Entanglement, a uniquely quantum phenomenon, enables two initially independent quantum systems to be interlinked. Entanglement forms the basis of why quantum technologies have an advantage to their classical counterparts, in particular for computing and networking. At Oxford, we have developed a quantum network of trapped ions, generating entanglement between ions in two macroscopically separate systems via a photonic interconnect. While this network was originally built for applications in quantum computing or cryptography, in this project, I aim to use our quantum network for entanglement enhanced sensing, using either the optical transition of these ions or their motion.
First, I aim to build a network of entangled optical atomic clocks for enhanced frequency comparisons of ions in two separate traps. Optical atomic clocks are used for the most precise measurements of time. Precision timing is essential for applications from global positioning satellites, precision agriculture, or for the timestamping of financial transactions to ensure security. Frequency comparisons between two atomic clocks are also used for geodesy, and to probe variations of fundamental constants, or the properties of dark matter. However, such frequency measurements are typically limited by the standard quantum limit. Entanglement provides a path beyond that limit, reducing the number of measurements required to achieve a given precision. An entangled network of clocks would also maximise the use of timekeeping resources and have potential security benefits as well. Our experiment is the first demonstration of such a network. Starting from our initial proof of principle demonstration, we will incorporate another ion species, showing the versatility of our system to extend the remote entanglement to transitions that are more stable and can be probed longer. In addition, we can build on our initial demonstration by adding local entangling operations in each network to further enhance the frequency measurements.
Second, I want to generate remote entanglement of the motional states of the ions. By coupling the ions' internal states to their motion, we can map the initial spin-spin entanglement to their respective motions. This demonstration would be the first for two ions in separate systems, generating entanglement between quantum states that have a spatial extent of a few nm even though they are separated by metres. Previous demonstrations have been done for two ions in a single trap. We can use this remote entanglement to enhance measurements of displacements in each of the ions. As the ions are charged, sensing small displacements of the ion can be used to measure small electric fields, with applications to detecting dark matter. The protocols we develop for entangling remote mechanical oscillators and measuring their displacements would also be applicable to other optomechanical systems. Further enhancements could be obtained by using nonclassical states of the ion motion such as squeezed states.
Finally, we can use the ions' motional degree of freedom to explore a different approach to quantum computing, where we encode the information in the ions' motion rather than their internal states. Here, the ion motion offers a larger Hilbert space to explore, with potential advantages for error correction. We can use the remote entanglement that is unique to our system to develop new protocols for continuous variable quantum computation, which would also be applicable to other systems such as superconducting qubits.
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