Deterministic ion-photon interface for entanglement distribution

We will develop an apparatus for transferring quantum states from an ion to a photon and the other way around, in order to send the quantum information content to other, distant ions. 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 do computation. Researchers use single ions stored in linear traps to replace the bits in a classical computer. Operations on these quantum bits are performed with laser light. In our project we will combine the areas of quantum computation and quantum communication by building a 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 systems. We can achieve this by surrounding the ion with a cavity, enhancing its interaction with the 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. Interesting cases are those where the atom is in a superposition of two possible states. This is allowed in quantum mechanics. Our interface will make sure that the quantum state of the photon looks like that of the ion and will therefore have a superposition of polarizations. Even more interesting are the cases when the ion doesn't surrender all information on its original superposition state to the photon. The ion and the emitted photon will then be in a linked state, called 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 place, while we will be able to distribute the entanglement to distant locations. This will be a major achievement of our project. We will also reverse the 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, with nothing in the process 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 be extremely good, 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 microscopic trap with a small mirror separation is the main experimental challenge of this project. The benefit is the development of a new quantum technology, linking quantum computing with quantum communication.