(Originally published by TU Delft)
April 24, 2013
Researchers at TU Delft have managed to bring two electrons, three metres from each other, into a quantum- entangled state. This result marks a major step towards realizing a quantum network that can be used to connect future quantum computers and to send information in a completely secure way by means of ‘teleportation’. The results have been published online on 24 April in Nature.
Entanglement is arguably the most intriguing consequence of the laws of quantum mechanics. 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. Einstein doubted this prediction, which he called ‘spooky action at a distance’, but experiments have proven its existance
Entangled states are interesting for computers as they allow a huge number of calculations to be carried out simultaneously. A quantum computer with 400 basic units (‘quantum bits’) could, for example, already process more bits of information simultaneously than there are atoms in the universe. In recent years, scientists have succeeded in entangling quantum bits within a single chip. Now, for the first time, this has been successfully achieved with quantum bits on different chips.
Prof. Ronald Hanson’s research group at TU Delft’s Kavli Institute of Nanoscience makes quantum bits from electrons in diamonds. ‘We use diamonds because they form ‘mini-prisons’ for electrons if there is a nitrogen atom in the position of one of the carbon atoms. As we can examine these mini-prisons individually, we can study and monitor an individual electron and even a single atomic nucleus. We can prepare the spin (direction of rotation) of these particles in a previously determined state, control the spin and subsequently read it out. We do all this using a material from which chips can be made. That is important it is believed that only chip-based systems can be scaled up to a practical technology’, Hanson explains.Video: The two electrons are stimulated by a laser pulse to emit a photon (particle of light). This photon carries with it information on the state of the electron. Both photons pass at the same time through a semi-transparant mirror. Following the laws of quantum physics, a photon detected behind the mirror came from both electrons at the same time. This way detection of a photon entangles the two electrons. Measurement of the state (‘spin’) of one of the electrons then instantaneously determines the state of the other electron. Compare this to tossing a coin. Tossing a single coin yields a random outcome. But for entangled coins, whenever one of the coins shows “heads”, the other one always yields “tails”, and vice versa.
Co-financed by the FOM Foundation and in cooperation entity. ‘Incidentally, the three-metre distance between the electrons was chosen quite arbitrarily. We could conduct this experiment over much larger distances’, Hanson adds.with the British firm Element Six, Hanson and his colleagues succeeded in bringing two electrons in different diamonds, situated at several metres’ distance from each other, into an entangled state. As the two electrons do not feel each other at this large distance, the researchers used light particles to mediate the required interaction. To prove the resulting entanglement, the spin orientation of both electrons was read out and compared. Although the spin orientation of each electron individually was completely random, exactly as predicted by quantum mechanics, the researchers found that the two orientations were always exactly opposite to each other. This proves that the two electrons are entangled and behave as a single entity. ‘Incidentally, the three-metre distance between the electrons was chosen quite arbitrarily. We could conduct this experiment over much larger distances’, Hanson adds.
TeleportationThe next step for the research is the teleportation of electrons. Hanson: ‘In theory it is possible to ‘teleport’ the state of a particle over a large distance to another particle by making smart use of entanglement. Quantum teleportation does not relocate the material itself, but only the state of that material. But given the fact that all elementary particles are identical, quantum teleportation of one electron to another has the same effect as relocating the electron.’
According to Hanson, in addition to new fundamental insights, there are two further reasons why the publication in Nature is likely to be an important impulse for the development of new technologies. ‘Firstly, because this is an important step towards creating a quantum network for communication between future quantum computers – a quantum internet. We are already working on expanding the experiments using more quantum bits per chip. Entanglement could be used to link such a network of quantum computers.’
‘Secondly, teleportation offers the possibility of sending information in a completely secure way. With teleportation, the information does not travel through the intermediate space and therefore cannot be intercepted.’