Free-Programmable Quantum Computing in a Segmented Ion Trap and Experimental Demonstration of a Heat Engine in the Quantum Regime

Entanglement is an important resource for applications such as quantum computation and high precision sensing. In a segmented Paul trap, entanglement can be created by combining laser-driven logic gates and ion-shuttling operations. We present how we encode a qubit with coherence times in the 1 s range in the valence-electron spin of ⁴⁰Ca⁺ ions [1]. The implementation of the set of shuttling operations required for scalable protocols is outlined. We show how to conduct high-fidelity gate operations, which are insensitive against motion excited by the shuttling operations. We combine gate and shuttling operations to generate a 4-ion GHZ state |↑↑↑↑⟩ + |↓↓↓↓⟩. By applying dynamical decoupling techniques, we can keep the entangled state alive for about 1 s. As an application of spatially distributed entanglement, we employ Bell states |↑↓⟩+|↓↑⟩ for sensing inhomogeneous magnetic fields. These states accumulate a phase, which depends on the magnetic field difference between the locations of the constituent ions. By measuring the accumulated phase, we map out dc magnetic fields with accuracies down to 270 pT [2]. Another application of essentially the same techniques is a single-ion heat engine in the quantum regime. The working medium is given by the spin and its temperature is cyclically modified by optical pumping and depolarisation. The ion is placed into a phase-stabilised standing wave of detuned light with a polarisation gradient along the trap axis [3]. By the ac Stark effect, this field exerts a spin-dependent optical dipole force and thus couples the spin to the axial motion, which serves as storage for the delivered work. We demonstrate the starting behaviour of the heat engine in the quantum regime of axial oscillation.

[1] T. Ruster et al., Appl. Phys. B 122, 1-7 (2016)
[2] T. Ruster et al., arXiv:1704.01793 (2017)
[3] C.T. Schmiegelow et al. PRL 116, 033002 (2016)