Wissenschaftler der ETH Zürich haben Fortschritte bei der Herstellung von Schrödingers schwereren Katzen erzielt, die gleichzeitig lebendig (oben) und tot (unten) sein können. Bildnachweis: Yiwen Chu/ETH Zürich

Forscher der ETH Zürich haben die bisher schwerste Schrödinger-Katze geschaffen, indem sie einen Kristall in eine Überlagerung zweier Schwingungszustände gebracht haben. Ihre Ergebnisse könnten zu leistungsfähigeren Qubits führen und helfen zu erklären, warum Qubits im Alltag nicht beobachtet werden.

• Forscher der ETH Zürich haben Schrödingers bisher schwerste Katze erschaffen.
• Dazu kombinierten sie einen Schwingquarz mit einem supraleitenden Schaltkreis.
• Sie hoffen, besser zu verstehen, warum Quanteneffekte in der makroskopischen Welt verschwinden.

Auch wenn Sie kein Quantenphysiker sind, haben Sie wahrscheinlich schon von Schrödingers Katze gehört. Erwin Schrödinger kam 1935 in einem Gedankenexperiment auf Katzen, die gleichzeitig lebendig und tot sein konnten. Das offensichtliche Paradoxon – schließlich sehen wir im Alltag nur Katzen, die entweder leben oder leben. oder Dead – veranlasste Wissenschaftler, ähnliche Situationen im Labor zu realisieren. Bisher gelang ihnen dies, indem sie beispielsweise Atome oder Moleküle in quantenmechanischen Überlagerungszuständen des gleichzeitigen Vorhandenseins an zwei Orten einsetzen.

An der ETH hat ein Forscherteam unter der Leitung von Yiwen Chu, Professorin am Labor für Festkörperphysik, eine dramatisch schwerere Schrödinger-Katze geschaffen, indem sie einen kleinen Kristall in eine Überlagerung von zwei Schwingungszuständen gebracht hat. Ihre Ergebnisse, die diese Woche in der Fachzeitschrift Science veröffentlicht wurden, könnten zu leistungsfähigeren Qubits führen und das Geheimnis lüften, warum Quantenüberlagerungen in der makroskopischen Welt nicht beobachtet werden.

Katze in einer Kiste

In Schrödingers ursprünglichem Gedankenexperiment ist eine Katze in einer Metallbox mit radioaktivem Material, einem Geigerzähler und einem Giftfläschchen eingeschlossen. In einem bestimmten Zeitrahmen – zum Beispiel einer Stunde -[{“ attribute=““>atom in the substance may or may not decay through a quantum mechanical process with a certain probability, and the decay products might cause the Geiger counter to go off and trigger a mechanism that smashes the flask containing the poison, which would eventually kill the cat. Since an outside observer cannot know whether an atom has actually decayed, he or she also doesn’t know whether the cat is alive or dead – according to quantum mechanics, which governs the decay of the atom, it should be in an alive/dead superposition state. (Schrödinger’s idea is commemorated by a life-​size cat figure outside his former home at Huttenstrasse 9 in Zurich).

In the ETH Zurich experiment, the cat is represented by oscillations in a crystal (top and blow-​up on the left), whereas the decaying atom is emulated by a superconducting circuit (bottom) coupled to the crystal. Credit: Yiwen Chu / ETH Zurich

“Of course, in the lab we can’t realize such an experiment with an actual cat weighing several kilograms,” says Chu. Instead, she and her co-​workers managed to create a so-​called cat state using an oscillating crystal, which represents the cat, with a superconducting circuit representing the original atom. That circuit is essentially a quantum bit or qubit that can take on the logical states “0” or “1” or a superposition of both states, “0+1”. The link between the qubit and the crystal “cat” is not a Geiger counter and poison, but rather a layer of piezoelectric material that creates an electric field when the crystal changes shape while oscillating. That electric field can be coupled to the electric field of the qubit, and hence the superposition state of the qubit can be transferred to the crystal.

Simultaneous oscillations in opposite directions

As a result, the crystal can now oscillate in two directions at the same time – up/down and down/up, for instance. Those two directions represent the “alive” or “dead” states of the cat. “By putting the two oscillation states of the crystal in a superposition, we have effectively created a Schrödinger cat weighing 16 micrograms,” explains Chu. That is roughly the mass of a fine grain of sand and nowhere near that of a cat, but still several billion times heavier than an atom or molecule, making it the fattest quantum cat to date.

In order for the oscillation states to be true cat states, it is important that they be macroscopically distinguishable. This means that the separation of the “up” and “down” states should be larger than any thermal or quantum fluctuations of the positions of the atoms inside the crystal. Chu and her colleagues checked this by measuring the spatial separation of the two states using the superconducting qubit. Even though the measured separation was only a billionth of a billionth of a meter – smaller than an atom, in fact – it was large enough to clearly distinguish the states.

Measuring small disturbances with cat states

In the future, Chu would like to push the mass limits of her crystal cats even further. “This is interesting because it will allow us to better understand the reason behind the disappearance of quantum effects in the macroscopic world of real cats,” she says. Beyond this rather academic interest, there are also potential applications in quantum technologies. For instance, quantum information stored in qubits could be made more robust by using cat states made up of a huge number of atoms in a crystal rather than relying on single atoms or ions, as is currently done. Also, the extreme sensitivity of massive objects in superposition states to external noise could be exploited for precise measurements of tiny disturbances such as gravitational waves or for detecting dark matter.

Reference: “Schrödinger cat states of a 16-microgram mechanical oscillator” by Marius Bild, Matteo Fadel, Yu Yang, Uwe von Lüpke, Phillip Martin, Alessandro Bruno and Yiwen Chu, 20 April 2023, Science.
DOI: 10.1126/science.adf7553