William Wootters

Professor of Physics

at Williams since 1982

Education/Experience

• Stanford University, B.S. 1973
• University of Texas, Austin, Ph.D. 1980
• Santa Fe Institute, visiting researcher 1989-90
• University of Montreal, visiting researcher 1994
• IBM Watson Research Center, collaborator 1995, 1998

Contact Information

• Office: Thompson Physical Laboratory 305
• Phone: (413) 597-2156
• E-mail: william.wootters@williams.edu

Courses given 2007-2008

• Physics 405T: Electromagnetic Theory
• Physics 142: Foundations of Modern Physics

Research interests

Quantum Information Theory

Information stored in quantum systems behaves very differently from ordinary information. It cannot be copied perfectly, for example, and it is usually degraded by measurement. Despite these restrictions, this ghostly sort of information could be of great value in quantum computation and quantum cryptography. My research aims at learning more about the fundamental properties of quantum information. In past years my students and I have studied the classical capacity of a quantum channel and the amount of information one can extract from a single quantum object. More recently I have been interested in finding quantitative rules governing the "entanglement" between two or more quantum objects.

Entanglement is a peculiarly quantum mechanical kind of correlation that has no analogue in classical physics. It is the essential ingredient in such phenomena as superdense coding, in which any of four possible messages can be transmitted via a single binary quantum object, and teleportation, in which a quantum state is transmitted from one location to another without passing through the intervening space. In recent years much progress has been made in developing a quantitative theory of entanglement. For example, we now have a well-justified analytic formula for entanglement between simple systems. My students and I have used these developments to identify certain "laws of entanglement." To give one example: two former students, Valerie Coffman and Joydip Kundu, and I, using a convenient measure of entanglement known as the "concurrence," showed that for any state of three binary quantum objects (qubits) A, B, and C, there is a simple trade-off between the AB entanglement and the AC entanglement. That is, qubit A has a limited capacity for entanglement, which must be split between the objects with which it is entangled.

Wigner Functions

The density matrix is the most commonly used representation of a general mixed or pure quantum state. However, there is an alternative representation, the Wigner function, which is a real function on phase space. The Wigner function acts in many ways like a probability distribution, but it is not a standard probability distribution in that it can become negative. A few years ago two students (Kathleen Gibbons and Matthew Hoffman) and I presented a discrete version of the Wigner function applicable to a system ofqubits, in which the phase space is a two-dimensional vector space over a finitefield. My current students are exploring this construction in more depth, with an eye towards applications in foundations of physics and quantum computation.

Selected publications

• W. K. Wootters and C. G. Langton, "Is There a Sharp Phase Transition for Deterministic Cellular Automata?" Physica D 45, 95-104 (1990).
• A. Peres and W. K. Wootters, "Optimal Detection of Quantum Information," Phys. Rev. Lett. 66, 1119-1122 (1991).
• C. H. Bennett, G. Brassard, C. Crepeau, R. Jozsa, A. Peres, and W. K. Wootters, "Teleporting an Unknown Quantum State via Dual Classical and Einstein-Podolsky-Rosen Channels," Phys. Rev. Lett. 70, 1895 (1993).
• L. P. Hughston, R. Jozsa, W. K. Wootters, "A complete classification of quantum ensembles having a given density matrix," Phys. Lett. A 183, 14-18 (1993).
• R. Jozsa, D. Robb '93, and W. K. Wootters, "A Lower Bound for Accessible Information in Quantum Mechanics," Phys. Rev. A 49, 668 (1994).
• W. K. Wootters, "Is Time Asymmetry Logically Prior to Quantum Mechanics?" in Physical Origins of Time Asymmetry, ed. by J. Halliwell, J. Perez-Mercader and W. Zurek (Cambridge Univ. Press, 1994).
• P. Hausladen '93 and W. K. Wootters, "A 'Pretty Good' Measurement for Distinguishing Quantum States," J. Mod. Optics, 41, 2385 (1994).
• P. Hausladen '93, R. Jozsa, B. Schumacher, M. Westmoreland, and W. K. Wootters, "Classical information capacity of a quantum channel," Phys. Rev. A 54, 1869 (1996).
• C. H. Bennett, G. Brassard, S. Popescu, B. Schumacher, J. A. Smolin, and W. K. Wootters, "Purification of Noisy Entanglement and Faithful Teleportation via Noisy Channels," Phys. Rev. Lett. 76, 722 (1996).
• S. Hill '97 and W. K. Wootters, "Entanglement of a Pair of Quantum Bits," Phys. Rev. Lett. 78, 5022 (1997).
• W. K. Wootters, "Entanglement of Formation of an Arbitrary State of Two Qubits," Phys. Rev. Lett. 80, 2245 (1998).
• V. Coffman, J. Kundu, and W. K. Wootters, "Distributed Entanglement," Phys. Rev. A 61, 052306 (2000).
• C. H. Bennett, D. P. DiVincenzo, J. A. Smolin, B. M. Terhal, and W. K. Wootters, "Remote State Preparation," Phys. Rev. Lett. 87, 077902 (2000).
• K. M. O'Connor '00 and W. K. Wootters, "Entangled Rings," Phys. Rev. A 63, 052302 (2001).
• W. K. Wootters, "Entanglement of Formation and Concurrence," Quantum Information and Computation 1, 27 (2001).
• W. K. Wootters, "Entangled Chains," in Quantum Computation and Information, S. J. Lomonaco and H. E. Brandt, editors (American Mathematical Society, Providence, 2002), pp. 299-310.
• K. A. Dennison '01 and W. K. Wootters, "Entanglement Sharing among Particles with More than Two Orthogonal States," Phys. Rev. A 65, 010301 (2002).
• W. K. Wootters, "Parallel Transport in an Entangled Ring," J. Math. Phys. 43, 4307 (2002).
• N. Linden, S. Popescu, and W. K. Wootters, "Almost Every Pure State of Three Qubits Is Completely Determined by Its Two-Particle Reduced Density Matrices," Phys. Rev. Lett. 89, 207901 (2002).
• N. Linden and W. K. Wootters, "The Parts Determine the Whole in a Generic Pure Quantum State," Phys. Rev. Lett. 89, 277906 (2002).
• S. R. Nichols '03 and W. K. Wootters, "Between Entropy and Subentropy," Quantum Information and Computation 3, 1 (2003).
• W. K. Wootters, "Why Things Fall," Foundations of Physics 33, 1549 (2003).
• W. K. Wootters, "Picturing Qubits in Phase Space,"IBM Journal of Research and Development 48, no. 1, p. 99 (2004).
• K. S. Gibbons '03, M. J. Hoffman '04, and W. K. Wootters, "Discrete Phase Space Based on Finite Fields," Phys. Rev. A 70, 062101 (2004).
• W. K. Wootters, "Quantum Measurements and Finite Geometry," Foundations of Physics 36, 112 (2006).
• W. K. Wootters, "Distinguishing Unentangled States with an Unentangled Measurement," Int. J. Quant. Inf. 4, 219 (2006).

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