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news feature The philosopher of photons From meeting the Dalai Lama to national media star, Anton Zeilinger is on a mission to bring physics to a wider audience. Quirin Schiermeier listens in. nlightenment is hard work — especially when you mix philosophy with quantum E ANOLIN.COM physics. But the Dalai Lama is Z always keen to investigate areas of science, no matter how complex they seem. And in 1997, when he wanted to delve into the quantum world, one of the men he invited to India was Anton Zeilinger. Based at the University of Vienna in Austria,Zeilinger is com- mitted to explaining his work to a broad audience. As a student, he was fascinated by both opera and the mathematical beauty of quan- tum mechanics. And although Zeilinger, who turns 60 in May, has just revived his old student hobby of playing walk-on parts at the Tough talking: Anton Zeilinger (left) discusses quantum physics and philosophy with the Dalai Lama. Vienna State Opera House, it was fundamental physics that won his heart.Over When the Dalai Lama made a return visit performed the first real transfer of money the years, his scientific achievements, as well to Zeilinger’s lab, then in Innsbruck, a year from a bank using quantum cryptography. as his communication skills,have made him a after their first meeting, he confessed to hav- The possibilities of multiparticle entan- media star,at least in his native Austria. ing difficulties with the philosophical impli- glement are endless,Zeilinger says.Until now, “Explaining science to the public is hard cations of quantum physics, especially the entanglement experiments, such as the ‘tele- work,” he says jovially, while clearing a spot role of chance and causality in nature. As the portation’ of a complete quantum state from on a visitor table that is overflowing with idea of determinism is central to Buddhism, one particle to another, have used up to four manuscripts. He knows that fellow the existence of purely random acts might call particles (usually photons). If it were possible researchers can be scornful of the media into question Buddhist doctrine,he said. to ‘connect’even more particles and maintain attention he gets. And he admits that he But Zeilinger is keen to stress that his high-quality entanglement over greater sometimes feels uneasy about popular por- research,and the ground-breaking quantum- distances,this may open up entire new worlds trayals of his work. When the leading Ger- optical experiments he has designed, are all of quantum communication,he says. man news magazine Der Spiegel last month explained by the known laws of physics. Less Zeilinger dreams of using satellites for featured him on its cover as ‘The Sorcerer mysterious,perhaps,but equally fascinating. quantum communication between any two from Vienna’,with a story full of references to spots on the globe. Although transmitting time travel, parallel universes and pop cul- A tangled tale entangled photons on the ground is limited ture,he was not too happy. When a photon of polarized laser light passes to distances of less than 20 kilometres, in But we need capable translators. Quan- into certain types of crystal, it can generate a space it is much easier. A feasibility study for tum mechanics so often contradicts our pair of lower-energy photons that are ‘entan- supplying satellites with high-precision lasers ideas of how cause and effect determine what gled’ in terms of their respective polarization to produce entangled photons and send them we see, that many physicists struggle to — the vertical or horizontal direction in to Earth has already been made. Zeilinger accept its description of the physical world. which they oscillate. In Zeilinger’s hands, hopes that the European Space Agency will Even Einstein felt notoriously spooked by the these entangled particles offer revolutionary fund a payload of such components to the concept of quantum entanglement,in which ways of communication and computation. International Space Station by 2010. independent quantum particles remain One application, cryptography, has The ultimate goal might be to connect a mysteriously linked. So journalists appreci- already been demonstrated, and is close to network of future quantum computers. ate Zeilinger’s ability to translate the math- being commercially exploited. Quantum Zeilinger’s group has recently demonstrated ematics into a working philosophy. cryptography cannot prevent a spy from try- the first one-way quantum computer, in Zeilinger is also happy discussing the ing to decode a secret transmission. But the which an entangled multi-photon state challenges of quantum mechanics — both spy’s actions will irreversibly modify a secret starts out with all of the possible calculation technical and philosophical. “I strongly feel quantum key sent from the sender to the results, and proceeds by irreversible steps to that we need to clearly tell the people what we receiver, providing cast-iron proof that an reach the final answer (P. Walther et al. don’t understand,” he says. For example, unwanted third party is listening in. Nature 434, 169–176; 2005). But a future quantum physicists have yet to find a satisfy- As quantum information is so easy to dis- ‘Internet’ based on the laws of quantum ing explanation for the randomness inherent turb, transmitting entangled photons over physics is currently pure science fiction. in the quantum world. “Mysteries of that useful distances is a considerable challenge. Something else for the Dalai Lama, and kind are among the biggest questions of the But the technology is making good progress. the rest of us,to meditate over. ■ twenty-first century,”Zeilinger says. Last year, for example, Zeilinger’s group Quirin Schiermeier is Nature’s German correspondent. 1066 NATURE | VOL 434 | 28 APRIL 2005 | www.nature.com/nature © 2005 Nature Publishing Group.

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