Quantum states of light are kinds of electromagnetic field with special quantum statistical properties. These quantum states can be used to improve measurement sensitivities of small phase and amplitude changes beyond the standard shot-noise limit. We are interested in finding practical applications of these quantum states (including squeezed states, twin-photon states, and states with sub-Poissonian photon statistics) in areas such as atomic spectroscopy, precision measurements, optical communication, optical computing, and electrical-optical devices. To work towards these goals, we are currently pursuing the following research directions:

(1) Electromagnetically induced transparency in multi-level systems. When coherent electro-magnetic fields interact with multi-level atomic systems, coherence between levels is induced. This atomic coherence is essential in creating novel effects, such as electromagnetically induced transparency (EIT) and lasing without inversion (LWI), enhanced dispersion without absorption, and enhanced nonlinear optical efficiencies. We have experimentally demonstrated the EIT effects with weak cw diode lasers in three-level cascade-type and lambda-type systems in rubidium atoms. We have measured dispersion properties of the EIT systems and demonstrated the enhancement of efficiency of nonlinear optical processes (such as non-degenerate four-wave mixing) by using EIT effect. We believe that these coherence and quantum interference effects in multi-level systems will have practical applications in photonic devices in solid materials (such as semiconductor quantum well structures and optical crystals) by modifying the absorption and dispersion properties of the media. We are working on the applications of these novel effects in electro-optical devices.

(2) Precision measurements beyond the shot-noise limit. In nonlinear interactions between electromagnetic fields and media, one can manipulate the quantum fluctuations in the system to suppress quantum noises in one quadrature of the field, such as to create certain kind of quantum states of light. These quantum states of light can be used to improve measurement precision beyond the standard shot-noise limit. Recently, we have employed amplitude- squeezed light from a quantum-well semiconductor laser and weak optical feedback from a highly dispersive grating in laser Doppler anemometry. Enhanced sensitivity is demonstrated in the Doppler measurement of a gas flow velocity with an improvement in the signal-to-noise ratio beyond the shot-noise limit. Many other practical applications (such as in optical communication and laser Doppler radar) of quantum states of light are currently under consideration.

(3) Quantum fluctuations in semiconductor nanostructures. To get higher density and more efficiency in photoelectric devices, the semiconductor nanostructures (such as quantum wells, quantum wires, and quantum dots) have recently attracted much attention. One can now grow quantum dots with an MBE machine quite easily. These quantum dots behave, in many aspects, like atoms and are called artificial atoms. Many optical properties of these quantum nanostructures were experimentally studied extensively. However, as system size decreases, the relative quantum fluctuations increase and become more important. We are interested in the quantum fluctuations of such small systems, especially with quantum dots inside micro-optical cavities. Working together with other faculty members in the department, we are developing practical photonic devices for optical communication and optical computing. We are using our strong background in multi-level atomic systems to manipulate the noise properties of the systems and eventually to make use of these quantum fluctuations in designing better electro-optical devices.

(4) Fundamental quantum optics. To understand quantum fluctuations better, we have been working on the fundamental theory of quantum optics for many years. We are interested in several very basic questions in quantum optics, such as generating quantum states of light in different nonlinear optical systems, interpreting of quantum phase operators in one and two field modes, and detecting quantum fluctuations.

(5) Applied optics. Other than the above mentioned basic research directions, we have worked on several applied projects in the past years. These projects include improvement of frequency stabilization of semiconductor diode lasers, ferroelectric domain structures in nonlinear optical crystals, nonlinear optical properties of polymers, optical tweezers for biological applications, and detection of contaminants in food products. We will continue to identify and work on these practical problems in the future.