(Press release material)
September 4, 2015
Nippon Telegraph and Telephone Corp. (NTT) (Head Office, Chiyoda-ku, Tokyo; Hiroo Unoura, President and CEO) and CEA Saclay have demonstrated for the first time that a graphene*1 p-n junction*2 can serve as an electronic beam splitter. The realization of electron beam splitters, key components for interferometry, is a crucial step towards the development of electron quantum optics experiments in graphene.
Owing to the probable long coherence length*3 of electrons, graphene is expected to be a useful material for electron quantum optics. In this work, we proposed that graphene p-n junctions can serve as beam splitters and substantiated the performance in large-area graphene grown on silicon carbide (SiC). Utilizing the long coherence length in graphene, it could be possible to make complicated interferometers with multiple beam splitters, which are not able to be fabricated in conventional semiconductors. Experiments using such interferometers will reveal decoherence mechanisms of electrons and enable the generation of quantum entangled electron pairs. These studies contribute to the development of quantum information processing.
This work will be reported in the UK science journal “Nature Communications” on the 4th of September, 2015.
Quantum optics is a field of research that investigates the wave-particle nature of light based on quantum mechanisms, using interferometers consisting of optical elements, such as mirrors and beam splitters. Knowledge obtained by quantum optics has been applied to quantum cryptography and quantum teleportation. An electron is also a quantum having wave-particle nature like a photon and thus can interfere. A difference between an electron and a photon is whether it is a Fermion*4 or Boson*5. This difference appears in the two-particle interference. Another important difference is the presence/absence of the Coulomb interaction*6. The presence of the Coulomb interaction for electrons has both positive and negative aspects: through the interaction, electron quantum state can be controlled but its coherence is easily destroyed.
The resultant short coherence length makes electron quantum optics experiments difficult. So far, electron quantum optics has been performed in the quantum Hall state*7, which appears in two-dimensional electron systems under magnetic field, formed in gallium arsenide (GaAs)/aluminum gallium arsenide (AlGaAs) heterostructures*8. In this system, a narrow channel called quantum point contact*9 with the electron transmission probability of 1/2 (Figure 1, left) is used as a beam splitter. A fundamental problem of this system is that the coherence length of approximately 10 µm is as long as the typical size of the interferometer, limiting experiments to the basic level. A way to solve this problem and carry out more advanced experiments is to use graphene, in which the coherence length is expected to be longer. However, since graphene is a gapless material, beam splitters based on quantum point contacts made by depleting local electrons do not work.
In this work, a team comprised of members from NTT Basic Research Laboratories and CEA Saclay proposed a new architecture of a beam splitter using a graphene p-n junction. In graphene, electrons injected from the n and p regions are mixed in the p-n junction and then partitioned at the exit of the junction (Figure 1, right). The electron mixing and the subsequent partitioning processes can serve as a beam splitter. The beam splitter behavior has been verified by the measurements of the current noise (shot noise*10).
If the p-n junction acts as a beam splitter, electrons are randomly distributed to the n and p current channels, generating the shot noise. We showed that, when the p-n junction is short, the shot noise is generated as expected. As it becomes longer, the noise becomes smaller. Our results indicate that the energy relaxation length, which is an indicator of the loss of the quantum information, is 15 µm and shorter p-n junction can serve as a beam splitter.
In general, graphene is obtained by exfoliation from graphite. A drawback of this technique is that the graphene size is limited to several 10 µm. Whereas, the graphene we used were grown by thermal decomposition of SiC substrate. By exploiting the wafer-size graphene, we fabricated devices with different p-n junction lengths. This enables us to measure the energy relaxation length.
The architecture of graphene p-n junction is simpler than that of quantum point contact, which is used as a beam splitter in conventional semiconductors, allowing us to integrate several beam splitters within a mesoscopic scale. Utilizing these beam splitters and the long coherence length in graphene, it would be possible to fabricate complicated electronic interferometers, which have not been able to be fabricated. Experiments in these devices would reveal effects of the Coulomb interaction on decoherence. Furthermore, generation of quantum entangled electron pair is expected to be achieved in such devices.
N. Kumada, F. D. Parmentier, H. Hibino, D. C. Glattli, and P. Roulleau
“Shot noise generated by graphene p-n junctions in the quantum Hall effect regime”
Nature Communications (2015).
Complementary results from a group of Osaka University, Kyoto University, and NIMS will be published simultaneously in Nature Communications.
Nippon Telegraph and Telephone Corporation
Science and Core Technology Laboratory Group, Public Relations
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