The Teleportation Tango

Quantum Teleportation has been in the news recently, so here is some background, from my book Computing With Quantum Cats.

Quantum teleportation is based on the spooky action at a distance that so disgusted Einstein but is demonstrated to be real in tests of the EPR “paradox” and measurements of the Bell inequality.  It rests on the fact — confirmed in those experiments — that if two quantum entities, let’s say two photons, are entangled, then no matter how far apart they are what happens to one of those two photons instantly affects the state of the other photon.  But the refinement is that by tweaking the first photon in the appropriate way (called a “Bell-State Measurement”) its quantum state can be transferred to the second photon, while the state of the first photon is, of course, changed by being tweaked.  In effect, the first photon has been destroyed and the second photon has become what is termed in common parlance a clone of the first photon.  Since the original has been destroyed, however, for all practical purposes the first photon has been teleported to the location of the second photon, instantly.  It is not a duplication process (and it has also been done with trapped ions!).

There’s one small catch.  In order to complete the transformation, information about the way the first photon was tweaked has to be transmitted to the location of the second photon by conventional means, no faster than the speed of light.  This information is then used to tweak the second photon in just the right way (not the same way that the first photon was tweaked, but in a kind of reverse process) to complete the transformation.  In effect, the conventional signal tells the system what tweak has been applied to photon number one, and the system then does the opposite to photon number two.  Quantum teleportation requires both a quantum “channel” and a classical “channel”; it takes two signals to dance the teleportation tango.

A large and successful research effort has gone into making this reality, not least because quantum information offers a way of transmitting information utterly securely using systems that cannot be cracked.  I have explained the details in my book Schrödinger’s Kittens, but the essential point is that information travelling by the quantum “channel” cannot be read by a third party; in addition, any attempt to eavesdrop will alter the quantum state of the photons, making it obvious that they have been interfered with.  This is not the reason why teleportation helps in the design of quantum computers; indeed, in recent times headline-making developments in quantum teleportation have concentrated on much larger scales than those appropriate for computation.  But their success emphasises the reality of the process, and how good scientists now are at working with quanta.

In 2012, two record-breaking experiments made those  headlines — by the time you read this, they will probably both have been superceded.  First, a large group of Chinese researchers succeeded in teleporting a quantum state through 97 kilometres of open air across Qinghai Lake, using a telescope to focus the photons.  Almost as an aside, the experiments confirmed the by-now-expected violation of the Bell inequalities, offering insight for the theorists into the foundations of quantum physics.  A few weeks later, a team from Austria, Canada, Germany and Norway teleported the properties of a photon across a distance of 143 km, from the astronomical observatory on La Palma, in the Canary Islands, to a European Space Agency ground station on the neighbouring island of Tenerife.  Both the transmitting station and the receiving station were located roughly 2,400 metres above sea level, where the air is thin and atmospheric interference is reduced.

But the air is even thinner at higher altitudes, so that in some ways it should be easier to carry out quantum teleportation, and achieve secure communication, by pointing the beams upward to a satellite.  The distances involved are very similar to those already achieved on the ground, and although there are, of course, many other problems involved in establishing this kind of satellite communication, the Chinese are already planning a satellite experiment, provisionally scheduled for launch in 2016 or 2017, to test the possibilities, using ground stations in Europe and in China to communicate with the satellite simultaneously for a few minutes in each orbit.  This is particularly important because this kind of quantum information is soon lost if the photons are sent through fibre optic cables.  The leader of the Chinese team, Pan Jianwei, of the University of Science and Technology of China in Hefei, envisages an eventual network of satellites acting as repeater stations for global coverage of a quantum communications network.  This could be the basis of an utterly secure quantum internet; and in all probability many of the computers plugged in to that internet will by then themselves be running on quantum principles, including teleportation.

In connection with this work, Chinese researchers have devised ever better techniques for entangling photons.  In 2004, they could produce a few four-photon entanglement events every second; by 2012, they could produce entangled groups of four photons at a rate of a few thousand per second.


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