Active Fellows

In my work as a theoretical astrophysicist, I focus on problems in fluid dynamics near black holes and in protoplanetary disks. My recent work includes studies of the formation and early evolution of extrasolar planetary systems; numerical simulations of turbulence in black hole accretion flows; and investigation of the astrophysical consequences of binary black hole mergers in galactic nuclei.

My research interests center around the behavior of extremely cold atomic gases. Recent developments in laser-cooling techniques have made possible new families of experiments at microKelvin temperatures. My group investigates techniques for manipulating cold atoms and studies interactions between trapped alkali atoms at collision energies below one microKelvin. I am best known for producing a Bose-Einstein condensate in a sample of trapped atoms. Most recently I've begun a project to measure the electric dipole moment of the electron, a project designed to investigate the particle physics concept known as "supersymmetry."

 My research interests include quantum coherence in nanoscale electrical circuits (solid-state qubits), dynamics of individual electrons in one-dimensional conductors, and ultrafast quantum-limited charge measurements. Recent advances in nanolithography and cryogenic electronics have made it possible to sense individual electrons as they move through nanometer-sized conductors. My research exploits this ability to detect the quantum superposition of macroscopically distinct states of electrical circuits and to detect the flow of single electrons through molecular wires. My current work addresses the following questions: What are the sources of decoherence in solid-state qubits? How efficiently and how quickly do electrons screen each other in nanoscale circuits? Can a sensor of charge reach quantum-limited sensitivity?

My group uses a two-step process to prepare ultracold molecules of NH, a simple free radical. The first step, supersonic expansion, forces NH molecules through a small opening into a vacuum system, where intermolecular collisions cool the rapidly expanding gas (400 m/s) to less than 1 K. The second step uses varying electric fields (Stark deceleration) to slow the cold molecules to rest. Once the molecules are cold and stopped, we can subject them to magnetic trapping, electrostatic trapping, laser cooling, or sympathetic cooling. I plan to work with theorist John Bohn to investigate collisions of cold polar NH radicals (produced in this process) with each other and with ultracold atoms such as Rb.

My research includes quantum-state-resolved laser spectroscopy and dynamics of van der Waals and hydrogen-bonded clusters, time-resolved kinetics of atmospheric radicals, crossed-beam studies of state-to-state inelastic and reactive dynamics, high-resolution laser spectroscopy of jet-cooled radicals and molecular ions, nonlinear frequency generation of narrowband tunable infrared laser sources, vibrationally mediated photochemistry in size/quantum state-selected clusters, alignment phenomena, collision dynamics of gases with thin films, and development of atomic force/scanning-tunneling methods for near-field-scanning optical microscopy (NSOM) of molecules on surfaces.

The biochemical cycle of mechanoenzymes generates a force and a displacement that can be measured at the single-molecule level. The outstanding question is how motor proteins transduce chemical energy into physical motion. To answer this question, we use optical tweezers, a focused laser beam that can manipulate micron-sized beads in solution, allowing measurement of position and force in the nanometer (nm) and piconewton (pN) ranges, respectively. Our research focuses on developing assays and precision instrumentation to measure the properties of single-DNA-based molecular motors. Typically, enzymatic motion along the DNA is measured by anchoring the enzyme to a surface and monitoring the position of an optically trapped bead attached to the DNA's distal end.

My main research interest is quantum systems of interacting atoms, photons, and phonons. I seek to engineer and explore new quantum systems with controlled connections for quantum information and quantum optics. In particular I focus on manipulating single and few ultracold neutral atoms and the quest to control mesoscopic mechanical oscillators in the quantum regime. My experiments draw on, for example, low-loss optical interfaces, high-Q mechanical oscillators, and laser cooling and trapping techniques.

My research focuses on understanding the interface between ultracold atoms and quantum optics - an understanding I plan to apply to the field of precision measurement. I am presently devising strategies to reduce the effect of the fundamental quantum noise that arises from Heisenberg's uncertainty relationship as applied to atomic spins. In one project, I work on non-destructively measuring and canceling out the quantum fluctuations in the collective spin state of an ensemble of laser-cooled ⁸⁷Rb atoms in a high-finesse optical cavity. By learning how to minimize the effect of quantum noise in this type of system, I hope to advance the precise measurements required for atomic clocks and in searches for permanent electric dipole moments in atoms and molecules.

My group combines mass spectrometry with laser spectroscopy to characterize positively and negatively charged ions and biomolecules. One project is concerned with infrared spectroscopy of molecular and metal containing cluster ions. These are produced in a pulsed supersonic expansion, and mass selected in a time-of-flight mass spectrometer. Photons absorption leads to evaporation of weakly bound ligands from the cluster. The absorption spectra of the clusters under study can be measured by monitoring the generation of photofragments as a function of the laser wavelength. In a second project, we are developing a three-stage photofragmentation spectrometer that uses an electrospray ion source to deliver ions into an ion trap (Step 1). There, the ions can be cooled or reacted with small solvent molecules such as water. Next, the products from Step 1 are mass selectred and illuminated with tunable pulsed laser light (Step 2). Finally, the light-induced photofragments are analyzed by means of a time-of-flight spectrometer (Step 3). This system will allow us to study the spectral and dynamical differences between unsolvated, sequentially solvated, and solution phase ion species. In the future, we plan to use it to investigate the photo physics and chemistry of structurally and/or electronically complex molecules such as multiply charged anions, cationic and anionic salt cluster ions, and charged biomolecules.