Limits to atomic qubit control from laser noise*

The use of laser radiation for high fidelity manipulation of atomic qubits presents a pathway to large-scale universal quantum computation. Technical noise from the laser source itself can erroneously couple to qubit rotations and therefore must be minimized to reach the highest fidelities. The ultimate fidelity floor for atomic qubits driven with laser radiation is due to spontaneous emission from excited energy levels. This work addresses the requirements to suppress control noise from the laser source to below the spontaneous emission floor such that it is no longer a limiting factor. It has previously been found for microwave sources that the spectral structure of the noise plays a critical role [1]. By considering the spectral structure of laser frequency noise, we find that, contrary to common belief, narrowing the laser linewidth alone is not sufficient for high fidelity qubit control. From these considerations we find that laser gain media with long relaxation times have an advantage in relaxing requirements on stabilisation bandwidths. For laser intensity noise, we find that errors from shot-noise limited light are always below the spontaneous emission floor, and we present requirements for the active stabilisation of laser noise to the shot noise limit. The requirements have wider implications for the generation of microwave local oscillators for both atomic and superconducting qubit control.

[1] H. Ball, W. D. Oliver, and M. J. Biercuk, The role of master clock stability in quantum information processing, npj Quantum Information 2, 1 (2016)