Entanglement, the quantum mechanical phenomenon, was coined as a term by Erwin Schrödinger in 1935 but is still not understood completely.    From an applied perspective, while entangled particles cannot be defined as single particles with defined states but are instead a whole system, that means by entangling single quantum bits, a quantum computer should solve problems considerably faster than conventional computers. 

But understanding entanglement when there are two particles is tough enough.   When there are many, it's even trickier, but a new experiment in the research group led by Rainer Blatt at the Institute for Experimental Physics at the University of Innsbruck may provide some insight.

Since 2005, Blatt's group has held the record for the number of entangled quantum bits realized experimentally - nobody else has been able to achieve controlled entanglement of eight particles, which represents one quantum byte.   Now, they write in Physical Review Letters, they have nearly doubled that, confining 14 calcium atoms in an ion trap, which, similar to a quantum computer, which they then manipulated with lasers.   The internal states of each atom formed single qubits and a quantum register of 14 qubits was produced. 

Controlled entanglement of 14 quantum bits (qubits) and, the largest quantum register ever  produced.  Credit: University of Innsbruck

They also found out that the decay rate of the atoms is not linear, as usually expected, but is proportional to the square of the number of the qubits. When several particles are entangled, the sensitivity of the system increases significantly. "This process is known as superdecoherence and has rarely been observed in quantum processing," explains Thomas Monz. It is not only of importance for building quantum computers but also for the construction of precise atomic clocks or carrying out quantum simulations.

They have succeeded in confining up to 64 particles in an ion trap but are not able to entangle that high number of ions yet.    A few weeks ago, in Nature, they showed that ions might be entangled by electromagnetic coupling. This enables the scientists to link many little quantum registers efficiently on a micro chip. All these findings are important steps to make quantum technologies suitable for practical information processing," Blatt is convinced.