Improving the state selectivity of field ionization with quantum control

Abstract

The electron signals from the field ionization of two closely-spaced Rydberg states of rubidium-85 are separated using quantum control. In selective field ionization, the state distribution of a collection of Rydberg atoms is measured by ionizing the atoms with a ramped electric field. Generally, atoms in higher energy states ionize at lower fields, so ionized electrons which are detected earlier in time can be correlated with higher energy Rydberg states. However, the resolution of this technique is limited by the Stark effect. As the electric field is increased, the electron encounters numerous avoided Stark level crossings which split the amplitude among many states, thus broadening the time-resolved ionization signal. Previously, a genetic algorithm has been used to control the signal shape of a single Rydberg state. The present work extends this technique to separate the signals from the 34s and 33p states of rubidium-85, which are overlapped when using a simple field ramp as in selective field ionization.

Perturbed field ionization for improved state selectivity

“…The perturbations that occur at intermediate times are not able to use either strategy and produce only minimal fitness score increases. Physically, the GA has more success when using the avoided crossings in the vicinity of ionization because of the widely varying ionization rates of neighboring states, as discussed previously in [26]. States where the Rydberg electron is concentrated on the up-field side of the ionic core ("red" states) are relatively easily ionized, while states with the Rydberg electron concentrated on the down-field side of the core ("blue" states) are harder to ionize.…”

“…Our primary motivation for controlling the shape of the time-resolved field ionization signal is to improve the selectivity of Rydberg state distribution measurements [26]. In this case our goal is to design a perturbation that best separates the signals that would otherwise overlap with standard SFI.…”

“…First, tournament selection is used to select a pair. Two non-overlapping random sets of pulses are selected and the best pulse is selected from each (lines [17][18][19][20][21][22][23][24][25][26][27]. A child pulse is created by randomly selecting a parent gene at each locus, a technique referred to as uniform crossover.…”

“…The small perturbation allows one to direct the pathway of the electron through the Stark map on the way to ionization so this technique is referred to as directed field ionization (DFI). DFI has also been used to improve the selectivity of experiments in which the electron is in a superposition of two states, allowing for precise quantification of the state distribution [26].…”

Time Dependence of Few-Body Förster Interactions among Ultracold Rydberg Atoms

“…A set of coaxial cylinders placed on either side of the MOT allow us to apply static and time varying electric fields [35]. To separate the p and p 0 states, atoms are excited in an electric field of 4.2 V=cm.…”