Diamond defects, previously applied in measurement of electromagnetic fields and temperature, can now be used to sense rotations.

We propose a scheme for the generation of optimal squeezed states for Ramsey interferometry. The scheme consists of an alternating series of one-axis twisting pulses and rotations, both of which are straightforward to implement experimentally. The resulting states show a metrological gain proportional to the Heisenberg limit. We demonstrate that the Heisenberg scaling is maintained even when placing constraints on the amplitude of the pulses implementing the one-axis twisting and when taking into account realistic losses due to photon scattering.

Nuclear spin ensembles in diamond are promising candidates for quantum sensing applications, including rotation sensing. Here we characterize the optically detected nuclear spin transitions associated with the ${}^{14}\mathrm{N}$ nuclear spin within diamond nitrogen-vacancy (NV) centers. We observe that the contrast of the nuclear-spin-dependent fluorescence is comparable to the contrast of the NV electron-spin-dependent fluorescence. Using Ramsey spectroscopy, we investigate the temperature and magnetic field dependence of the nuclear spin transitions in the 77.5–420 K and 350–675 G range, respectively. The nuclear quadrupole coupling constant $Q$ was found to vary with temperature $T$, yielding $d\left|Q\right|/dT=-35.0\left(2\right)\mathrm{Hz}/\mathrm{K}$ at $T=297\mathrm{K}$. The temperature and magnetic field dependencies reported here are important for quantum sensing applications such as rotation sensing and potentially for applications in quantum information processing.

Considering the unique energy level structure of the one-axis twisting Hamiltonian in combination with standard rotations, we propose the implementation of a rapid adiabatic passage scheme on the Dicke state basis. The method permits to drive Dicke states of the many-atom system into entangled states with maximum quantum Fisher information. The designed states allow us to overcome the classical limit of phase sensitivity in quantum metrology and sensing. We show how to generate superpositions of Dicke states, which maximize metrological gain for a Ramsey interferometric measurement. The proposed scheme is remarkably robust to variations of the driving field and has favorable time scaling, especially for a small to moderate ($\sim 1000$) number of atoms, where the total time does not depend on the number of atoms.

We study the role of average concurrence in entanglement swapping in quantum networks. We begin with qubit pure states, and there is a very simple rule governing the propagation of average concurrence in multiple swaps. We find a similarly simple rule for average concurrence when creating a Greenberger-Horne-Zeilinger state from three entangled pairs. We look at examples of mixed qubit states and find that the relation for pure states gives an upper bound on what is possible with mixed states. We then move on to qudits, where we make use of the I-concurrence. Here the situation is not as simple as for qubits, but in some cases relatively straightforward results can be obtained.

The robustness of an atomic fountain interferometer with respect to variations in the initial velocity of the atoms and deviations from the optimal pulse amplitude is examined. We numerically simulate the dynamics of an interferometer in momentum space with a maximum separation of $20\hslash k$ and map out the expected signal contrast depending on the variance of the initial velocity distribution and the value of the laser field amplitude. We show that an excitation scheme based on rapid adiabatic passage significantly enhances the expected signal contrast, compared to the commonly used scheme consisting of a series of $\pi /2$ and $\pi$ pulses. We demonstrate further substantial increase of the robustness by using optimal control theory to identify splitting and swapping pulses that perform well on an ensemble average of pulse amplitudes and velocities. Our results demonstrate the ability of optimal control to significantly enhance future implementations of atomic fountain interferometry.