What is the quantum tool of time

Artificial crystal: magnetism in World Cup fever

Magnetic crystals can be generated in an optical lattice with Rydberg atoms

You know it from your own living environment: Relationships between neighbors can be intense and at the same time characterized by sensitivities. Complex quantum systems can be imagined in a similar way - especially when it comes to magnetism. A team led by Christian Groß in the department of Immanuel Bloch, director at the Max Planck Institute for Quantum Optics in Garching, is researching such a system. It is inspired by the crystals of magnetic solids. But Garching's artificial crystal consists of a grid of laser light that traps rubidium atoms. The researchers use special laser light to pump some of these atoms up into giant exotic atoms. These form quantum crystals whose behavior can not only answer fundamental questions about magnetism.

Normal magnetism, such as that found in iron, for example, is similar to the state of a row of houses during a soccer world championship. Live broadcasts are on in every house, and as soon as the German national team scores a goal, collective cheers can be heard from the open windows. In crystalline solids, which include normal magnets, the atoms that contribute to the magnetism represent an order reminiscent of row houses. Certain electrons in these atoms align themselves in one direction like tiny compass needles. They unite to form collective magnetism, just as the cheers from the houses build up into a powerful collective outcry.

This well-known magnetism is called ferromagnetism, from the Latin word ferrum for iron. There are also other forms: In antiferromagnetism, the neighboring electronic compass needles align in opposite directions. That would correspond to a row of houses in which every second neighbor would be a supporter of the opposing soccer team - and the game would end in a draw.

Physicists learn a lot by tinkering with quantum systems

There are many intermediate forms of magnetism between these two magnetic extremes. In addition, such collective quantum effects also play an important role in other physical phenomena, for example in superconductivity: a state in which some substances conduct electricity without resistance at low temperatures. That is why researching them is so important. But real crystals made of firmly connected atoms have several disadvantages: It is difficult to look deep into them in experiments, and there are always a gigantic number of atoms and electrons involved. Most importantly, the researchers can barely influence the interactions that lead to a collective phenomenon like magnetism. Physicists can learn a lot about the quantum world by tinkering with quantum systems.

Scientists are therefore placing hope in artificial crystals made up of a manageable number of atoms that are easily accessible. Above all, their interactions can be manipulated wonderfully. Christian Groß's team in Garching works with such a system. It consists of a cloud of 250 to 700 rubidium atoms that are frozen at very low temperatures. Because the individual atoms move relatively slowly, they can be easily captured. The particles are caught by a grid of laser beams: an atom gets stuck at each of its intersection points, creating an order reminiscent of a crystal. Above all: by cleverly irradiating additional laser light, the physicists can now manipulate the interactions between the atoms - and even the atoms themselves.

“This is a very clean system in which we can study the individual processes in detail at the quantum level,” says Christian Groß. This new quantum tool is so flexible and powerful that it has long since emancipated itself from the pure simulation of real solid-state crystals. This is particularly true of the latest research by the team around the postdoctoral fellow.

Magnetic interactions as a troubling collective

Together with theorists led by Thomas Pohl from the Max Planck Institute for Physics of Complex Systems in Dresden, the Garching-based researchers asked themselves the following: What would happen if the interaction between magnetic atoms in the quantum collective extended very far? In normal magnetism, this interaction is limited to short distances: the direct neighbors in particular influence each other. In our hypothetical row house settlement, a long range would correspond to a situation in which every tenth neighbor would have a vuvuzela. This would allow them to get in the mood for a particularly loud whistling collective across the intermediate houses. If, however, the neighbors in between would also resort to vuvuzelas, then the peace would again be so permanently disturbed that order could no longer be established in the settlement.

The quantum system behaves in a very similar way, with which Groß ‘Team has now formed a completely new type of magnetic crystal. The Garching team irradiated the atoms trapped in the light grid with a special laser light. With its energy, they pumped some of the atoms - in simple terms - into exotic giant atoms. Such Rydberg atoms, like Vuvuzela owners in the settlement, are able to influence other atoms beyond many neighboring atoms.

“Such a giant atom is a thousand times larger than a normal atom,” explains Groß. Its outermost electron is extremely far from the nucleus and makes the giant atom into a kind of antenna. In this way, it can influence other Rydberg atoms, which also act like antennas, so that they form a common, crystalline order - just as the Vuvuzela owners come together in a coordinated roar across many houses. “The atoms only remain in the Rydberg state for a few millionths of a second,” explains Groß: “But that is an extremely long time in the quantum world.” It is enough for a nice order.

Giant atoms form a magnetic crystal

As when baking cookies, the Garching team punch out a few hundred rubidium atoms trapped in the light grid, either oblong or circular, in which they use their laser to pump individual atoms into Rydberg atoms. In the elongated shapes, one-dimensional chains of giant atoms emerged, which together formed a magnetic crystal, and two-dimensional crystals of up to eight Rydberg atoms from the circular disks. It turned out that the size of this section determined how many giant atoms were involved in magnetism. The distance between them always remained the same and corresponded to about ten atoms of the light grid. In the one-dimensional section, two, then three, and finally four Rydberg atoms formed a quantum crystal in stages.

Now you have to know that normal magnetism at the quantum level of individual electrons in solids only knows two states: Like rotary switches, the electrons can only lock in parallel or anti-parallel to the applied magnetic field. In the Garching system, Rydberg atoms represent the switching state parallel to the magnetic field. Antiparallel, on the other hand, corresponds to the rubidium atoms in the light lattice if they are not excited to form giant atoms. The physicists can use the special laser light to switch between these two quantum states.

The people of Garching have their system perfectly under control. In doing so, they have created a tool with which they can research the collective behavior of such quantum systems more precisely. It's not just about a deeper understanding of magnetism. In principle, this tool can simulate the behavior of many complex quantum systems. Perhaps as “quantum simulators” they can even help to answer profound questions in other areas such as particle physics.