Dance of the Photons: From Einstein to Quantum Teleportation

  • Anton Zeilinger
FSG: 2010. 320 pp. $26 9780374239664 | ISBN: 978-0-3742-3966-4

The only way to understand the quantum world is to measure it. This empirical view is dear to the heart of Anton Zeilinger, now at the University of Vienna, a leading figure in quantum physics through his work on correlated photons. In Dance of the Photons, he explores the phenomenon of quantum entanglement, the quantum correlations in the properties of particles.


When two photons are made to interact, they share their quantum information and become 'entangled'. If one travels off, it retains knowledge about its counterpart. So measuring one can determine the state of the other, even if they are far apart. Albert Einstein was worried by such reasoning: instant messaging between entangled particles contradicted his theory of relativity, which stated that signals cannot travel faster than the speed of light, unless you allow the crazy idea that particles do not have real properties independent of measurement. Quantum mechanics, he decided, was not up to explaining the world.

Zeilinger explains that Einstein was wrong. Experiments in the 1980s and 1990s proved the weird predictions of quantum entanglement to be true. Putting the reader in the role of discoverer, he describes these tests through the eyes of fictional students Alice and Bob, namesakes of the characters regularly put to work in explaining quantum physics. Examining the philosophical and technological implications of spooky quantum phenomena, he points to big issues that demand further thought — the inherent randomness of quantum physics and the role of the observer in determining a quantum particle's reality.

As well as giving an overview of other work, Zeilinger relates in detail his own group's research. For instance, he describes a 'delayed choice entanglement swapping' experiment he has carried out using four photons (1, 2, 3, 4). Two pairs share prior information: photons 1 and 2 are entangled, photons 3 and 4 are also entangled, but there is no correlation between those pairs. Making a particular type of quantum measurement — known as a Bell measurement — jointly on photons 2 and 3 entangles them and then destroys them. Through their prior links, this connection then entangles the states of photons 1 and 4, even though they have never interacted and may be very distant from one another. This remarkable property also has practical significance — the ability for two parties to share entanglement over long distances could have applications in secure communications and powerful distributed processing.

Even stranger things can happen. It is possible to delay the measurement on photons 2 and 3 until after photons 1 and 4 have been detected. One need not even decide whether to make that measurement until after 1 and 4 are detected. Yet the experiment seems to 'know' what you will do in advance: 1 and 4 appear entangled if a later measurement of 2 and 3 is made; they are not entangled if not. It is as if photons 1 and 4 knew the future — whether or not the measurement would be made at a later time. The state of the photon not only seems to depend on the choice of measurement, but also on measurements that are yet to be made. This has implications for our ideas about reality and time, but Zeilinger reminds us that we must always make a careful accounting of the data. The reward for following Alice and Bob's reasoning as they teach us how to puzzle out these types of result is a rich understanding of entanglement beyond the simplified picture.

Zeilinger adds local colour throughout the book. In his tale, however, the real treasure of Vienna is not its opera, nor Ludwig Boltzmann's blackboard (which was used for the book's sketches), but a set of dark tunnels under the River Danube. These are home to a photon teleportation experiment, in which the quantum polarization state (which shows the orientation of the plane in which the light wave oscillates) of a photon on one side of the Danube is instantaneously transferred to a photon on the other side. Again, the author gives the science a human face: we meet Rupert, possibly a caricature of Zeilinger's postdoc, who is condemned to the tunnels to keep the equipment running. Fortunately, Zeilinger instils him with a sense of humour.

The Vienna group's latest entanglement experiments are performed on a far larger scale — between two of the Canary Islands. A telescope with a one-metre-diameter mirror is used to catch an entangled photon that has travelled 144 kilometres through the turbulent atmosphere. Optimizing the optics, stabilizing the pointing systems and synchronizing the electronics over picoseconds make these experiments challenging, but they have enabled even more careful tests of the counter-intuitive features of quantum entanglement. By using satellites to send the quantum signals, such techniques will one day allow us to distribute entangled information between far-distant locations on Earth.

The book concludes with an outlook of where entanglement will and won't take us. Teleporting humans may be out, as we can't entangle two atom-for-atom clones of a person. But the powerful way in which quantum states carry information opens the path to quantum computing and quantum cryptography. By sharing entanglement over optical fibres (as in the Danube experiment), secret keys can be distributed over short distances. Using entanglement swapping (as in the delayed choice experiment), we might build a quantum repeater — a device for extending key distribution over much longer ranges. Using satellites, secure worldwide communication networks between classical and quantum computers will become possible.

Dance of the Photons is an enjoyable introduction to the strange world of quantum phenomena and the technologies they empower. It gives a foundation from which to ponder the nature of randomness and reality — and whether, in Vienna, the photon dance is performed to a Strauss waltz. Maybe Rupert can tell us over a lager, if he's ever allowed out of the tunnels.