Quantum Networking with Fibre-Coupled Ions

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.

1.) Academic Impact
The research described in this proposal is expected to have immediate impact in the academic area. The expected results are highly relevant to researchers in quantum communication and quantum computation, since in our project explores the boundary between the two areas. Examples for themes on which our research will impact are distributed quantum computation, quantum memory for photons, quantum repeaters and long-distance entanglement. The deterministic transfer of quantum states between atoms and photons should be important to researchers interested in the foundations of quantum mechanics. Finally, we expect to develop important technologies during the project which would be highly relevant to experimentalists in the atomic physics community. Novel concepts of optical micro-resonators and microscopic traps will be beneficial to physicists working on single particle detection and manipulation.

2.) Economic and industrial Impact

Immediate Impact: Some equipment which will be designed for this project may become viable products especially for specialised companies. The funding for this project is likely to catalyse the development of devices which may become commodities and may lead to spin-offs.

Long-term Vision: The exchange of quantum information between nodes of a network will enable distributed quantum processing which may revolutionise the processing of information in the future. Quantum computing is expected to be far superior to current conventional computers in solving particular problems. The number of algorithms for which quantum computers may surpass conventional computers is rapidly growing so that they might replace them in the long term for a large number of application. An important component of quantum information processing will be the link between them to form large clusters of processors or networks in which quantum information can be exchanged. The proposal addresses the task to create the technology and deeper understanding which is required for interlinking quantum processors on the basis of trapped ions. Trapped ion QIP experiments are currently the most advanced systems and thus focusing on this technology appears to be very good investment into the emerging area of QIP on a large scale. The results obtained in this project will be highly relevant for quantum networking in general, as they will also be applicable to a large variety of other quantum technologies.

3.) Skills, training and knowledge economy
The current proposal employs cutting edge techniques in atomic and laser physics. The student and postdoc engaged with this project will benefit from the high quality training in these methods. The postdoc will have the opportunity to benefit from the project by gaining invaluable experience and skills. Also undergraduate and postgraduate students will benefit from the training they will receive during the project. All personnel will be able to take part in the transferable skills training programmes at Sussex. They will also be exposed to and interact with the international community of scientists working in this field which comprises many of the leading physics groups in the world.

4) Impact in society
The proposed research has great potential to engage the public in the fascination of science and we will use the results as a springboard to public outreach work. The interpretation of this work is strongly linked to the 'mysterious world' of quantum physics. We have the opportunity to convey the excitement of the application of quantum mechanics in information processing.