Trapped ions with microwave radiation are a promising platform for universal quantum
computing. However, a major obstacle in the way of scalability is the coupling of the qubits
to their noisy environment. This thesis offers means to improve the fidelity of two-qubit
entangling gates. To this end, we investigate noise from classical control hardware and
study quantum control methods that increase the gate’s robustness.
The noise spectrums of classical control hardware typically exhibit non-Markovian
behaviour. Therefore, a transfer function in frequency space is derived for each source,
transforming hardware noise to qubit-frame noise. It is found that voltage noise on the
electrodes is a significant contribution to decoherence as it displaces the ions within the
static magnetic field gradient. We propose and demonstrate a voltage noise cancellation
scheme that is compatible with microfabricated surface traps.
We then identify a library of quantum control methods that increase the robustness
of a bichromatic interaction to both spin and motional decoherence. We also propose a
novel σz ⊗ σz entangling gate which makes use of the intrinsic J-coupling interaction of
ions in a static magnetic gradient. The resulting interaction is virtually insensitive to
motional decoherence, which alleviates stringent experimental requirements. We finally
demonstrate a bichromatic interaction that is simultaneously robust to spin and motional
decoherence, by means of continuous dynamical decoupling and phase modulation on the
sidebands.
Recalling that noise in the ion’s position couples into magnetic field noise due to the
static magnetic field gradient, we use this noise mechanism as the basis of a promising
electric field sensor. We experimentally demonstrate AC electrometry with a sensitivity
of S = 7.0(5)mVm−1Hz−1/2. Noise spectroscopy was also demonstrated and was limited
by the noise floor, where the minimum sensitivity was 545 nVm−1Hz−1/2.