Fast Gates and Mixed-Species Entanglement with Trapped Ions

Abstract

Building a quantum computer is one of the outstanding contemporary goals in physics. Trapped-ion qubits are among the most promising contenders, achieving the highest precision gates out of all platforms. However entangling gates have previously only been performed in a regime where their speed is limited by the motion of the ions in the trap. The work in this thesis demonstrates entangling operations beyond this speed limit. In addition, entanglement of different species of ion has been performed. This is an important step towards building a scalable quantum computer, by being able to interface and connect trapped ion qubits via photonic links. We demonstrate mixed-species entangling gates using a method that requires only a single laser for both species. In an experiment demonstrating the technique with two isotopes of calcium, 40Ca+ and 43Ca+ , we achieved a Bell state fidelity of F = 99.8(6)%. We also perform a Bell test with the two different isotopes, achieving a violation of the CHSH inequality by 15 standard deviations with CHSH parameter S = 2.228(15), without making a fair-sampling assumption. We then use the same gate mechanism to perform an entangling gate on two different atomic species, 43Ca+ and 88Sr+ . In a proof-of-principle experiment we achieved a Bell state fidelity of F = 83(3)%. For achieving faster two-qubit gates we shape the amplitude of the laser pulses driving the gates. The pulse shapes are specifically designed to yield low error rates and be insensitive to the optical phase of the driving field. We perform an entangling gate with fidelity F = 99.78(3)% in tg = 1.6 μs, a factor of 20 − 60 times faster than the previously highest-fidelity trapped ion gates with only a factor ≈ 2 increase in gate error. We also demonstrate entanglement generation within 480 ns, less than a single period of the centre-of-mass motion of the ions.