• What is the problem/issue being addressed?
Long distance quantum communication, i.e. the transfer of information by means of quantum mechanics, is presently limited to a few hundred kilometers because quantum states cannot be copied and amplified. That is a huge dilemma as quantum communication is the only known method that can in principle guarantee fundamentally secure communication—it would therefore be important to extend present typical quantum communication distances of several ten kilometers to several thousand kilometers. This requires a quantum repeater, the quantum-analogue to a classical repeater within a classical communication channel. A few years ago, a new method to realize such a quantum repeater was theoretically introduced. It is based on the application of photonic cluster states to establish a long-distance communication channel. The generation of such cluster states is the objective of this project.
• Why is it important for society?
Secure communication is at the heart of our modern communication-based society. We as members of the European society communicate permanently with each other: through emails, voice calls, and social media; we communicate with our banks, our employers, and our governments. These communicate with each other. It is therefore inevitable to guarantee that private communication stays private and cannot be eve-dropped by any unknown third party. Quantum communication is presently the only known method that can in principle guarantee fundamentally secure communication—we therefore need to develop the required quantum technologies that will enable to apply such secure communication schemes in our everyday life.
• What are the overall objectives?
The overall objective of this project is the efficient generation of photonic cluster states based on self-assembled quantum dots in nanophotonic systems. A self-assembled quantum dot is a point-like structure with dimensions of about 10 nanometers in all dimensions. Due to its small dimensions and combination of materials, electronic energy levels are discretized—and the quantum dot behaves like an atom in the solid state. The quantum dot, equivalent to an atom, can be used as a very special “quantum” light source. This quantum light source emits only one light particle, i.e. a photonic qubit, at a time. A qubit (from quantum-bit) is the quantum-analogue of the classical bits that are the basis of our computers, cell phones, basically any digital device. Is the quantum dot furthermore charged with a single electron, the two spin states of this electron build a stationary qubit which can be used as additional quantum-mechanical resource. In such a charged quantum dot, the emission of a photonic qubit is coherently coupled to the spin state of the electronic qubit—the electron and photon can be entangled. Because the electron is quasi-permanently present in the quantum dot, the emission of a photon can, if carried out within the so-called coherence time, be directly coupled to the emission of the previous photon. If done in the required way by applying particular quantum gates, i.e. a coherent operation on the electron, the entanglement can be extended from one electron and one photon to one electron and two photons, and so on, and finally to one electron and n photons. By performing a specific, final quantum gate on the electron qubit, the electron is decoupled from the photons, and the n photons are left in an entangled state. This entangled state is the so-called cluster state. Such a cluster state can then be used for a variety of important quantum technologic applications, for example for the aforementioned quantum secure long-distance communication.
