Entanglement is the strange quantum phenomenon in which objects become so closely linked that they share the same existence. In the language of physics, they are described by the same wavefunction.
Entangling things isn’t so difficult really. Most interactions involve entanglement of one sort or another.
The trouble is pinning it down. Entanglement is a fragile and fleeting phenomenon. Blink and it leaks into the environment. That’s why it’s so difficult to preserve, to observe and ultimately so difficult for physicists to play with.
In recent years, physicists have learnt how to entangle all kinds of objects in pairs–photons, electrons, atoms and so on. In 1999, they created a qutrit by entangling three photons. Last year, they even entangled 6 photons.
Today, however, Xing-Can Yao and buddies at the University of Science and Technology of China in Hefei, say they’ve smashed this record by entangling 8 photons, then manipulating and observing them all simultaneously.
That’s no easy feat. Getting eight photons exactly where you want them at the same time is the quantum mechanical equivalent of herding cats (clearly of the Schrodinger variety).
The trick is to first send a high energy photon through a nonlinear crystal that converts it into two entangled but lower energy photons. One of these, photon A, enters the experimental apparatus while the other is split again into two by another crystal.
This pair is, of course, entangled with photon A. One of this pair then enters the apparatus while the other is again split, creating yet another pair that are entangled with photon A. One of these enters the apparatus while the other is split and so on, until there are eight photons in the apparatus, all entangled with each other and photon A .
The trouble with this process is that it results in a very weak beam. With the kind of lasers available until recently, the best that could be managed was a count rate of about 10^-5 hertz. That’s one simultaneous strike of 8-photons every hundred thousand seconds or about one count a day. Even postdocs don’t have that kind of patience.
Xing-Can Yao and co say they’ve got around this using a much brighter ultraviolet laser source that produces entangled photon pairs at a much higher rate. Of course, they’ve also had to learn how to manipulate eight entangled photons too.
That’s significant. Having eight entangled photons is the closest physicists have come to having a Schrodinger’s cat in the lab. This “may provide new insights into our understanding of the intriguing questions of classical to quantum transition,” say Xing-Can Yao and co.
But it also allows a host of other quantum tricks too. For example, an 8-photon state should allow them to demonstrate a powerful way of correcting quantum errors called topological error correction. Many physicists think that topological error correction will be one of the enabling technologies of large scale quantum computing but nobody has been able to test it, until now.
And being able to manipulate an 8-photon state will allow them to simulate other quantum systems. That should make it possible to simulate for the first time various phenomena in quantum chemistry and even in biophysics.
And to do it using nothing but light (with a little smoke and mirrors thrown in).
Ref: arxiv.org/abs/1105.6318: Observation Of Eight-Photon Entanglement