Our conversation with Anton Zeilinger, quantum mechanics pioneer and FiO keynote speaker.
Anton Zeilinger was always interested in quantum mechanics. In 1971, he received his Ph.D. from the University of Vienna in Austria, where his research was focused on neutron interferometry. In the late 1980s, he began studying quantum entanglement. Today, as a professor of physics at the University of Vienna and director of the Institute of Quantum Optics and Quantum Information at the Austrian Academy of Sciences, Zeilinger is a leader in breakthrough photonic entanglement research.
To explain the strange phenomenon of quantum entanglement, Zeilinger frequently uses the analogy of rolling a pair of dice. If the numbers on each die always come up as a match (double sixes, double twos, etc.), the dice are behaving as if they are entangled: Each die is independent, but the outcome of both is always the same. In quantum entanglement, dice represent particles and the numbers represent the properties of the particles, such as polarization or energy; once entangled, the properties of each particle are always perfectly correlated.
Optics plays an important role in entanglement. “The benefit of entangled photons is that they can propagate through free space over large distances, practically without their state being disturbed,” he says. “Photons provide many more possibilities for fundamental experiments because of the possibility of producing beams of much higher intensity and of creating rather complicated entangled states.”
Therefore, he says, photons are the only particles one can expect to use for long-distance quantum communication. In his teleportation experiments, Zeilinger uses a pair of entangled photons (A and B) along with a third photon (X) that he wants to teleport. He allows photon X to interact with photon A and measures the two interacting photons, which can then cause photon B to take on the properties of photon X. So, the properties of photon X are teleported from one location to another—which, in quantum physics, is equivalent to teleporting the photon itself.
Now that Zeilinger’s team has figured out how to teleport photons, their focus is on quantum communication over longer and longer distances. In his plenary session keynote address at Frontiers in Optics, OSA’s annual meeting in Rochester, N.Y., this month, Zeilinger will discuss his team’s most recent success with long-distance quantum communication experiments between two Canary Islands.
His quest for ever-longer quantum communication took off in 2005, when his team successfully teleported photons a length of 600 m under the Danube River in Vienna. Shortly thereafter, they increased the distance across the city of Vienna, and, just last year, they set a new world record by distributing entangled photons 144 km between two Canary Islands. According to Zeilinger, these advances are the first steps toward quantum satellite communication.
Zeilinger can claim several other firsts in the field. In 1989, he and colleagues Daniel M. Greenberger and Michael A. Horne published the first paper describing entanglement involving more than two particles. This resulted in the GHZ theorem, which demonstrates the contradiction between the classical physics concept of local realism and the fundamentals of quantum mechanics.
Throughout the 1990s, his team was the first to teleport an independent photon, to demonstrate entanglement-based quantum cryptography, to show quantum purification of entangled states and to demonstrate what is called hyper-dense coding, to name a few. In 1998, Zeilinger confirmed the GHZ theorem experimentally.
Zeilinger believes that John Bell made the most significant contribution to quantum mechanics in the past 50 years when he discovered that entanglement makes the classical physics concept of local realism untenable. “Some of the most exciting developments in the field concern fundamental issues about whether quantum physics is about reality or about information,” Zeilinger says.
In a 2006 interview with the Swiss magazine Die Weltwoche, when asked about the counter-intuitive nature of entanglement, Zeilinger said, “The spooky effect at a distance is a process outside time and space that even I can’t really imagine. But I believe that quantum physics tells us something very profound about the world, and that is that the world is not the way it is independently of us...”
For Zeilinger, these basic, and sometimes mind-boggling, fundamentals of quantum science are what make quantum physics fun.
Anton Zeilinger will deliver a plenary session keynote address, “Photonic Entanglement and Quantum Information” at OSA’s Annual Meeting, Frontiers in Optics, Oct. 19-22 in Rochester, N.Y. For more information, visit www.frontiersinoptics.org.
Angela Stark is the public relations coordinator at OSA.