Entanglement is the strange phenomenon in which quantum particles become so deeply linked that they share the same existence. Once rare, entangling particles has become routine in labs all over the world.
Physicists have learned how to create entanglement, transfer it from one particle to another, and even distil it. Indeed, entanglement has become a resource in itself and a crucial one for everything from cryptography and teleportation to computing and simulation.
But a significant problem remains. To carry out ever more complex and powerful experiments, physicists need to produce entanglement on ever-larger scales by entangling more particles at the same time.
The current numbers are paltry, though. Photons are the quantum workhorses in most labs and the record for the number of entangled photons is a mere eight, produced at a rate of about nine events per hour.
Using the same techniques to create a 10-photon count rate would result in only 170 per year, too few even to measure easily. So the prospects of improvement have seemed remote.
Which is why the work of Xi-Lin Wang and pals at the University of Science and Technology of China in Heifu is impressive. Today, they announce that they’ve produced 10-photon entanglement for the first time, and they’ve done it at a count rate that is three orders of magnitude higher than anything possible until now.
The biggest bottleneck in entangling photons is the way they are produced. This involves a process called spontaneous parametric down conversion, in which one energetic photon is converted into two photons of lower energy inside a crystal of beta-barium borate. These daughter photons are naturally entangled.
By zapping the crystal continuously with a laser beam, it is possible to create a stream of entangled photon pairs. However, the rate of down conversion is tiny, just one photon per trillion. So collecting the entangled pairs efficiently is hugely important.
That’s no easy tasks, not least because the photons come out of the crystal in slightly different directions, neither of which can be easily predicted. Physicists collect the photons from the two points where they are most likely to appear but most of the entangled photons are lost.
Xi-Lin and co have tackled this problem by reducing the uncertainty in the photon directions. Indeed, they have been able to shape the beams of entangled photons so that they form two separate circular beams, which can be more easily collected.
In this way, the team has generated entangled photon pairs at the rate of about 10 million per watt of laser power. This is brighter than previous entanglement generators by a factor of about four. It is this improvement that makes 10-photon entanglement possible.
Their method is to collect five successively generated pairs of entangled photons and pass them into an optical network of four beam splitters. The team then introduces time delays that ensure the photons arrive at the beam splitters simultaneously and so become entangled.
This creates the 10-photon entangled state, albeit at a rate of about four per hour, which is low but finally measureable for the first time. “We demonstrate, for the first time, genuine and distillable entanglement of 10 single photons,” say Xi-Lin and co.
That’s impressive work that immediately opens the prospect of a new generation of experiments. The most exciting of these is a technique called boson sampling that physicists hope will prove that quantum computers really are capable of things classical computers are not.
That’s important because nobody has built a quantum computer more powerful than a pocket calculator (the controversial D-Wave results aside). Neither are they likely to in the near future. So boson sampling is quantum physicists’ greatest hope that will allow them to show off the mind-boggling power of quantum computation for the first time.
Other things also become possible, such as the quantum teleportation of three degrees of freedom in a single photon and multi-photon experiments over very long distances.
But it is the possibility of boson sampling that will send a frisson through the quantum physics community.
Ref: arxiv.org/abs/1605.08547: Experimental Ten-Photon Entanglement