This research program aims to understand and control quantum coherence and entangled states in atomic, molecular, and optical (AMO) systems. An entangled state of a compound subsystem is one that cannot be described in terms of separate realistic descriptions of its subsystems. Entangled states are central in studies of quantum information processing, which in addition to showing the way towards new technologies, such as quantum computers or quantum cryptography, may lead to deeper understanding of quantum mechanics. An important development in AMO Physics is the study of collective atomic-ensemble variables, which provide an interface between microscopic degrees of freedom, such as single-photon wave packets, and macroscopic degrees of freedom, such as electronic, vibrational or rotational, in a gas or vapor. Two themes are being pursued: (1) Mesoscopic-level entanglement of collective electronic excitation in the ground states of rubidium vapor, and (2) Exploration of a new platform--hollow core photonic crystal fibers--for use in quantum optics.

Theme (1) seeks to extend recent experiments on number-state entanglement of atomic ensembles and optical fields at the one- and two-photon level to the 5-to-20 photon level. This requires state-of-the-art advances in both photon-number-resolving detection and in the measurement of higher-order field statistical moments. This tests the hypothesis that entanglement is a robust feature of nature even in the macroscopic world, but that detecting it becomes progressively more difficult technically as the excitation number becomes large. At some high excitation number, the quantum theory could either become irrelevant or break down. The experiment has several unique aspects: i) A new type of photon-number-resolving detector--a back-illuminated drift silicon photomultiplier--is being used to measure Stokes light pulses, thereby creating non-Gaussian atomic-ensemble entanglement in the 5-20 excitation-number regime. ii) To verify entanglement the researchers are developing balanced homodyne correlation, using a more practical scheme based on a CCD camera rather than multiple beam splitters as originally conceived.

Theme (2) extends the research into a new promising area: hollow-core photonic crystal fibers (HC-PCF). In collaboration with a group at Bath University, the research team will undertake exploratory studies of rubidium-filled, hydrogen-filled, and xenon-filled HC-PCF. The long path lengths and tight, controlled optical confinement make these systems promising for ultralow-intensity nonlinear quantum optics, including creation of ultrawide-band comb-spectrum fields, photon pair generation, four-mode optical entanglement, atom-field entanglement, and others.

The research will contribute broadly to the understanding of quantum entanglement and its measurement. This may impact studies of the foundations of quantum physics as well as quantum information science. Education of graduate students and undergraduate students is foremost in the planning of the project. Undergraduate students will be involved through REU programs. Important collaborative aspects of the project include close interaction with the theory group of Prof. Steven van Enk at the University of Oregon, experimental collaboration with Prof. F. Benabid at Bath University in England on HC-PCF, and technical collaboration with H.-G. Moser at Max Planck Institute in Garching, Germany on photon-number-resolving detectors. The PI is involved in various outreach efforts, including public lectures, new course development and textbook authoring, in an effort to bring physics to a wider audience. He is involved in international outreach through summer schools and research collaborations. And, the PI was founding Director of the Oregon Center for Optics at the UO, a synergistic center involving faculty and students from physics and chemistry.

Project Report

Laser light is used in a huge range of technologies and sciences, as well as in consumer products such as compact disk players. Precisely constructed light beams can be used as ‘measuring sticks’ or ‘clocks’ to measure quantities such as distance, time, or color (which is technically determined by the oscillation frequency or equivalently the wavelength of the light). The biggest breakthrough in recent years in this ability was the invention of the ‘optical frequency comb,’ which is a light beam containing many precisely known colors, or wavelengths, of light, and can be used to measure the unknown wavelength of a light beam, much as a ruler with many tick marks can be used to measure distance. Such a frequency comb also can create a steady stream of ultrashort bursts or pulses of light, each having duration around 1 femtosecond, but this occurs only if the oscillations of all of the different-color components in the light are synchronized, or ‘in-phase.’ The PI on this grant, along with several undergraduate and graduate students, and collaborators at the University of Bath in the United Kingdom, created a frequency comb by adding hydrogen gas to the inside of a hollow optical-fiber cable and sending a short burst of red laser light through the gas inside the fiber. By sending in laser light of just a single color, a frequency comb was created having many colors present within the visible spectrum of light. The figure shows the spectrum of the frequency comb, photographed after spreading it with a prism. The mechanism by which the different colors of light are produced in the hollow fiber is the following. As the short burst of red laser light passed through the gas, the hydrogen molecules in the gas were caused to vibrate with a particular frequency specific to this type of molecule. The vibration of the molecules can either increase the frequency of the light or decreases its frequency by the frequency at which the molecules vibrate. This process can repeat itself as the light travels in the fiber, leading to emission of strong light of many colors. This observation, and the theoretical understanding that the researchers developed to explain them, provides groundwork for future research in creating ultra-short light pulses. Such pulses could be used for studying the behavior of large molecules such as DNA—the main ingredient of all life—which is a far more complex molecule than is hydrogen, the gas used to create the frequency comb and the short light pulses. The broader impact of this research project and the activities of the PI lie in science training and education. Laser physics and optical physics offer excellent opportunities to integrate research with science education. PhD students currently involved in the PI’s research have participated in the NSF’s GK-12 Program, which pairs PhD students with high schools and middle schools, exposing their students to the ideas of science and scientific research as a career. High-school students, undergraduate students, Masters and PhD students, as well as visiting scientists, have all been involved in the groups’ research in recent years. Students also participate as co-instructors of courses in a new Science Literacy Program at the University of Oregon, created and co-directed by the PI.

National Science Foundation (NSF)
Division of Physics (PHY)
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Siu Au Lee
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University of Oregon Eugene
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