“Flying” excitons propelled by gigahertz (GHz) surface acoustic waves (SAWs) are outstanding messengers to fulfill these requirements. A main challenge for the implementation of flying excitonic quantum bits is the creation of two-level states for single excitons interconnected by a transport channel, where the particles can be stored, manipulated, and converted to single photons. Recently, researchers at the Paul-Drude-Institut für Festkörperelektronik and collaborators have realized a major step towards this goal by demonstrating the remote pumping of single-exciton centers by flying indirect excitons propelled by GHz SAWs in a gallium arsenide (GaAs)-based semiconductor platform. Furthermore, the centers can follow the high-frequency (3.5 GHz) acoustic pumping rate, leading to the emission of single photons synchronized with the acoustic phase. The results were published in the journal ACS Photonics.
The SAWs used here are mechanical vibrations traveling along the surface of a solid. They can be envisioned as microscopic earthquakes on a chip. The moving deformation caused by the micro earthquake changes the properties of a semiconductor and can create a moving potential to capture and transport excitons - neutral electron-hole pairs bound by the Coulomb force. This unique feature can be exploited for the controlled transfer of information between locations on a chip using “flying” excitons as messengers. The studies were carried out on an epitaxial (Al,Ga)As heterostructure consisting of two GaAs quantum wells separated by a thin barrier. The application of a transverse electric field across the structures creates a special type of long-living exciton – the indirect exciton – consisting of an electron and a hole residing in different quantum wells.
The flying excitons were launched to pump a remotely located two-level center for excitons, consisting of a shallow impurity center embedded in the quantum wells (denoted here a DB center). Semiconductors come with many impurities. Some of them (like, e.g., the DB centers) can capture and store single excitons: they can thus act as single-photon sources emitting one photon at a time. Differently from the everyday light sources found in our household, which generates bunches of photons at a time, the emission of single-photons is non-classical and builds the foundation of photon-based quantum communications. In this work, it was shown that the DB centers can be efficiently populated by flying excitons transported by the high-frequency SAW. More importantly, it was also demonstrated that the pumped DB centers emit single photons at a rate of 3.5 GHz determined by the acoustic frequency, thus becoming one of the fastest so-far realized single-photon sources.
The results demonstrate the feasibility of exciton manipulation as well as of exciton-based, high-frequency single-photon sources driven by acoustic waves. They also pave the way for on-chip transfer of quantum information between different locations with a natural electrical to optical interface between GHz and THz excitations.
