“Mr. Beam”: Sailing’s most important milestones

One of the central quantum physics phenomena Zeilinger deals with is the so-called entanglement, which Erwin Schrödinger (1887-1961) called the “essence of quantum physics”. Albert Einstein, on the other hand, spoke scornfully of “ghostly long-range effects” – and in fact the phenomenon is barely comprehensible with the horizon of everyday experience.

Particles stay connected
Because in the quantum world, two or more entangled particles remain strongly connected even at a distance and share their physical properties. Whatever you do with one particle, it seems to immediately affect the state of the other particle, no matter how far away it is.

The starting point for Zeilinger was a conversation with American physicists Daniel Greenberger and Michael Horne about the phenomenon from the late 1970s at the Massachusetts Institute of Technology (MIT). In 1986, the three then described a specific type of three-particle entanglement in a small four-page conference paper. Today it is called “GHZ condition” after the first letters of their name.

From theory to experiment
At the time, there was still a heated debate about whether the “ghostly action at a distance” couldn’t be explained by properties that entangled particles carry that are just not yet known — physicists call them “hidden variables.” In 1964, the physicist John Bell (1928-1990) formulated a theorem (“Bell inequality”) that made it possible to determine by experiment whether there were actually hidden variables: numerous well-defined measurements on two entangled particles would show a clear difference between classical systems and quantum systems.

In contrast, in the three-particle entangled thought experiment designed by Greenberger, Horne and Zeilinger, a single measurement is enough to clarify this conceptual difference between classical and quantum physics. Based on this theoretical work, “My goal over the next decade was to experimentally realize these GHZ states with three entangled particles,” Zeilinger explained in an interview with APA.

But this required pioneering, “nothing existed at all: the type of photon sources was not clear, we did not know how to generate the GHZ states, how to manipulate them, etc,” Zeilinger recalls. In 1999 the time had come: they published the breakthrough in the journal “Physical Review Letters” (“Observation of three-photon Greenberger-Horne-Zeilinger entanglement”).

“Bright” – but unfortunately not like in “Star Trek”
But Zeilinger had already become famous two years earlier: It started in 1993, when physicists Charles Bennett and Gilles Brassard first theoretically described the possibility of quantum teleportation. “For me at the time, it was a typical paper from pure theorists who didn’t see that it wasn’t possible — not knowing that we in my group were already developing the methods for this while we were working on the GHZ states,” says Zeilinger. “Then we saw that teleportation was easier than the entanglement of many particles that we were aiming for, so we did that.”

In 1997 the scientists transferred the exact quantum state of a photon A at an arbitrary distance to a particle B. The state to be transferred was the polarization, the vibrational plane of the light particle. Since particle A is destroyed during the measurement, but B then carries exactly the same information as A and is indistinguishable from it, the process was called “beaming” – based on a matter transport system in the science fiction series “Star Trek”. .

Since then, Zeilinger has been repeatedly referred to as “Mr. Beam” and to this day has to explain why we can’t “beam” on holiday in the future either. To this day it is Zeilinger’s most cited work.

In the years that followed, Zeilinger and his team continued to push the alleged boundaries of entanglement and teleportation. Leaving the lab, “official” particles passed through sewers under the Danube, sending entangled photons through the atmosphere, first over Vienna and finally between two Canary Islands. When China launched the first quantum communication satellite “Micius” in 2016 to send entangled photons to Earth from there, Zeilinger was there as a collaboration partner, the instruments on board the satellite were originally “developed by us in Austria”.

Another spin-off from the fundamental work in quantum physics is quantum cryptography, which also uses entanglement to enable absolutely tap-proof encryption of messages and data transmissions. In 2004, he demonstrated the first transfer from Vienna City Hall to a bank, encrypted using quantum cryptography. In 2017, the first quantum-encrypted video call Zeilinger had with his Chinese counterpart as OeAW president received worldwide attention. The keys were exchanged using “Micius”.

Zeilinger also contributed to the much-praised successes of his students and collaborators: Markus Arndt and Markus Aspelmeyer, for example, with their spectacular experiments to demonstrate quantum phenomena on increasingly larger objects, or Philip Walther with his concepts for quantum computers. These works are also a kind of spin-off from Zeilinger’s research into fundamental questions in quantum physics.