Broadband coherent sub-picosecond far-infrared pulses, known as "half-cycle" pulses, are elegant tools for both measuring and manipulating Rydberg wave packets. We will show new results on phase-control of Rydberg states with half-cycle pulses, and interactions with Stark wave packets.
The field of coherent control has today been seeing significant progress, in a wide array of physical systems. One such system is the Rydberg atom. The long lifetime, strong dipole interactions, multiple degrees of freedom, and simple atomic structure make Rydberg atoms particularly appealing for experiments involving coherent control. The same properties have led to the exploration of Rydberg atoms as candidates for quantum information storage, processing and retrieval. Quantum information processing is a particular application of coherent control. Rather than being disparate subjects of study, coherent control of Rydberg atoms and processing of information stored as phase in different Rydberg levels are two sides of the same coin. In this dissertation, we experimentally characterize the effect of a half-cycle pulse on the phases of the states of a Rydberg wave packet and their mutual coherence. This has bearing both on our ability to control the states of the atom and on our ability to measure the information stored as quantum phase. We observe delay dependent phase shifts and losses of coherence. Despite a loss of information due to transfer of quantum amplitude to states outside the data register, we demonstrate partial retrieval of that information by the application of a second half cycle pulse. We also present an essential states model of the interaction, and we compare results from the model to experimental features observed. Although the particulars may vary, the concepts involved are quite general and may be applied to multilevel systems other than Rydberg atoms.
The short-pulse dynamics of excitation and stimulated emission between highly excited states (15<n<60) is studied with picosecond far-infrared pulses (40--60 $\ensuremath{\mu}\mathrm{m}$) originating from a free electron laser. We measured the excitation of a Rydberg wave packet in rubidium around n=40 excited from a deeper bound $(n\ensuremath{\approx}20)$ Rydberg state by a picosecond far-infrared laser pulse. Moreover, starting from a stationary n=40 state we found that upon irradiating with a short far-infrared laser pulse, adjacent Rydberg states are populated by resonant Raman transfer via a deeper (n=20) bound state. Finally, we discuss the dynamics of wave-packet excitation at a Cooper minimum in lithium, where sign change of the excitation matrix elements causes excitation at large distances from the atomic core. Possible measurement methods for this effect are discussed and found to yield different information.
Ultrashort, nearly unipolar electric field pulses are used to produce novel dynamic states in Rydberg atoms. A single 500 fsec field pulse redistributes essentially all of the population from a single stationary Rydberg state to hundreds of different final states. The dynamic evolution of the nonstationary wave packet is monitored using a second, time-delayed pulse. The time-dependent momentum-space probability distribution of the wave packet can be retrieved experimentally using a classical impulse approximation.
Quantum excitation is usually regarded as a transient process occurring instantaneously, leaving the underlying physics shrouded in mystery. Recent research shows that Rydberg-state excitation with ultrashort laser pulses can be investigated and manipulated with state-of-the-art few-cycle pulses. We theoretically find that the efficiency of Rydberg state excitation can be enhanced with a short laser pulse and modulated by varying the laser intensities. We also uncover new facets of the excitation dynamics, including the launching of an electron wave packet through strong-field ionization, the re-entry of the electron into the atomic potential and the crucial step where the electron makes a U-turn, resulting in twin captures into Rydberg orbitals. By tuning the laser intensity, we show that the excitation of the Rydberg state can be coherently controlled on a sub-optical-cycle timescale. Our work paves the way toward ultrafast control and coherent manipulation of Rydberg states, thus benefiting Rydberg-state-based quantum technology.