Quantum tricks up the spectroscopist sleeve

Multiple methods have been developed in the context of Quantum Information Science to extend the time over which quantum superpositions remain coherent. While negating the effect of noise, all these methods allow for the application of quantum logical gates in their logic subspace. Many such methods, including Dynamic Decoupling (DD) protocols, Decoherence Free Subspaces (DFS), and Quantum Error Correction Codes (QECC), were demonstrated experimentally in several systems. The successful implementation of these methods is in some sense boring – they were designed to be successful. Their failure (and they always fail at some point) is interesting. This is because their failure exposes a subtle part of the environment: that which overlaps with the protected qubit logical operations. This part of the environment is normally masked by the much larger noisy background. These methods can therefore be used for the metrology of weak non-trivial signals. In this talk I will review the application of methods such as above in trapped-ion qubit arrays. Examples include the use of DD for precision magnetometry and light-shift spectroscopy. The use of a two qubit DFS for the measurement of the magnetic interaction between two electronic spins at a micrometer-scale distance and the use of repetitive QECC for Heisenberg-limited metrology.