Cavity quantum electrodynamics (QED) is one of the main tools of physicists to explore the non-classical quantum world. The high quality factors of modern optical cavities combined with the ability to achieve strong coupling between a cavity mode and a single quantum emitter have made cavity QED the perfect laboratory for exploring unitary quantum mechanics with an eye towards directly testing fundamental principles of quantum mechanics and developing quantum information processing devices. Traditional studies of cavity QED have focused on achieving the strong coupling regime where the quantum mechanical interaction with the vacuum field is stronger than all decay processes. Experiments have only recently demonstrated continuous strong coupling to an individual two-level emitter in the optical regime. Here we propose a research program that studies new applications of cavity QED based on novel extensions of the Jaynes-Cummings model. Just as cavity QED has evolved along two tracks based on the type of emitter and cavity, atoms in a Fabry-Perot cavity or quantum dots grown inside of a semiconductor microcavity, the proposed research will look at new applications in both areas. Specifically, the proposed research will develop new theoretical models to describe Raman photoassociation of ultracold atomic gases via a cavity field. The coherent quantum dynamics of quantum degenerate ultracold gases along with the feedback of an optical resonator lead to a novel nonlinear quantum system that mixes atomic, molecular, and optical waves. The feedback effects of a driven cavity mode provides an extra level of control over the photoassociation dynamics that will lead to precise control over the number of molecules created or dissociated in a way that is impossible using conventional free space photoassociation. A semiclassical analysis of this system by the PI already indicates the presence of bifurcations and bistable regimes. The PI plans to develop and investigate a fully quantum model of cavity assisted photoassociation (CAP). Of particular interest will be the effect of quantum noise on the semiclassical bifurcations and bistable states. The second direction will be to study a single quantum dot embedded in an optical microcavity with electrical leads connected to the dot that allow controlled charging of the dot. The possibility of electrical current fowing through a cavity coupled dot opens up new directions, which have hitherto not been considered. The PI has recently shown that such a system can be an efficient source of pure spin current. The ability to use the cavity field to control the spin current and its statistics will be explored.

Project Report

Here we carried out a research program that pushed the interactions of matter inside of an optical resonator into hitherto uncharted territo- ries by examining its use in molecule production from ultracold quantum degenerate gases such as Bose-Einstein condensates (BECs) and separately, mesoscopic electrical transport in semiconductors. Specifically we focused on optical fields inside of resonators with limited numbers of photons to study the quantum mechanical effect that the light can have on either molecule formation or electron transport in quantum dots including quantum entanglement between the electrons and/or molecules and resonator photons. Optical cavity assisted formation of ultracold molecules had been an unexplored field that offered the potential of greater control over the molecule formation process as well as the potential for entangling light with the quantum state of the molecules. Our research developed new numerical and analytical mathematical techniques for studying the dynamics of the coupled atomic, molecular, and optical quantum fields. These techniques were then used to understand the experimental parameters that influence molecule formation rates including cavity field strength and scattering between atoms and molecules. Subsequent work showed the existence of non-classical entanglement between photons and molecules and squeezed quantum fields. Entangled states have applications to quantum computing and quantum cryptography while squeezing is important for high precision low noise measurements. The second direction of the project was mesoscopic transport of electrons through quantum dot structures coupled photons in optical microcavities in which the dots are embedded. This work was was inspired by recent experimental work showing controlled charging by a doped lead of a quantum dot in a microcavity and quantum dot single photon sources. We addressed the question of how are both the electrical current and shot noise from the quantum dot(s) modified by the coupling to the light inside of a microcavity?? First it was shown that such devices did not readily produce ordinary electrical currents but rather spin currents that involve zero flow of charge but a net flow of electron spin. Spintronics is an emergent field of electronics that utilizes the spin degree of freedom of the charge carriers in semiconductors and metals for information processing. Manipulation of the spin degrees of freedom rather than the charge has the advantage of longer coherence and relaxation times. Currently there is a great deal of interest in developing devices that can create, measure, and manipulate spin currents in order to realize this new technology. Spin currents can be considered to be the 'flying qubits' for solid state quantum information networks and therefore spin batteries play a role analogous to single photon sources for optical quantum networks. Our work showed how dot-cavity configurations could be used as a controlled source of pure spin currents for spintronic devices, essentially a "spin" battery. The spin battery configuration after a simple modification to the cavity field becomes an optically controlled spin current switch where by light incident on the microcavity mirrors can control the flow of an electron spin current through a quantum dot. Both of these devices are important contributions to the toolbox of spintronics. Subseuquent work studied the quantum mechanical limits of classical bistability by showing that an individual quantum dot produces a spin current with statistical behavior analagous to classical bistability as a function of the photon number in the microcavity. Finally, a spin based Aharanov-Bohm interferometer was studies in which it was shown that the cavity field could be used to induce couplings between electrons in the separated arms of the interferometer. Moreover, when two electrons are simultaneously in the interferometer a new type of interference as a function of magnetic flux appears that is only visible in the shot noise spectrum.

Agency
National Science Foundation (NSF)
Institute
Division of Physics (PHY)
Application #
0757933
Program Officer
Richard Houghton Pratt
Project Start
Project End
Budget Start
2008-06-01
Budget End
2011-05-31
Support Year
Fiscal Year
2007
Total Cost
$188,569
Indirect Cost
Name
Stevens Institute of Technology
Department
Type
DUNS #
City
Hoboken
State
NJ
Country
United States
Zip Code
07030