Any experiment is perfect in theory, but in practice there are many aspects that are different than we would like them to be. For example, lasers do not have a perfectly stable amplitude or frequency, and the interaction between the laser light and the ions is only approximated by our favourite theoretical model.

Many such deficiencies can be overcome by carefully controlling the dynamics of the ions, and the central goal of optimal control is to figure out how this can be done. We are working on the design of temporally shaped laser pulses that make ions robust against drifts in laser parameters and that realise quantum gates in parameter regimes where simplifying approximations break down.

Within this agenda we have devised control pulses that minimise the detrimental effect of motional heating that typically degrades the achievable fidelities of entangling gates. Currently we are working on statistical tools that allow us to efficiently track the drift in fluctuation parameters. With this, we expect to reduce the impact of these parameter fluctuations on gate operations further.

The central goal of our current EPSRC project is the identification of gate operations beyond the Lamb-Dicke approximation. This approximation is valid only when the coupling between qubit and motional degrees of freedom is weak.  This weak coupling, in turn, implies slow gate operation and thus limits the number of gates that can be executed within the decoherence time.  The ability to perform high-fidelity gates outside this weak-coupling regime should therefore allow us to realise more complex gate sequences than currently possible.

References: 

Haddadfarshi F, Mintert F. High fidelity quantum gates of trapped ions in the presence of motional heatingNew Journal in Physics. 2016;18(12): 123007. doi: 10.1088/1367-2630/18/12/123007. 

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