Robust rotations & coherent quantum states with a single trapped ion

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Coherent operations necessary for the implementation of single and multi-qubit quantum gates with trapped ions, that are robust against variations in experimental parameters and intrinsically indeterministic system parameters, are demonstrated. In particular, single qubit gates developed using optimal control theory are demonstrated for the first time with trapped ions. Their performance as a function of error parameters is systematically investigated and compared to composite pulses.

A two level quantum mechanical system is realized on the S1/2 F = 0 → S1/2 F= 1 mF = 0 transition in 171Yb+ confined in a Paul trap, driven by microwave radiation close to 12.6 GHz. Shaped pulses and composite pulses have been realized that are specifically designed to tackle off-resonance errors, timing errors or power variations of the driving field. Good agreement is seen between the simulated results and measured ones. Higher experimental fidelities are obtained with the aforementioned shaped and composite pulses over an extended parameter regime than with a simple pulse.

The experiments have baselines as low as 76 %. The suspected culprit of this result is the preparation procedure. To this end the preparation process was changed to be faster (0.5 ms) and more efficient (97 %). The essential role of effective preselection of the data is highlighted whilst investigating unwanted optical pumping effects.

Coherence time is an important property of a quantum computer. A weakness of an ion trap quantum computer is its potential dependence on magnetic field sensitive levels. Ambient magnetic field noise is the cause of the shorter coherence times of magnetic field sensitive levels (5 ms) compared to their field insensitive counterparts (500 ms) as observed in 171Yb+.

At an avoided crossing the energy separation of a dressed state should be robust to small changes in detuning. Using a microwave field and the ground state hyperfine levels of 171Yb+, such dressed states are prepared and Rabi oscillations are observed between them. The coherence times of systems built on this principle are measured.