Electrons jump between quantum dots

Just days after TU Delft scientists entangled quantum bits on distant chips, another group has successfully allowed electrons to jump between quantum dots. Both groups of the Kavli Institute of Nanoscience lay a basis for quantum information processing.


Researchers, led by Professor Lieven Vandersypen, have showed that they could make an electron jump between the ends of a chain of three small semiconducting islands (so-called quantum dots) without crossing the island in the middle.

The researchers published their findings on 28 April online in Nature Nanotechnology, just three days after Prof. Ronald Hanson’s research group published an article (an advanced online publication) in Nature showing how they entangled quantum bits on distant chips. Both approaches seem promising for future quantum information processing.

The approaches differ significantly. The most important difference is the way in which informationis read out. Hanson’s group uses light, whereas Vandersypen’s group uses electronics.

Hanson’s qubits are diamond-based and housed in cryostats. The electron-spins of the electrons in these qubits can be set, read out and – as Hanson recently showed – also be entangled by fibre-coupled lasers. When two particles become entangled, their identities merge. Their collective state is precisely determined but the individual identity of each of the particles has disappeared. The entangled particles behave as one, even when separated by a large distance.

Vandersypen’s group works with chips. In their experiment with three adjacent quantum dots, the scientists adjusted the voltage such that the middlemost quantum dot was not accessible for electrons. To their surprise they discovered that an electron could still disappear from the left dot and subsequently reappear on the right dot. That is possible due to the Heisenberg uncertainty principle, which states that energy and time cannot exactly be determined at the same time.

“Both our work is about long distance coupling of quantum dots”, says PhD student Floris Braakman (MSc), the first author of the jumping electron article (entitled ‘Long-distance coherent coupling in a quantum dot array’). “Hanson however showed that he could entangle electrons at distance of three meters, which is gigantic. In analogy to humans it is like two people communicating over a distance of a quadrillion light years. The distance between our quantum dots was just a micrometre or so. Yet in a sense the communication between our quantum dots was also long distance, be it in a more conceptual way, since the quantum dots could communicate with one another regardless of the quantum dot that was set in the middle.”

Braakman and his colleagues also demonstrated that this process can generate superpositions: the electron is simultaneously present in both the first and the third quantum dot. “This means that the interesting physical phenomena we observe in adjacent quantum dots also appears to occur in quantum dots that are located further away from each other”, says Braakman.

The findings could have consequences for the use of a future type of quantum computer based on quantum dots. Calculations can only be performed on such computers if pairs of electrons can be brought close together. Previously this could only be realised between adjacent quantum dots. Now that it appears that electrons from more distant quantum dots can also be brought together, the scaling up to larger chains of quantum dots has become far easier.

F.R. Braakman, et. al., ‘Long-distance coherent coupling in a quantum dot array’, 28 April, Nature Nanotechnology.

H. Bernien, B. Hensen, R. Hanson et. al., ‘Heralded entanglement between solid-state qubits separated by 3 meters’, Nature AOP

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