconditions of the quantum theory for the occurrence of the effect:

Analogy, a reverse Zeno effect in optics

A writable within the framework of classical physics experiment that serves to approach an understanding of the Zeno effect, consists of a polarized light source and several polarizers, as shown in the adjacent figure.

First (Fig. (0)) the light from the light source is polarized purely vertical. For free propagation, this orientation does not change, will therefore never horizontally polarized. Therefore, a horizontal polarizer always leads to extinction.

Adds one now one against the direction of polarization of the light by twisted polarizer added, the intensity decreases proportionally to the observer, since only the projection of the plane of vibration is transmitted to the polarizer. But it is interesting first of all that this polarizer is a quantum mechanical measurement. Then the plane of polarization that is parallel to the polarizer ( Figure (1)), corresponding to a quantum mechanical state of preparation.

Is now added after the other polarizers, in the limiting case: respectively mutually rotated only by an infinitesimal angle, the loss per polarizer is minimal and is in the limit of zero. Thus, one can rotate the polarization direction purely by consecutively executed lossless measurements, that is, change the size of the observable expectation. This scenario corresponds approximately to the continuous measurement as described above.

Criticism and other aspects

So far no stopping the radioactive decay has been confirmed by experimental measurements of an ensemble of radioactive atoms or even a single radioactive atom, as it would require the theory of the quantum Zeno effect. Above all is the opposite, the reversal of the Zeno effect, no analogy, but only a contrarian or polar contrast represents the experiments of Itano and colleagues refer to stable isotopes of beryllium -9 in mixtures with magnesium -26, with transitions in stimulates the UV range and were observed. Since the quantum mechanical system has been defined or disturbed in this case a priori by the observer, can not be assumed that the observation of an indeterminate system principle, whereby the experimental approach is called into question. In fact, it is more likely that the corresponding quantum mechanical process, in particular the radioactive decay, even accelerated when it is analyzed with a high frequency monitoring.

Literature and links

• Christian memory: " Illusory motion in the quantum world, a modern version of Zeno's paradox measurement as engagement with far-reaching consequences. .. " - Nature and Science ( supplement of the Frankfurter Allgemeine Zeitung), April 6, 1994 N1 f
• PDF, quantum mechanical paradoxes (1.01 MB)
• NZZ Research and Technology, populärwiss. Article on the QM - Zeno effect

Swell

• Paradox
• Quantum physics