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arXiv:1602.07791 · Physical Review A

Stimulated Raman adiabatic passage for improved performance of a cold-atom electron and ion source

Ben M. Sparkes, D. Murphy, Robert J. Taylor, Rory W. Speirs, A. J. McCulloch, Robert E Scholten

Published Aug 8, 2016• 14 citations
arXivHTMLPDF

Abstract

We implement high-efficiency coherent excitation to a Rydberg state using stimulated Raman adiabatic passage in a cold-atom electron and ion source. We achieve an efficiency of 60% averaged over the laser excitation volume with a peak efficiency of 82%, a 1.6 times improvement relative to incoherent pulsed-laser excitation. Using pulsed electric field ionization of the Rydberg atoms we create electron bunches with durations of 250 ps. High-efficiency excitation will increase source brightness, crucial for ultrafast electron diffraction experiments, and coherent excitation to high-lying Rydberg states could allow for the reduction of internal bunch heating and the creation of a high-speed single-ion source.

DOI
10.1103/physreva.94.023404

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We implement high-efficiency coherent excitation to a Rydberg state using stimulated Raman adiabatic passage in a cold-atom electron and ion source. We achieve an efficiency of 60% averaged over the laser excitation volume with a peak efficiency of 82%, a 1.6 times improvement relative to incoherent pulsed-laser excitation. Using pulsed electric field ionization of the Rydberg atoms we create electron bunches with durations of 250 ps. High-efficiency excitation will increase source brightness, crucial for ultrafast electron diffraction experiments, and coherent excitation to high-lying Rydberg states could allow for the reduction of internal bunch heating and the creation of a high-speed single-ion source.
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We present an ionization mechanism for use in a cold atom electron source with the goal of producing highly monochromatic electron beams. We experimentally produce a map of the Stark states of 85Rb^{85}\mathrm{Rb}85Rb below the ionization threshold and identify states that undergo selective field ionization. The properties of an electron beam produced by field-assisted ionization of such states are quantified. A theoretical framework is established to predict the improvement to beam quality when ionization is conducted above the ionization threshold, where ionization conditions are typically more favorable than below the threshold. Calculations suggest that selective ionization of Rydberg states may offer a pathway to the production of high-brightness, highly monochromatic ion and electron beams.
Atoms excited into states of high principal quantum number, Rydberg atoms, have exaggerated properties such as large size and dipole moments, long lifetimes, and high polarizabilities. These properties can be used to realize a Rydberg excitation blockade in narrow-band photo-excitation (∼ 1 MHz bandwidth) of laser-cooled and trapped atoms. Under ideal conditions, the blockade phenomenon leads to an inhibition of more than one Rydberg excitation in dense samples of atoms. The blockade is useful in a number of applications including quantum information processing, quantum cryptography, and atomic clocks. In this thesis, I discuss experimental investigations of the dependence of the blockade effectiveness on the principal quantum number of the excited Rydberg level, and the Rydberg excitation laser's detuning and intensity. Rydberg-atom counting statistics is used as a technique for measuring the effectiveness of the Rydberg blockade. An important aspect in experiments on the blockade effect is the efficiency of the photo-excitation process used for Rydberg-atom generation. Therefore, the described work deals, in part, with methods of coherent excitation of atoms into and out of Rydberg states. I show that STIRAP (stimulated Raman adiabatic passage) allows us to transfer atoms into Rydberg levels with about 70% excitation efficiency, under the constraints of the present experiment. It is also seen that the excitation efficiency is insensitive to variations in laser pulse intensity and width, as expected for STIRAP. The experimental techniques used to measure the blockade effect using counting statistics and to achieve coherent excitation into Rydberg states, are important for future studies of small atomic systems blockaded to one Rydberg excitation.
Accurate control of a quantum system is a fundamental requirement in many areas of modern science ranging from quantum information processing to high-precision measurements. A significantly important goal in quantum control is preparing a desired state as fast as possible, with sufficiently high fidelity allowed by available resources and experimental constraints. Stimulated Raman adiabatic passage (STIRAP) is a robust way to realize high-fidelity state transfer but it requires a sufficiently long operation time to satisfy the adiabatic criteria. Here we theoretically propose and then experimentally demonstrate a shortcut-to-adiabatic protocol to speed-up the STIRAP. By modifying the shapes of the Raman pulses, we experimentally realize a fast and high-fidelity stimulated Raman shortcut-to-adiabatic passage that is robust against control parameter variations. The all-optical, robust and fast protocol demonstrated here provides an efficient and practical way to control quantum systems.