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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - FLYELEC (Quantum Optics with single flying electrons)

Teaser

Summary

Manipulation of quantum information coded into a quantum state of a single photon has been well established in quantum optics. For example, quantum cryptography, a secure way to transmit information, is a well known application. Such a development is owing to a long coherence time of photons due to the fact that photons are basically non-interacting quantum particles. On the other hand introducing correlations between photons at the single photon level is a challenging task due to the fact that they are non-interacting particles. In analogy to photons similar experiments should be possible with single flying electrons in a solid-state device. The strong Coulomb interaction between electrons allows us to envision new quantum entanglement schemes, which understanding is a key for quantum information processing. The overall objective of this project was to perform quantum optics like experiments with single flying electrons. A decisive step in this direction was the development of basic tools to coherently manipulate single flying electrons and to control the Coulomb interaction between such single flying electrons. In particular the research group aimed at realising a coherent beam splitter as well as a phase shifter for single flying electrons. As a result of this project the efficiency of single-electron transfer in the integrated circuit was improved from previously 90% to more than 99% what opens new application possibilities and impacts the quantum information processing technology development.

Work performed

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 â€œDevice.pngâ€. 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 t

Final results

In the field of quantum electron optics, up to now several coherent single-electron sources have been realised. Prime examples are an electron emitted from a QD into edge channels of a quantum Hal system or a Lorentzian voltage pulse applied to an ohmic contact of a two dimensional electron system. However in those approaches a single-electron detector is at present not available. In contrast, the present study exploits another approach, namely single electron transport using SAWs. Using this approach, very efficient single-electron sources and detectors have already been realised. This opens the possibility to perform quantum electron optics with single-shot detection. New tools such as a directional coupler and a technique to synchronise different single electron sources, which are important elements towards performing various quantum electron optics experiments with SAWs, have been developed during this project. Quantum electron optics is a promising candidate to realise a scalable quantum network. In addition, quantum electron optics experiments using this novel approach will allow for developing quantum electric circuits, which works at the single electron level. Future single electron circuits will have a huge potential for quantum engineering as well as information technology.