Towards the goal of a large-scale quantum computer based on trapped ions,
near-field microwaves represent a promising approach to perform the techni-
cally challenging key operation of a two-qubit entangling gate. In this thesis
we present the first microwave-driven two-qubit gate in 9Be+ ions employing a
first-order field-independent qubit transition and a scalable surface-electrode
ion trap at room temperature. We test the quality of the gate operation
by producing a maximally entangled state and measuring the resulting state
preparation fidelity in a reduced tomography procedure. For the best two-
qubit gate achieved in the system we find this fidelity to be F = 98.2 ± 1.2 %.
Following a comprehensive error analysis based on numerical simulations and
experimentally determined input parameters, we identify current infidelity con-
tributions of the apparatus. Here, the natural error source of the microwave
near-field approach, namely fluctuating AC Zeeman shifts, could be reduced
to the 10^−4 level due to the optimized design of the employed microwave con-
ductor. As we find that the three largest errors can all be reduced upon purely
technical improvements, higher fidelities are feasible in the future. Besides the
gate realization, the thesis also comprises the initial characterization of the
employed ion trap as well as the design and construction of a Raman laser
at ∼ 313 nm which is utilized to perform near ground state cooling of radial
modes of a single- and two-ion crystal. Here, ensuing heating rate measure-
ments on a single-ion’s radial modes show good agreement with the electric-
field noise spectral density expected from literature given the chosen mode
frequency and nearest ion-to-electrode distance of 70 µm. Finally, we present
the operation of a first multi-layer ion trap whose electrode layout features a
3-dimensional microwave conductor intended for performing microwave-driven
two-qubit gates. Following a characterization of the resulting near-field pat-
tern using a single ion as a local field probe, we find the multi-layer conductor
to have significantly better field properties when compared to an equivalent
single-layer design. Given these promising results, future work will focus on
the integration of similar microwave circuitry in a multi-zone trap array as
envisioned by the QCCD architecture.
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