Single Photon Reveals Entangle­ment of 16 Million Atoms

Partial view of the source producing the single photons that were stored in the quantum memory to produce entanglement between many atoms inside the memory. (Source: U. Geneva)

Quantum theory predicts that a vast number of atoms can be entangled and inter­twined by a very strong quantum relation­ship, even in a macro­scopic structure. Until now, however, experi­mental evidence has been mostly lacking, although recent advances have shown the ent­anglement of 2,900 atoms. Scientists at the Univer­sity of Geneva (UNIGE), Switzer­land, recently reengineered their data processing, demon­strating that 16 million atoms were entangled in a one-centi­metre crystal.

The laws of quantum physics allow imme­diately detecting when emitted signals are inter­cepted by a third party. This property is crucial for data pro­tection, especially in the encryp­tion industry, which can now guarantee that customers will be aware of any inter­ception of their messages. These signals also need to be able to travel long distances using special relay devices known as quantum repeaters – crystals enriched with rare earth atoms and cooled to 270 degrees below zero, whose atoms are entangled and unified by a very strong quantum relation­ship. When a photon pene­trates this small crystal block, ent­anglement is created between the billions of atoms it traverses. This is explicitly predicted by the theory, and it is exactly what happens as the crystal re-emits a single photon without reading the infor­mation it has received.

It is rela­tively easy to entangle two particles: Splitting a photon, for example, generates two entangled photons that have identical properties and beha­viours. Florian Fröwis, a researcher in the applied physics group, says, “But it’s impos­sible to directly observe the process of ent­anglement between several million atoms since the mass of data you need to collect and analyse is so huge.” As a result, Fröwis and his colleagues chose a more indirect route, pondering what measure­ments could be under­taken and which would be the most suitable ones.

They examined the charac­teristics of light re-emitted by the crystal, as well as analysing its sta­tistical proper­ties and the proba­bilities following two major avenues – that the light is re-emitted in a single direction rather than radia­ting uniformly from the crystal, and that it is made up of a single photon. In this way, the researchers succeeded in showing the entangle­ment of 16 million atoms when previous observations had a ceiling of a few thousand. In a parallel work, scientists at University of Calgary, Canada, demon­strated ent­anglement between many large groups of atoms. “We haven’t altered the laws of physics,” says Mikael Afzelius, a member of Pro­fessor Nicolas Gisin’s applied physics group. “What has changed is how we handle the flow of data.”

Particle entangle­ment is a prerequisite for the quantum revo­lution that is on the horizon, which will also affect the volumes of data circu­lating on future networks, together with the power and operating mode of quantum computers. Now, imagine there are two physi­cists in their own labora­tories, with a great distance separating the two. Each scientist has a a photon. If these two photons are in an entangled state, the physi­cists will see non-local quantum corre­lations, which conventional physics is unable to explain. They will find that the pola­risation of the photons is always opposite, and that the photon has no intrin­sic polari­sation. The polarisation measured for each photon is, there­fore, entirely random and funda­mentally indeter­minate before being measured. This is an unsys­tematic pheno­menon that occurs simul­taneously in two loca­tions that are far apart. (Source: Unige)

Reference: F. Fröwis et al.: Experimental certification of millions of genuinely entangled atoms in a solid, Nat. Commun. 8907 (2017); DOI: 10.1038/s41467-017-00898-6

Link: Dept. of Applied Physics, Université de Genève, Genève, Switzerland


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