Controlling the shape of a quantum wavefunction

Abstract

The ability to control the shape and motion of quantum states¹,² may lead to methods for bond-selective chemistry and novel quantum technologies, such as quantum computing. The classical coherence of laser light has been used to guide quantum systems into desired target states through interfering pathways^3,4,5. These experiments used the control of target properties — such as fluorescence from a dye solution⁶, the current in a semiconductor⁷,⁸ or the dissociation fraction of an excited molecule⁹ — to infer control over the quantum state. Here we report a direct approach to coherent quantum control that allows us to actively manipulate the shape of an atomic electron's radial wavefunction. We use a computer-controlled laser to excite a coherent state in atomic caesium. The shape of the wavefunction is then measured¹⁰ and the information fed back into the laser control system, which reprograms the optical field. The process is iterated until the measured shape of the wavefunction matches that of a target wavepacket, established at the start of the experiment. We find that, using a variation of quantum holography¹¹ to reconstruct the measured wavefunction, the quantum state can be reshaped to match the target within two iterations of the feedback loop.