Quantum Networking with Fibre-Coupled Ions

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In the course of the project, we will develop a system to transfer quantum information between distant ions deterministically, using single photons in an optical fibre as carrier. This device will be a key building block of a so-called quantum network, which in the future will connect quantum computers like the internet does with present-day computers.Already today single quanta of light (photons) are used to transmit information securely over long distances, a process called quantum communication. The laws of quantum mechanics would foil any attempt of eavesdropping. Quantum effects can also be used to perform computations. Re-searchers use single ions stored in linear traps as quantum bits, replacing the classical bits in ordi-nary computers. These quantum bits are manipulated with laser light. In our project, we will com-bine the areas of quantum computation and quantum communication by building an efficient user-controlled interface (a quantum link) between ions and photons. The transfer of quantum states from ions to photons requires that we strongly couple the two sys-tems. We can achieve this by surrounding the ion with two mirrors, forming a cavity and enhancing the interaction of ions and photons. The conversion process from the ion-qubit to the photon-qubit will be steered with a suitable laser pulse applied to the ion. A photon will be generated with one of its properties (polarization) depending on the state of the ion. Particularly interesting are cases where the atom is in a superposition of two possible states, a situation that is allowed in quantum mechanics. Our interface will make sure that the quantum state of the photon is identical that of the ion and will therefore have a superposition of polarizations. Even more interesting are the cases when the ion doesn't transfer all information on its original su-perposition state to the photon, but retains some of it. The ion and the emitted photon will then be in a linked or entangled state, where the outcome of a measurement on the separate components is unpredictable, but combining the results of the two systems one always finds perfect correlation. Previously these states have been produced in a controlled way only in one location, while we will be able to distribute entanglement over long distances. This will be one of the major achievements of our project. We will also reverse this process and transfer the quantum state of an incoming photon to that of an ion in our cavity. Combining the two processes, we can transfer quantum states from one ion to a distant ion, or entangle their quantum states. This is done very efficiently, as nothing in the process is left to chance. This kind of entanglement is an important resource for performing efficient quantum computation in the future.To achieve our goals, we have to master two technologies: first we need to store a single ion in a very small region of space (less than 40 nm) for a long time (hours). This is possible with the help of a microscopic ion trap. The mirrors of the cavity surrounding the ion must have extremely high quality, allowing 200,000 reflections of the photon without loss. In addition, we must put the mirrors very close to the ion, to enhance the interaction between ion and photon. Combining the micro-scopic trap with a small mirror separation is the main experimental challenge of this project. By employing laser machined end facets of optical fibres as mirrors, we can achieve the ultimate miniaturization of the cavity. Furthermore, the cavity emission will be coupled directly into the fibre for reliable long distance transmission.