Quantum Physicist Kurt Jacobs
December 2011
As a quantum, or nano-, physicist, Kurt Jacobs attempts to measure and thus understand the role and relevance of the smallest discrete values, or quanta, of matter and energy—and in so doing measure the effects of energy, externally applied and internally generated, to map and enhance our understanding of the behavior of stochastic systems.
According to Webster's On-line Dictionary, "A stochastic process is one whose behavior is non-deterministic, in that a system’s subsequent state is determined both by the process’s predictable actions and by a random element."
Feedback control is the process of monitoring a system and using the information as it is being obtained in real time to apply forces to the system to control its behavior,” explains Jacobs. “Generally the objective is to get the system to maintain a desired evolution in the presence of noise or other uncertainties. The subject of feedback control in classical systems is well-developed, and feedback control is essential in many electrical and mechanical devices. The application of feedback control to quantum systems is the subject of quantum feedback control.”
The key property that distinguishes quantum feedback control from its classical counterpart is that, in general, measurements cause disturbance in quantum systems. That is, the measurement that is part of the feedback loop will introduce noise into the system. Understanding feedback control in quantum systems therefore involves understanding not only how best to use the information obtained by the measurement, but how to optimize the measurement to minimize the disturbance.
In fact, Jacobs has developed and now teaches the new graduate course Introduction to Stochastic Processes. “The class fills two roles,” says Jacobs. “Firstly, the subject is central to much of my own research, and it is therefore very useful for graduate students who wish to pursue research with me. Secondly, this subject has a wide range of applications in interdisciplinary fields, such as neuroscience, finance, and biophysics.”
In 2009, the National Science Foundation awarded Jacobs a grant to determine the ability of time-dependent controls to engineer nonlinear dynamics in, and nonlinear probes for mesoscopic quantum resonators. In 2010, he partnered with physicist Fred Strauch of Williams College, who recently published a paper describing a new theoretical analysis of the routing of information in quantum networks.
A grid of mesoscopic quantum resonators might form the basis for a new type of computer that has ground-breaking potential for harnessing quantum mechanics to perform certain computations far, far better than even today’s supercomputers. So Jacobs is on the cutting-edge of what could very well be a “quantum” leap in our ability and speed to more accurately model, or predict, the behavior of systems, biological, financial, and ecological, to name just a few.
Another important application of feedback control in quantum systems is adaptive measurement. In this case the measurement (or equivalently the system) is altered in real time so as to achieve an effective measurement process that may be difficult to obtain in any other way. Jacobs and his UMass Boston colleagues, Professor Bala Sundaram and post-doc Stephen Choi, are working on the theory of feedback for stabilization and on adaptive measurements, as well as specific applications in atom optics and quantum nano electro mechanical systems.
