In many applications within the field of quantum information processing, quantum states are stored and processed as stationary qubits at the nodes of a quantum network. Within the network quantum information is exchanged between the nodes via flying qubits. Photons are a natural choice for the flying qubits whereas electronic spin states often are chosen as stationary qubits. In order to facilitate a functional quantum network we are developing an efficient and accurate quantum interface between flying and stationary qubits.
Our quantum interface is based on semiconductor quantum dots. The polarisation quantum state of a photon is automatically transcribed into the spin state of an electron-hole pair when the photon is absorbed by a quantum dot. However, in a single quantum dot the electron-hole pair rapidly recombines to recreate a photon and the stationary qubit can only be stored for a short period of time.
In order to extend the storage time we split up the electron and hole and store them in two separate quantum dots. With a voltage applied across the two quantum dots we can controllably move the electron between the two quantum dots. When the electron and hole are residing in different quantum dots recombination is effectively prevented for a time period several hundred times longer than the natural lifetime of the electron-hole pair. The photon can be recreated, on demand, when the electron is moved back to the quantum dot containing the hole. The figure schematically illustrates the operation of the quantum interface. The two quantum dots (QD1 and QD2) are here represented by their electronic band structure.