Control of ion+photoelectron entanglement in attosecond experiments

Quantum mechanical entanglement is a vibrant research topic, which received a lot of attention in recent years and was rewarded with last years Nobel prize. Entanglement is not only a key element in quantum computing or teleportation. It also plays a crucial role in pump-probe experiments, which involve ionization.

In attosecond science, it is common to use radiation in the extreme ultra-violet (XUV) regime. Due to their high photon energy, attosecond pulses are highly ionizing radiation for any sample placed in their way, creating multicomponent quantum systems. Entanglement between different sub-systems can have measurable consequences, especially when the experiment only includes measurements in one of the sub-systems, which means for example in the case of molecular photoionization only measuring the ion or photoelectron.

To emphasize the role of entanglement in photoionization, we designed an experimental protocol, which utilizes a pair of phase-locked XUV pulses and a near-infrared (NIR) pulse to ionize hydrogen molecules. We investigated the limiting role entanglement between the ion and photoelectron has in regards to vibrational coherence and electronic coherence. In the former case the two XUV pulses generate a coherent superposition of vibrational states in the 1sσ_g state of the H₂⁺ ion, which is subsequently probed by the NIR pulse. In the latter case the XUV pulses create an entangled ion+photoelectron system, which is converted by the NIR pulse into a coherent superposition of the gerade and ungerade electronic states of the H₂⁺ ion.

In both cases the degree of ion+photoelectron entanglement is controlled by changing the time delay between the two XUV pulses and as a consequence the degree of vibrational or electronic coherence is altered. Our studies show the crucial role of entanglement in attosecond science and are a first approach to link ultrafast science with quantum information.