Schrödinger's cat of light

Transmission of superimposed state to laser pulse makes "cat" mobile

Like the famous cat in Schrödinger's experiment: researchers have linked a light pulse with the superposition state of an atom. © Christoph Hohmann, Nanosystems Initiative Munich (NIM)
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At the same time dead and alive: Physicists have for the first time realized a special variant of Schrödinger's cat - the famous thought experiment on quantum physical superposition. In their experiment, the researchers transmitted superposition states from a trapped atom to a laser pulse. This represented an optical version of the "cat", which is still mobile and can be shipped in quantum networks, as the researchers report in the journal "Nature Photonics".

The phenomenon of superposition is an unusual peculiarity of the quantum world: In it, atoms or other particles occupy several possible states simultaneously until their status is measured. The physicist Erwin Schrödinger explained this phenomenon with a famous thought experiment. In a box sits a cat whose life depends on the decay of a radioactive atom: If it decays, poison is released and the cat dies. But as long as you do not look into the box, the cat is both dead and alive at the same time.

Since then physicists have experimentally simulated such quantum physical states of superposition with several systems and particles - with electrons, groups of photons and even with molecules of more than 400 atoms.

Photon pulse in the superposition state

Researchers led by Gerhard Rempe from the Max Planck Institute for Quantum Optics have now implemented a new variant of Schrödinger's cat. The peculiarity here: Your cat can leave the box and still remains in the overlay state. "Optical cat states, as we have realized, are not locked in a box, but fly free, " Rempe explains. "Nevertheless, they remain isolated from their environment and can thus be maintained over long distances."

In the experiment, the "box" consists of an optical resonator, a cavity with two domed mirrors. In this a rubidium atom is captured whose quantum states bring the researchers into a superposition state. Now, the "cat" comes into play: It consists of a laser pulse, which is limited by the interaction with the resonator with the atom - and also assumes its superimposition state. display

Optical cats learn to fly

The photons of the laser pulse became, through this interaction, one equivalent of Schrödinger's cat. "We were able to produce flying optical cat states and show that they correspond to the predictions of quantum mechanics, " says co-author Stephan Welte. "We were thus able to prove that our method works and continues to investigate which parameters are crucial."

An atom is caught in the resonator between two mirrors (left). The reflected light pulse is confined with the atom, the two hills show the overlay (right). Bastian Hacker / Max Planck Institute for Quantum Optics

However, this new method is not only suitable for transferring the superposition of atoms onto moving photons; the nature of the states can also be controlled in a targeted way: In our experiment k Not only can we create a particular cat state, but any number of different phases of superimposition - that is, a whole zoo, as it were, explains Weltes colleague Bastian Hacker.

Transmission in the quantum network

The exciting thing is that because the "cats" are mobile in this system, they could in future be used to encode and transmit quantum information. Because the photons retain their superimposed state even if they are sent through a fiber, for example, as the researchers explain. In addition, such "cats" could be used as qubits in quantum computers.

"In the future, we want to use this technology to build entire quantum networks where flying optical cats transmit information, " says Rempe. (Nature Photonics, 2019; doi: 10.1038 / s41566-018-0339-5)

Max Planck Institute for Quantum Optics

- Nadja Podbregar