Quantum networks is a promising field of research which aims to the development of infrastructures devoted to the execution of quantum informational tasks, such as secure quantum communication and distributed quantum computing. A quantum network is made up of nodes, connected together through classical and quantum channels. Using photons for the encoding of information, quantum networks need single-photon sources as essential building blocks. A complete quantum network should be composed of many interconnected single-photon sources, allowing the full exploitation of complex quantum schemes.
In recent years important progresses have been made in the generation of single photons with quantum dots (QDs). The interest in this platform is due to the fact that they can produce on-demand single photons through quantized exciton recombinations. The advantage with respect to other on-demand single-photon sources, like single atoms, is that they do not require demanding cooling techniques.
The sharp absorption line of an atomic gas can be used as frequency reference in quantum optics experiments. This fact can be extremely useful in solving one of the major issues affecting QDs: each QD is different from the others, showing different energies for the emitted photons and thus reducing the indistinguishability, which is indeed essential for quantum communication and computation.
In this project we want to build and test a quantum node based on single-photon QD sources coupled to a 87Rb vapor cell. In particular it will be tested for the first time the possibility of using two distinct QDs for the implementation of a teleportation protocol, where the two single-photon sources will be frequency locked to the absolute frequency reference represented by the absorption line of the vapor cell.
Solid state nanostructures, like for example semiconductor quantum dots, represent one of the most promising platform for the generation of on-demand single and entangled photons to be used as quantum bits for quantum communication and computation protocols. The research in this field is already advanced if we consider single independent quantum emitters, and all the efforts are concentrated on the improvement of photon indistinguishability, purity and brightness [9,10]. Despite these great advances, the investigation of multiple QDs, as a collective apparatus for the generation of qubits to be used by different parties, is still at a preliminary level. This is an extremely important step to be done for the development of future quantum technologies. In particular it will be fundamental for the quantum network infrastructure, where several nodes are connected together and, ideally, several of these nodes should have their own qubit source, which has to be compatible with all the other sources of the network. Given all the recent advances concerning QDs, considering that they can be easily integrated in the current semiconductor technology, and counting on the fact that, unlike other single photon sources such as single atoms, they do not require very complex schemes to operate, we are convinced that QDs have the chance to represent the final physical system for the generation of on-demand qubits in the context of the upcoming second quantum revolution [8].
The great challenge in coupling two or more QDs together is represented by the fact that these nanostructures are still macroscopic objects, meaning that every QD is different from the others and include some imperfections. This non-ideality implies degradation of the quantum nature of the emitted photons and sample-dependent emission properties. In particular, in presence of anisotropies, a splitting of the X level energy called fine structure splitting is created. In this way a not negligible phase factor is acquired by the entangled photon pair. Since the phase factor depends on the time of permanence in the X level, the final emitted state will be a mixture of all these possible time-dependent phase factors. Even in presence of zero FSS, due to the structural differences between QDs, different samples emit photons at different random energies distributed around the typical emission wavelength of the particular QD type. Thus, to obtain indistinguishable and pure photons one has to deal with all these aspects.
Our project will represent a great opportunity to synthesize several techniques that for the moment have been only demonstrated in isolated experiments, like for example the 6-legged device [7] and the hybridization between QDs and ACs [5,6]. The demonstration of the quantum teleportation protocol, with the aforementioned schemes and physical systems, would be an important sign that our approach can play an important role in the ultimate quantum network infrastructure.