Also quantum particles tremble

Quantum simulation of a relativistic particle succeeded

Christian Roos in the laboratory © Lackner
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Researchers used an ion to simulate a relativistic quantum particle and were able to detect a phenomenon that was never directly observed in nature: the so-called dithering motion. They report about it in the current issue of the scientific journal "Nature".

After the quantum mechanics had established itself in the 1920s, the British physicist Paul Dirac 1928 succeeded for the first time to combine this theory with the postulated by Albert Einstein Special Relativity. In this way, quantum physics could also be applied to particles for which relativistic conditions apply, ie which move at extremely high speeds - close to the speed of light.

Groundbreaking findings

From the equation formulated by Dirac emerged some groundbreaking new insights, such as that there is also an antiparticle - antimatter - for each particle, as well as a natural explanation for the existence of electron spin.

The Austrian Nobel laureate Erwin Schrödinger postulated in 1930 as the existence of the so-called dithering movement, a kind of fluctuation in the movement of relativistic particles. "According to the Dirac equation, such a particle does not move rectilinearly in free space, but 'trembles' in all three spatial dimensions, " explains Christian Roos of the Institute of Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences (ÖAW). "It is unclear whether this dithering movement can be directly observed in nature."

Quantum simulation of a quantum particle

Physical phenomena are often described by equations that are too complicated to solve exactly. In this case, scientists often rely on computer simulations to get answers to open questions. Since this strategy fails even for relatively small quantum systems due to the lack of computer power, researchers like Richard Feynman have proposed experimentally simulating these phenomena in other quantum systems. The prerequisite for this are, of course, very detailed knowledge of the physics of these systems and an extremely good command of the technology. display

All this has been built up by the research team around Professor Rainer Blatt with his experiments on quantum computers in the past years in Innsbruck and is therefore now able to perform such quantum simulations in the laboratory. "The challenge is to emulate the equations in the quantum system well, to control the various parameters over a wide range, and to measure the results, " explains Roos.

Calcium atom in the trap

The Innsbruck experimental physicists have captured a calcium atom in an ion trap and cooled it down with lasers. In this well-defined state, the properties of the relativistic particle to be simulated were written into the particle with the help of additional lasers. "Our quantum system now behaves just like a free, relativistic particle that obeys the laws of the Dirac equation, " explains Rene Gerritsma of the IQOQI and first author of the Nature study.

With the help of measurements scientists were finally able to characterize the properties of this simulated particle. So we succeeded in proving the dithering movement in the simulation. We were also able to determine the probability with which the particle is in a certain place, "explains Gerritsma.

In the very small quantum system, physicists modeled the Dirac equation only for a spatial dimension. This is a demonstration experiment, says Roos, which can be applied to three-dimensional conditions with the appropriate technological effort.

Also antiparticles simulated

The Innsbruck experiment is characterized by an extremely good command of the physical properties of the simulated particle. For example, physicists were able to change the mass of the object and also simulate antiparticles.

"After all, our approach was very simple, but you first have to think of doing it, " says Roos, whose team was inspired by the theoretical proposal of a Spanish research group.

(Institute of Quantum Optics and Quantum Information (IQOQI), 07.01.2010 - DLO)