Tangling with Entanglement: Polar Molecules as Qubits

By Dudley R. Herschbach

Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA

View Presentation (SWF)

Additional materials available (4)

Licensed under General Performance Usage.

Published on


Over the past decade, arrays of ultracold (< mK) polar molecules have come to be considered among the most promising platforms to implement a quantum computer. In principle, such a computer can perform a variety of calculations with exponentially fewer steps than a classical computer. A requisite for quantum computing is entangled states. For a pair of entangled states, the wavefunction cannot be separated into a product of functions of the individual partner states. (Einstein found quantum entanglement intolerable, because it allowed “spooky action at any distance.”) For arrays of polar molecules, entanglement is supplied by dipole-dipole interaction. Previous studies of prospects for computing with diatomic polar molecules had specified experimental conditions deemed suitable and accessible. This talk reports the first calculations of entanglement pertaining to such conditions. The results identify a key frequency shift parameter, Δω, and show that entanglement would be very small for the ground eigenstate. Yet, in principle, such weak entanglement would be sufficient for operation of logic gates, provided Δω can be detected unambiguously. Also discussed are prospects for using as qubits instead of diatomic molecules, symmetric tops, in states that precess rather than pinwheel.


Dudley Herschbach Third-generation native of San Jose, California. Grew up near Cupertino, in what was then rural area; for years milked cows, fed chickens and pigs, picked prunes, apricots, and walnuts in summers. First in his family to attend college, recruited as a football player. Earned B.S. in math and M.S. in chemistry at Stanford, mentored by George Polya and Harold Johnston; A.M. in physics and Ph.D. in chemical physics from Harvard, mentored by E. Bright Wilson. Appointed assistant professor of chemistry at U.C. Berkeley in 1959, undertook experiments to probe reaction dynamics of molecules in single collisions. Returned to Harvard in 1963. With Yuan Lee and John Polanyi, shared Nobel Prize in 1986. Taught graduate courses in quantum mechanics, molecular spectroscopy, and collision theory; undergrad courses in physical chemistry and for twenty years freshman chemistry.

Emeritus at Harvard since 2002, have continued since to teach freshman seminar (“Molecular Motors: Wizards of the Nanoworld”) there; also have visiting appointments in physics and astronomy at Texas A & M University and at the geophysical laboratory at the Carnegie Institution of Washington. Current research topics: pursuing an unorthodox dimensional scaling approach to electronic structure; elucidating interaction of molecules with superintense laser fields; analysis of wave function entanglement that pertains to proposed quantum computers; experiments striving to produce very slow, cold molecular beams which act like waves.

Efforts to enhance science education and public understanding have centered for 20 years on the Society for Science and the Public, which publishes Science News and conducts the Intel Science Talent Search and the Intel International Science and Engineering Fair. Have had many radio and TV appearances, including as a guest voice on The Simpsons. Also a life member of Friends of Franklin and of the Sierra Club, and for many years chaired the Hans Bethe Center for Arms Control and Nonproliferation.


The work described in this seminar was carried out in collaboration with Dr. Qi Wei, recently minted Ph.D. from Purdue, Prof. Sabre Kais of Purdue, and Dr. Bretislav Friedrich of the Fritz-Haber Institute, Berlin.

Sponsored by

Purdue Lectures in Theoretical Chemistry

Cite this work

Researchers should cite this work as follows:

  • Dudley R. Herschbach (2011), "Tangling with Entanglement: Polar Molecules as Qubits," https://nanohub.org/resources/11191.

    BibTex | EndNote



WTHR 104, Purdue University, West Lafayette, IN