Optical crosstalk mitigation methods for a trapped ion qubit array*

We present experimental work performed in a cryogenic apparatus exploiting a segmented ion trap architecture for the implementation of quantum algorithms. The quantum register consists of a linear string of ⁴⁰Ca⁺ ions which are individually controlled by tightly focused laser beams perpendicular to the crystal axis. Light is delivered by a waveguide array allowing to individually feed each ion with a separately controlled laser beam.

Reducing ion spacing allows for faster and higher fidelity entangling gates. It increases the spectral separation of radial motional modes and the resulting stronger axial confinement reduces the detrimental effects from noisy transverse motion. However, the spatial profile of the addressing beams is fundamentally limited by diffraction, therefor leading to significant optical crosstalk for closely spaced ion strings. This will lead to unwanted errors on spectator qubits which can be suppressed at the gate level. 

We implement a novel coherent optical cancellation method by applying parallel compensation pulses on spectator ion(s). We compare this method against known techniques such as composite pulse sequences and noise refocusing methods using randomized benchmarking for one and two-qubit gates.