Faculty

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."

Institute of Astronomy
Madingley Road
Cambridge, CB3 OHA
UK

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 research focuses on understanding the statistics of precision clocks and frequency standards and on developing methods for distributing precise time and frequency information. The statistical studies are the basis for the NIST time scale, AT1, which computes the weighted average of the  times of an ensemble of cesium standards and hydrogen masers, and uses this weighted average to realize the NIST time scale, UTC(NIST). This time scale is the basis for all of the NIST time and frequency services and is also used to evaluate precision oscillators that are being developed by other groups at NIST and at JILA. For example, the time scale output can be used to characterize an optical frequency using a frequency comb, which links the microwave frequency output of the time scale to the optical frequency. It is also used to characterize oscillators based on cooled, trapped ions. The output of the time scale is typically within a few nanoseconds in time and less than 2 X 10^{^-15} (for up to 30 days of averaging) in frequency relative to UTC, the international time scale computed by the International Bureau of Weights and Measures (BIPM).

A second important application of the time scale is to provide a reference time for the NIST time services, including the radio stations WWV, WWVH and WWVB, which transmit time and frequency information from transmitters in Fort Collins, Colorado and Hawaii. The transmissions from station WWVB in Colorado are very widely used to set consumer devices such as wall clocks, microwave ovens and similar systems. In this application, the advance notice of the transitions to and from daylight saving time and the traceability of the time signals to a national standard are more important than the accuracy of the time signals as received by the users, which is typically on the order of 0.05 s. A related application uses the time signals from the NIST time scale to distribute time and frequency information in digital formats using dial-up telephone lines and the Internet. These services are widely used to synchronize e-mail networks and network routers and gateways. The signals are also used by commercial and financial institutions, and are particularly relevant to providing time stamps for rapid, automated trading in stocks, bonds, and commodities. The Internet time service also supports authentication based on one-way hash functions. This authentication guarantees that the time signal originated from a NIST server and was not modified (either accidentally or maliciously) in transit. The service uses an geographically diverse ensemble of systems that are synchronized to UTC(NIST) in Boulder, and the servers currently receive about 8 X 10^{^9} requests per day. These services are more fully described on the time and frequency web page, http://www.nist.gov/pml/div688.

A third research effort is a study of methods for improving the accuracy of transferring time and frequency information so as the enable the comparison of current primary frequency standards, which can realize the SI second with an uncertainty of less than 10^{^-15}, and the next generation of these devices, which may have uncertainties a factor of 10 or more smaller than this value. One promising approach is to use the phase of the carrier of the signals from GPS satellites to perform these comparisons, but there are a number of challenges that must be overcome before this technique can realize the required accuracy level. Although the short-term (second to second) fluctuations in the carrier-phase data can satisfy the transfer requirements, the noise spectrum is not white phase noise, so that the variance does not improve with averaging. The flicker and random-walk nature of the noise actually causes the accuracy of the frequency transfer to degrade as the averaging time increases.

Many of these issues are more fully discussed in my review article, which appeared in the February 2012 issue of the Review of Scientific Instruments and in the references listed at the end of that article.

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.

Astrophysical fluid dynamics is my primary research interest, and my work currently centers on theoretical treatments for compressible convection in stars. The nonlinear theory using computational fluid dynamics is complemented by detailed observations of velocity fields on the Sun, from both ground-based and satellite instruments. My interests extend to helioseismology, in which the five-minute oscillations of the Sun are used as probes of velocity and temperature structures within the interior. My group seeks to understand the coupling of global-scale convection with rotation by using a series of experiments flown on the space shuttle. My nonlinear dynamical studies in geophysics include thermohaline convection in the oceans and stratified shear flows.