Over the entire action the fellow has demonstrated a highly efficient (>99%) electron transfer in an integrated circuit to realise the new tools mentioned above. The fellow has also shown an important step to realise both a coherent beam splitter and a phase shifter for single flying electrons. Furthermore, the fellow has transferred his knowledge about coherent manipulation of ballistic electrons to an ESR in the group, which will help to keep on-going this challenging project. The obtained results of the action have made quantum optics experiments with single flying electrons within close reach.
To be more precise, in this project a single-electron transfer circuit developed in the previous experiment is integrated with a tunnel-coupled wire (TCW), where two-parallel quantum wires are coupled via a narrow tunnel-barrier, to realise a beam splitter as well as a phase shifter for single flying electrons. A TCW has been shown to work as a beam splitter for ballistic electrons by the fellow and his collaborators. The device is based on a GaAs/AlGaAs heterostructure that hosts a two-dimensional electron gas (2DEG) about 100 nm below the surface. The device geometry is defined by locally depleting the 2DEG using voltages on surface Schottky gates. The device also consists of an interdigital transducer (IDT) to generate surface acoustic waves (SAWs).
Firstly the fellow investigated several different geometries of the integrated device described above to find a geometry allowing for a highly efficient single-electron transfer. The investigations were carried out by electron-transport experiments at low temperatures (<100 mK) in a dilution refrigerator. After testing several different geometries and solving issues coming from the integration of a TCW, the fellow came up with the design shown in “Device.png”. With this device the fellow demonstrated highly efficient electron transfer (>99%).
Then the fellow worked on the realisation of a coherent beam splitter as well as a phase shifter for single flying electrons. By controlling the voltages on the TCW it was shown that an electron can be distributed to different paths with arbitrary probabilities. This operation formally corresponds to a directional coupler for single flying electrons. Further more the fellow demonstrated a technique to synchronise different single-electron sources. It is realised by synchronising a SAW burst with a sub-ns short voltage pulse on the gate of a source quantum dot (QD). The fellow showed that an electron can be loaded into a specific potential minimum of SAWs by simply controlling the delay time of the pulse with respect to the SAW burst.
In addition to the experiments to develop the new tools, the fellow worked on transferring his knowledge about coherent control of ballistic electrons using a TCW to an ESR in the group. For that purpose transmission phase measurements across a large QD (N~300) have been realised in a quantum two-path interferometer in order to solve a long standing puzzle of the transmission phase across a large QD. By careful and precise investigations important new insights towards the full understanding of this fundamental scattering problem across a QD have been obtained.
In summary the fellow has improved the efficiency of single-electron transfer in the integrated circuit from previously 90% to more than 99% which represents an important achievement towards the full control of single electrons in an on-chip device. The directional coupler developed in this project is also an important step to demonstrate a coherent beam splitter operation. Another important result is the synchronisation technique of different single-electron sources. It is required to demonstrate a phase shifter by controlling Coulomb interaction between single electrons from two different sources.
The work achieved by the fellow has been disseminated by several means and will have an important impact in the physics community:
- oral presentation at the International Conference on the Physics of Semiconductors (ICPS 2016), Beijing, China (participants >1000).
- Department seminar at the Institute of Solid State Physics (ISSP), University of Tokyo, Japan.
- Institute seminar at the “Institut de Physique et Chimie des Matériaux” (IPCMS), Strassbourg, France.
- Oral presentation at the yearly meeting of the French mesoscopic community (GDR2016).
Several publications have been realized during the project. In addition an invited review article on “Coherent control of single electrons” has been submitted to IOP and another manuscript on transmission phase measurements through a QD is under review.
