That makes entanglement important stuff.
“Stuff” is the way many physicists are beginning to think of entanglement: as a resource, rather like water or energy, to be called upon when needed in the new quantum world. These physicists want to be able to create entanglement, use it and store it whenever they need to.
The first two of these–creating and using entanglement–has been the subject of intense research for the last 30 or 40 years. But the ability to store entanglement in a useful way has eluded physicists. Until now.
Today, Christoph Clausen and buddies at the University of Geneva demonstrate not only how to store entanglement but how to release it again in fully working order.
Their device consists of a load of neodymium atoms buried in a crystal of ytterbium silicate, which when cooled, can absorb and store photons. The question that Clausen and co attempt to answer is whether this device can store entanglement too.
So they created a pair of entangled photons, sent one into the crystal and waited until it was emitted again. They were then left with this new photon and the original member of the pair. They then carried out a standard experiment, known as a Bell test, and proved that the pair were still entangled.
That’s impressive for several reasons. For a start, for the entanglement to be preserved, the entire crystal has to be involved. This crystal is about a centimetre in size and the idea that entanglement can be exchanged between a photon and an object of this size is amazing.
Next is the ability to transfer entanglement form a flying qubit–the photon–to a stationary one, the crystal. And to do it with photons with a wavelength of 1338nm, the so-called telecommunications wavelength that can pass easily through fibre optic cables. Any other wavelengths are interesting but practically useless for communications.