Quantum Odyssey in the ion trap

Scientists demonstrate "quantum migration" of single trapped ions

In a quantum maze, all paths are in a superposition state and can therefore be walked on simultaneously. The interference that occurs as a result of the superposition leads to strange phenomena such as the self-encounter of the quantum walker. With these "tricks", the exit from the maze, eg the desired solution of an algorithm or even the most efficient form of energy transfer in plants, can be found many times faster than in the classical way. © Tobias Schätz / MPQ
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Max Planck scientists have for the first time demonstrated the phenomenon of quantum migration on single trapped ions. They report on the results of their new experiment in the journal Physical Review Letters.

Many classical algorithms contain so-called "random walks", in which possible solutions are randomly selected. Such algorithms find application in a number of fields - for example, in physics, biology, economics or even psychology.

If one transfers random walks to quantum systems, then such decision-making becomes superfluous. Because in contrast to the classical method, the paths in question are in a superposition state, so that in "quantum migration" all can be taken simultaneously. The resulting interference leads to novel phenomena: for example, the "quantum walker" can encounter itself at some intersections.

Difference between classical and quantum mechanical "Odyssey" revealed

Quantum walks could considerably speed up computational algorithms for quantum systems. But they could also contribute to a better understanding of the borderline between the classical and the quantum mechanical world, which has become apparent in mesoscopic systems.

With a new experiment in an electromagnetic trap, Tobias Schätz and his colleagues at the Max Planck Institute of Quantum Optics in Garching have now clearly demonstrated for the first time the difference between the classical and the quantum mechanical "Odyssey", with an ion as a wanderer. display

Every time we come to an intersection, we have to decide between several ways, perhaps by coin toss. After several intersections and decisions we will have gone just a few of many possible paths. It may happen that some ways are used more often than others.

Weird situations

In contrast, a quantum walker does not have to decide, because he has no choice. Each coin toss creates a superposition of head and number so it can follow all paths at the same time. This can lead to strange situations, for example, the quantum walker, when paths collide at later intersections, meet themselves.

Due to interference effects, the likelihood of being at this intersection may increase but also decrease to the point where it completely disappears from there.

In the new experiment, a single magnesium ion trapped in a linear electromagnetic trap plays the role of the quantum walker. Its basic state of motion is, so to speak, the starting point from which it starts to march. By radiofrequency pulses, the researchers create a superposition of two electronic states. This process corresponds to the M nzwurf, through which you get a superposition of linker and rechter Wegentscheidung - head and number.

Ultraviolet light shoves

The necessary push to move is given to the ion by ultraviolet light of a precisely tuned frequency. Depending on its electronic state, the ion is pushed by the UV light to the left and sometimes to the right. Since the two electronic states - head and number - are in a superimposition state, the two movement possibilities of the ion - step to the right and / or step to the left - are superimposed. In the case of quantum migration, therefore, the two mean values ​​with the two movement possibilities of the ion are highly restricted.

The processes M ntwurf and position changes are repeated a total of three times, only then can quantum effects become visible. After completing this "quantum evolution", the scientists measure whether the coin shows head or figure and where the ion is located. This exploits the fact that the ion only emits fluorescent light in one of the two "magnetic states".

Imbalance of both directions

After about a thousand measurements, the physicists receive a statistical statement as to how frequently the ion has gone to the right or left. Their data confirms the theoretical prediction of an imbalance in both directions, in contrast to what one would expect from a classical system.

The team of Tobias Sch tz has clearly revealed the differences to the classical counterpart with this experiment, in which the hiker / the ion can go all the ways at the same time: The quantum interference strengthens asymmetric, non-classical distributions in the mutually highly constrained M nzwurf- and Bewegungszust nden. Currently, the number of repetition steps is still limited by nonlinear effects. The scientists therefore propose a new concept that allows quantum migration to be extended to many, and in principle even several hundred, steps.

Many possible applications

Quantum migration could be of fundamental interest to a number of applications. In this way, the speed of finding the right path can be vastly increased if one does not have to try one after the other at random, but at the same time can tread all. The performance of search algorithms in information processing could be considerably increased, according to the researchers.

There are also considerations that this quantum mechanical behavior is also responsible for the energy transfer in plants, which is distributed far more effectively in many ways than would be achievable with classical methods.

(idw - Max Planck Institute for Quantum Optics, 31.08.2009 - DLO)