An unexpected turn in the quantum system

Scattering in a quantum system leads to the fact that cold atoms are arranged alternately in two different figures of a checkerboard: either atoms are present only on white squares (in front), or squares are occupied by atoms with opposite directions of rotation. Courtesy: ETH Zurich.

Physicists from ETH Zurich have observed an amazing turn in the quantum system caused by the interaction between energy dissipation and coherent quantum dynamics. To explain this, they found a concrete analogy with mechanics.

“No scientist thinks in formulas,” Albert Einstein allegedly once said to his colleague Leopold Infeld. In fact, especially for physicists who do abstract things like the quantum physicsIt is often very useful to work with specific images rather than with mathematical symbols. A team of researchers led by Tilman Esslinger, a professor at the Institute of Quantum Electronics at ETH Zurich, experienced this when they recently discovered a new effect in their quantum mechanical systemAlthough they studied tiny atoms and light particles in their experiment, they were able to better understand their observations thanks to the catchy image: a shaft rotating inside the bearing. Their results were recently published in a journal. The science,

Sophisticated Quantum System

“In fact, we were not looking for this effect at all,” says Esslinger. “Only in hindsight did we understand what our data means.” He and his colleagues decided a very complicated topic: quantum system in which individual particles interact strongly with each other and which simultaneously move from the outside and also scatter. “Dissipative” means that the quantum states of particles do not just coherently develop in time, that is, so that their superpositional states remain unchanged. Rather, controlled communication with the outside world causes superposition states to fade. If the dispersion is very strong, they disappear very quickly, and as a result the particles behave almost the same as in classical physics, which we know from everyday experience. On the other hand, without any scattering, the way a system of particles develops in time is dictated by purely quantum mechanics – an ideal case that physicists use, for example, to create quantum computers.

Atomic patterns

“These two extremes can be calculated and understood quite well,” explains Tobias Donner, who works as a senior fellow at Esslinger’s laboratory. “On the contrary, it is much more difficult to deal with systems in the middle where coherent evolution and dispersion are equally important.” To build such a quantum system in the laboratory, physicists cooled the atoms to temperatures close to absolute zero at about -273 degrees Celsius, and exposed them to a focused laser beam that captures and directs atoms into the lattice from the light. Each atom also has A “rotation” that can point up or down (very similar to a compass needle that points north or south). In addition, cold atoms are surrounded inside the cavity by two mirrors that reflect the light scattered by the atoms back and forth.

The interaction between the atoms, the laser beam and the light in the cavity now makes the atoms spontaneously stagger. This can happen in two different ways. In one of them, atoms are present only on the “white” squares, while the black squares remain empty (see. Figure). In another case, there are also two types of squares, red and green, but now red squares are occupied only by atoms whose spins are directed upwards, while on green squares there are only atoms whose backs are directed downwards.

Amazing turn

Which of the two options is preferred by atoms depends on the direction of oscillation of the laser beam that irradiates them, strictly in accordance with the rules quantum mechanics– at least if the atoms do not undergo any dissipation. When physicists conducted the experiment in a regime where the influence of dissipation (caused by the loss of photons from the resonator) was large enough, something unusual happened. “Our data no longer showed us one of the two models; rather, it seemed that the atoms "Esslinger describes the unexpected results." It was an exciting discovery, but we absolutely did not know why this was happening. "

Unusual power

By simplifying the quantum-mechanical equations describing their experiment, physicists were eventually able to find an analogy with the mechanical system. In fact, the formulas were strikingly similar to the formulas describing a shaft rotating inside a bearing. Between the shaft and the bearing is a viscous lubricant that must ensure uniform rotation. However, if the shaft is slightly offset from the center of the bearing, a rather unusual friction force arises, which depends on the position of the shaft. The force arises because in one direction the distance between the rotating shaft and the stationary bearing decreases, and therefore, different friction forces act on the shaft and bearing. The resulting force, depending on the position, is perpendicular to the direction of movement of the shaft. As a result, the center of the shaft begins to rotate around the center of the bearing.

Now that physicists are able to describe the unexpected quantum effect in a concrete way, they are already thinking about the next step: use it to consciously control and manage quantum systems. “Usually dissipation alters or weakens existing quantum effects – but here we have an effect that actually owes its existence dispersion“Says Esslinger. Can such effects be more common in quantum systems and how can they be used in quantum technologies that are currently being developed, so the questions that concern researchers now.

Excellent rotation in nuclear transport

Additional Information:
Structural instability caused by dissipation and chiral dynamics in quantum gas. The scienceVol. 366, Issue 6472, pp. 1496-1499 (2019) DOI: 10.1126 / science.aaw4465

An unexpected turn in the quantum system (2020, January 10)
retrieved January 10, 2020

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