This CAREER project focuses on achieving a better understanding of continuous quantum measurements of the motion of ultracold atoms. The research will characterize continuous quantum measurements in detail through experiments on the free-space dynamics of atoms interacting with classical laser fields. This research direction begins with a relatively simple experiment to study measurement-induced interference in the resonance fluorescence of two atoms. The emergence of an interference pattern in the scattered light is the signature of a quantum measurement of relative position, a dynamical collapse process that counters the dispersive tendencies of the atomic motion. The next goal is to demonstrate a time- and space-resolved measurement of a single atom by imaging light scattered from a probe laser - a realization of a continuous Heisenberg microscope. This will allow a thorough characterization of a continuous measurement process, including studies of the emergence of quantum correlations and classical-like trajectory behavior in the measurement record. A long-term goal of this research plan is to study the feedback control of a single quantum system. The educational component involves outreach to high school teachers and the development of a mad scientist course aimed at non-science majors.
The theme of this project was to explore and characterize the influence of quantum-mechanical measurements on the dynamics of quantum systems, and in particular, ultracold atoms. Theoretical accomplishments include detailed models of position measurements on atoms, raising the possibility of observing exotic transport (diffusion) phenomena, engineering quantum states of atoms by tailoring the measurements, and connecting position measurements to the well-known quantum Zeno effect. Our main experimental accomplishment is a demonstration of the feasibility of a laser "trap-door" for ultracold atoms. In principle the idea is fairly simple: atoms hitting the barrier on one side are allowed to transmit, but if they hit the barrier from the other side they bounce off of it. We accomplished this with (among many others) three laser beams: one traps and guides the atoms to the barrier; a second make the atoms transmit or reflect depending on the orientation of the atoms; and a third ensures that once the atoms have transmitted, their orientation changes so that they reflect off the barrier when they return. These results are significant first, as a fundamental tool for transporting and controlling ultracold atoms: this one-way barrier acts on atoms in just the same way as the ubiquitous diode for electrical current. Even more importantly, they realize a textbook situation in the field of statistical mechanics known as "Maxwell's Demon," a paradox posed by James Clerk Maxwell more than a century ago for how a fictitious "demon" can use information to refrigerate a gas of atoms without expending any energy, a situation that appears to violate the laws of thermodynamics. The corresponding technological and scientific implications of this research are that this barrier can be adapted to create new methods for cooling atoms and molecules, most of which cannot currently be cooled to very low temperatures (within millionths of a degree above absolute zero) by present techniques. Since present laser cooling and trapping techniques have literally revolutionized the field of atomic physics, new techniques should have a similarly big impact on atomic and molecular physics in the future.
