Ulvi H. Yurtsever

His current research interests are focused on exploring the interface between physics and computation, specifically on the computational and information-theoretical implications of quantum theory. His recent research efforts have been directed at various theoretical questions of importance in quantum metrology, an area that lies at the interface between quantum information theory and sensor technologies. An intriguing new quantum algorithm discovered at JPL, quantum clock synchronization, has been the catalyst in initiating this research effort, which has started as the study of possible quantum information theory protocols to synchronize atomic clocks nonlocally, but has now grown into a full-fledged research program to study relativistic quantum information theory. This program comprises three major research areas: (i) development of a relativistic theory of quantum information, with emphasis on the Lorentz-invariant formulation of distributed-entanglement-based algorithms of significance in quantum metrology applications, (ii) discovery of a practical quantum clock synchronization protocol, and (iii) solutions to problems (such as decoherence of entanglement) related to the actual physical implementations of these algorithms and of entanglement distribution in general.

During his career as a postdoctoral researcher, Yurtsever's research interests were centered around general relativity and quantum field theory in curved spacetime. His joint work with Kip Thorne and Mike Morris raised the possibility of sustaining traversible wormholes in spacetime via exotic matter made out of non-standard quantum field states. Yurtsever continued to investigate fundamental physics questions inspired by this wormhole paper as a postdoctoral research fellow at various institutions. These investigations involved the study of the Cauchy problem in spacetimes with closed timelike curves (spacetimes which contain time machines), stability properties of Cauchy horizons which inevitably form at the interface of regions in spacetime with and without closed timelike curves, and other ramifications of these and related questions. Yurtsever then turned his attention to the following question: Do the laws of physics as we understand them today allow the kinds of exotic matter field configurations (in other words, the kinds of quantum states of matter) necessary to sustain traversible wormhole topologies? In a number of publications over a four-year period of research, Yurtsever essentially answered this question in the negative, modulo some caveats that can be characterized precisely. During this phase of his career, Yurtsever also developed and pursued an original approach to the problem of quantum gravity (quantization of spacetime), one of the most perplexing and difficult problems in the history of science.

During his post-doctoral research as a National Research Council fellow at JPL, Yurtsever explored the possibility of chaotic dynamics in general relativity, and continued to investigate the issue of fundamental limits to properties of quantum matter in curved spacetime. Although in classical physics this issue can be avoided by assuming spacetime to be empty, in quantum theory absolute vacuum is meaningless, and questions about how matter fields contribute to the semiclassical Einstein equations are unavoidable whenever quantum effects are important gravitationally. In deep contrast with the classical picture of matter obeying a number of the "standard" energy conditions, in quantum theory no such compact characterization is yet available for the right hand side of the semiclassical Einstein equations. In the absence of a workable, complete characterization of regularized stress-energy tensors in quantum field theory, many of the basic questions about global spacetime structure in semiclassical gravity remain unanswered. For example, can spacetime singularities, generically unavoidable with classical matter, be avoided when quantum effects make the dominant contribution to stress-energy? Are classically forbidden configurations of spacetime curvature (such as traversable wormholes, certain kinds of topology change) allowed in semiclassical gravity? Is the total mass of a bounded lump of quantized matter positive as measured from infinity? More generally, is any conserved, symmetric tensor realizable as the regularized stress-energy tensor of some quantum state? If this were the case, semiclassical gravity would have almost no physical content. If it is not the case, then what are the general constraints on realistic regularized stress-energy tensors? Yurtsever's most recent work in general relativity was devoted to answering these questions.