Heisenberg-limited Rabi spectroscopy
The use of entangled states was shown to improve the fundamental limits of spectroscopy to beyond the standard-quantum limit. In these Heisenberg-limited protocols the phase between two states in an entangled superposition evolves N-fold faster than in the uncorrelated case, where N for example can be the number of entangled atoms in a Greenberger-Horne-Zeilinger (GHZ) state. Here we propose and demonstrate the use of correlated spin-Hamiltonians for the realization of Heisenberg-limited Rabi-type spectroscopy. Rather than probing the free evolution of the phase of an entangled state with respect to a local oscillator (LO), we probe the evolution of an, initially separable, two-atom register under an Ising spin-Hamiltonian with a transverse field. The resulting correlated spin-rotation spectrum is twice as narrow as compared with uncorrelated rotation. We implement this Heisenberg-limited Rabi spectroscopy scheme on the optical-clock electric-quadrupole transition of $^{88}$Sr$^+$ using a two-ion crystal. We further show that depending on the initial state, correlated rotation can occur in two orthogonal sub-spaces of the full Hilbert space, yielding Heisenberg-limited spectroscopy of either the average transition frequency of the two ions or their difference from the mean frequency. The potential improvement of clock stability due to the use of entangled states depends on the details of the method used and the dominating decoherence mechanism. The use of correlated spin-rotations can therefore potentially lead to new paths for clock stability improvement.