A cherished viewpoint in physics, and even in daily life for that matter, is that any action on something from a distance must somehow travel through the intervening space. This was put into question in 1935 when Albert Einstein, Boris Podolsky and Nathan Rosen discovered strong correlations between distant particles that had interacted before. The Austrian physicist Erwin Schrodinger coined the notion "entanglement" to describe them. The situation might be compared to identical twins. If you observe one twin to have, say, blue eyes, you know the other will also have blue eyes, no matter how far away she is. Yet, while for identical twins we easily understand these perfect correlations because they carry the same genes, such an explanation is not possible for entangled pairs of particles. For example, photons can be entangled so they will always be found to have the same polarisation no matter how distant they are. The polarisation of each individual photon is always completely random and it carries no hidden property. But when one photon's polarisation is measured, the other one will instantly assume the same. This is why Einstein spoke of "spooky action at a distance".
In my laboratory we routinely play with such entangled quantum states of two, three, or even four photons and we are beginning to put them to work to produce novel methods of communication. For example, quantum teleportation permits the direct transfer of the quantum state of a photon to a distant one by directly exploiting entanglement.
Another example is in cryptography. The security of certain quantum cryptographic schemes depends solely on the features of entangled states - the randomness of an individual result and the perfect correlations between two distant measurements. Using entangled pairs of photons, my group was recently able to securely and secretly transmit a picture of the Venus of Willendorf effigy over a distance of about 350m. The cryptography experiments send individual photons to a distant laboratory, employing very fast electronics that operate on a timescale well below a microsecond. Our next plan is to verify teleportation across the campus of the University of Vienna.
Quantum spookiness is even stranger for states where more than two particles are entangled. For three photons, the polarisation of the first can have two definite but different values, depending on what measurement is performed on the other, distant two. Such multiparticle entanglements will be of crucial significance in novel quantum communication and computation schemes.
What remains to be done is to extend such experiments to larger and more complex systems. Some day they will certainly be done with atoms or even large molecules. In a recent experiment we were very surprised that individual molecules as massive as fullerenes, consisting of 60 or 70 carbon atoms, showed perfect quantum coherence even at temperatures as high as 1,000oK.
While the demonstration of quantum phenomena for living objects, such as the cat of Schrodinger's famous paradox, is still far away, we are working at finding ways to demonstrate quantum phenomena for large biomolecules or even small viruses.
In my group, the next generation of experimentalists has a completely new, intuitive access to quantum phenomena as a result of its day-to-day work. As we shed our classical instincts, I expect more puzzling quantum experiments will be invented and realised.
Anton Zeilinger is professor of physics at the University of Vienna, Austria.
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