High-fidelity microwave-driven quantum logic in intermediate-field 43Ca+

Marius Weber, Clemens Löschnauer, Jochen B. W. Wolf +6 more

T. P. Harty, Harty, Thomas Peter

This thesis is concerned with the development of an intermediate magnetic field "clock-qubit" in 43Ca+ at 146G and techniques to manipulate this qubit using microwaves and lasers. While 43Ca+ has previously been used as a qubit, its relatively complicated level structure - with a nuclear spin of 7/2 and low-lying D-states -- makes cooling it in the intermediate field an intimidating prospect. As a result, previous experiments have used small magnetic fields of a few gauss where coherence times are limited and off-resonant excitation is a significant source of experimental error. We demonstrate a simple scheme that allows 43Ca+ to be cooled in the intermediate field without any additional experimental complexity compared with low fields. Using the clock-qubit, we achieve a coherence time of T*2 = 50 (10)s - the longest demonstrated in any single qubit. We also demonstrate a combined state preparation and measurement error of 6.8(6)x 10-4 - the lowest achieved for a hyperfine trapped ion qubit [NVG+13] - and single-qubit logic gates with average errors of 1.0(3) x 10-6 - more than an order of magnitude better than the previous record [BWC+11]. These results represent the state-of-the-art in the field of single-qubit control. Moreover, we achieve them all in a single scalable room-temperature ion trap using experimentally robust techniques and without relying on the use of narrow-linewidth lasers, magnetic field screening or dynamical decoupling techniques. We also present work on a recent scheme [OWC+11] to drive two-qubit gates using microwaves. We have constructed an ion trap with integrated microwave circuitry to perform these gates. Using this trap, we have driven motional sideband transitions, demonstrating the spin-motion coupling that underlies the two-qubit gate. We present an analysis of likely sources of experimental error during a future two-qubit gate and the design and preliminary characterisation of apparatus to minimise the main error contributions. Using this apparatus, we hope to perform a two-qubit gate in the near future.

Christopher J. Ballance

R Hanley, Jochen. Wolf, Clemens Löschnauer +5 more

We present the design and initial characterisation of a next-generation surface-electrode ion-trap designed for room-temperature or cryogenic operation, that will aim to improve both the fidelity and speed achieved in microwave-driven quantum gates.

Marius Weber, Clemens Löschnauer, J Wolf +6 more

Abstract We report the design, fabrication, and characterization of a cryogenic ion trap system for the implementation of quantum logic driven by near-field microwaves. The trap incorporates an on-chip microwave resonator with an electrode geometry designed to null the microwave field component that couples directly to the qubit, while giving a large field gradient for driving entangling logic gates. We map the microwave field using a single 43Ca+ ion, and measure the ion trapping lifetime and motional mode heating rates for one and two ions.

Tyler Keating, Robert L. Cook, Ivan Deutsch +3 more