Physicists in Switzerland and Denmark have created a device that can separate pairs of entangled electrons. The device, which is based on a superconducting "Y" junction, should pave the way for tests of the so-called non-locality of quantum mechanics in the solid state.

In the theory of quantum mechanics, when two particles are entangled the measurement of one can affect the state of the other, no matter how far they are separated. Such non-locality would seem to go against Einstein’s theory of relativity, which implies that no information can travel faster than light. Even so, tests of non-locality using entangled pairs of photons have so far shown quantum mechanics to be correct.

But tests of non-locality using electrons – that is, matter in the solid state – has proved trickier. Unlike photons, which are relatively easy to create and manipulate in isolation, electrons in materials reside en masse in a "Fermi sea", making it difficult to isolate a well defined pair.

On solid ground

"It is important [to check non-locality] for electrons in a solid because these are so-called quasi-particles that live in an environment of many electrons," explained Christian Schönenberger at the University of Basel. "Quantum phenomena in a background of strongly interacting matter are very different from the existing studies with photons in a vacuum."

Schönenberger’s group, which includes others at Basel and at the University of Copenhagen, has found a way to extract entangled pairs of electrons, and separate them, using a superconducting Y-junction. An important property of the superconductor is that electrons can exist in entangled "Cooper pairs". Such pairs cannot enter the Y-junction without passing through a barrier. Because of the low probability of passing this barrier, Cooper pairs tend to enter the junction one at a time.

The next step is to ensure that the pairs split, rather than having both electrons travelling down just one arm. They do this by placing a tiny piece of semiconductor – a quantum dot – at the end of each arm. A lone electron can pass through a quantum dot but it is unlikely that two electrons (which repel each other electrically) will squeeze through at once.

Non-local correlation

The team confirmed that entangled Cooper pairs were indeed being separated by adjusting the resistance of one of the quantum dots while monitoring the conductance of each arm. When the electron source was in a superconducting state, a "non-local correlation" between these parameters was seen suggesting that entangled pairs were being separated. However, when a magnetic field was applied to the electron source – destroying its superconductivity and Cooper pairs – the non-local correlation vanished.

Takis Kontos, whose group at the École Normale Supérieure in Paris has submitted a similar study to Physical Review Letters (preprint at arXiv:0909.3243) that uses carbon nanotubes in place of superconducting wires, thinks Cooper-pair splitting is "an important step forward".

"It opens the avenue for the implementation of much more advanced quantum-optics-like experiments in electronic systems," he said. "One could, for example, envision correlation experiments together with the use of spin filters in order to probe quantum entanglement in a very elegant way…The findings presented in this paper bring on very exciting perspectives and are very likely to generate a renewed and intense experimental and theoretical activity."

Schönenberger told that his and other researcher groups are now pursuing tests of non-locality, in particular using statistical studies of so-called Bell’s inequalities, which reveal whether the behaviour of two entangled particles is correlated.

The research is published in Nature.