To be dead and alive at the same time, is that possible? Even though it contradicts conventional human understanding, in the case of Schrödinger’s cat it is possible – if only in theory. Already in 1935, the Viennese physicist Erwin Schrödinger (1887-1961) thought up the following quantum physics paradox: In a nutshell, a cat could be dead and alive at the same time by employing an entangled atom-light state – a mind game that has kept researchers busy ever since.

A group of researchers led by Gerhard Rempe, Director of the Division of Quantum Dynamics at the Max Planck Institute of Quantum Optics in Garching near Munich, has now got to the bottom of the question of how such an entangled state can be experimentally realized. However, the scientists did not carry out the experiment with a cat, but optically in the laboratory, where pulses of laser light played the role of the cat. The insights gained from the project would open up new prospects for enhanced control of optical states, that can in the future be used for quantum communications, the researchers say.

“According to Erwin Schrödinger’s idea, a microscopic particle, such as an atom, can be in two different states at the same time. This is called superposition,” explains Professor Gerhard Rempe. “If it is also entangled with a macroscopic object, it can also pass on its superposition to this object. This results in the example of a cat that, depending on the decay of a radioactive atom, can be both alive and dead – an idea that contradicts any everyday experience.”

“According to Schrödinger‘s idea, it is possible for a microscopic particle, such as a single atom, to exist in two different states at once. This is called a superposition,” Professor Rempe explains. “Moreover, when such a particle interacts with a macroscopic object, they can become ‘entangled’, and the macroscopic object may end up in a superposition state. Schrödinger proposed the example of a cat, which can be both dead and alive, depending on whether or not a radioactive atom has decayed – a notion which is in obvious conflict with our everyday experience.”

Schrödingers Katze

© Bastian Hacker, Max-Planck-Institut für Quantenoptik (MPQ)

In addition to an optical resonator, consisting of two mirrors separated by a slit only 0.5 mm wide, the laboratory at the Max Planck Institute also has a vacuum chamber and high-precision lasers with which scientists can isolate a single atom and manipulate its state. A laser pulse is fed into the resonator and reflected, and thereby interacts with the atom trapped in the resonator. As a result, the reflected light gets entangled with the atom, and by performing a suitable measurement on the atom, the optical pulse can be prepared in a superposition state.

The scientists were not completely convinced that this experiment would succeed and whether these quantum-mechanically entangled “cat states” could be generated and reliably detected with the available technology. The major difficulty lay in the need to minimize optical losses in their experiment, but eventually, all measurements were found to confirm Schrödinger’s prediction.

The cat is out of the box

“One special feature of the experiment is that the entangled states can be generated deterministically. In other words, a cat state is produced in every trial,” the researchers explain. We have succeeded in generating flying optical cat states and demonstrated that they behave in accordance with the predictions of quantum mechanics. These findings prove that our method for creating cat states works, and allowed us to explore the essential parameters,” says Ph.D. student Stephan Welte.

The scientists explain the “flying cat states” using the two illustrations: “Image 1 shows that Schrödinger’s cat is entangled with an atom. If the atom is excited, the cat is alive. If it has decayed, the cat is dead. Since a microscopic particle like an atom can be in two states at the same time, the same applies to the cat.”

“In the experiment, a light pulse simulates the two states (peaks in image 1) and may be in a superposition of both, just like the cat. Specifically – as shown in image 2 – n atom is trapped in the resonator between two mirrors (left). A light pulse, which is reflected off the resonator, gets entangled with the atom and may fly freely as a superimposed cat state (right).”

More precisely, flying optical cats are superpositions of two light states of different phases. “It turns out that these light states can also be used to encode information in the form of a so-called qubit. A qubit is the quantum mechanical analog to a classical bit. Since the qubit in our case is encoded with light states, it can be sent from a transmitter to a receiver simply like a light pulse through a glass fiber and thus transmit information.”

In the experiment, they succeeded “not only in creating one specific cat state but arbitrarily many such states with different superposition phases – a whole zoo, so to speak,” adds Bastian Hacker, who is also a doctoral student at the Institute.

And there was another difference to Schrödinger’s gedanken experiment. “Schrödinger’s cat was originally locked in a box to rule out any interaction with the environment. Optical cat states, as we have realized them, are not locked into a box, but fly freely,” “Nevertheless, they remain isolated from their surroundings and can thus be maintained over long distances.

“Schrödinger‘s cat was originally enclosed in a box to avoid any interaction with the environment. Our optical cat states are not enclosed in a box. They propagate freely in space. Yet they remain isolated from the environment and retain their properties over long distances,” Gerhard Rempe explains the importance of the successful experiment. “In the future, we could use this technology to construct quantum networks, in which flying optical cat states transmit information.”

For future research this successful experiment means that science could not only gain clarity over the old philosophical question of Erwin Schrödinger, the researchers say. “We have also successfully opened up a new way of controlling light states, which has so far only been possible to a very limited extent. Based on the results of this experiment, we can continue to test the validity of quantum mechanics and this capability could in the future be utilized to encode quantum information.”