David E. Pritchard

Pritchard completed his PhD in 1968 in Harvard University under the supervision of Daniel Kleppner. For his thesis he built the first atomic scattering machine with polarized atoms to study differential spin exchange scattering - the process that excites the 21 cm hydrogen radiation.[1]

Pritchard was a pioneer in the application of tunable lasers to physics and chemistry, being the first to demonstrate high resolution spectroscopy when two laser photons are absorbed simultaneously. He used both laser and radio-frequency spectroscopy to study weakly bound van der Waals molecules like NaNe[3] and KAr[4] made in cold supersonic molecular beams.

Exploiting the ability of tunable lasers to transfer large amounts of momentum to atoms, Pritchard performed classic demonstrations of the diffraction of atoms from a standing wave of light (denoted Kapitza-Dirac or Raman-Nath regimes) and Bragg scattering[5] of atoms from light gratings, founding the field of coherent atom optics.[6] This led to the first atom interferometer[7] where the atom wave passed on both sides of a metal foil before recombining, so that different interactions on the two sides produced a fringe shift of the atomic interference pattern.[8] This allowed precise measurements of atomic polarizability, the refractive index of gasses for atom waves, and fundamental tests of quantum decoherence, as well as the first demonstration of the power of atom interferometers to measure rotation like a gyroscope and to work for complex particles like Na₂ molecules.[9]

A singularly important development from atom optics is Pritchard’s invention of the magneto-optical trap[10] that captures and cools atoms to sub-milikelvin temperatures and the “dark spot MOT”[11] that can compress ~ 10¹⁰ cold atoms in the same small volume. Together with a magnetic atom trap Pritchard independently re-invented (sometimes called the Ioffe-Pritchard trap to honor its plasma physics origin). These traps are the workhorses in the field of cold atom research and are the foundational tools for the MIT-Harvard Center for Ultracold Atoms.

In 1990, he brought Wolfgang Ketterle to MIT as a postdoctoral researcher to work on atom cooling. To induce Ketterle to stay at MIT, in 1993 he gave him his experimental cold atom program (with two students and two grants) and stepped aside from that field to allow Ketterle to be appointed to the faculty. Ketterle pursued atom cooling to achieve Bose–Einstein condensation in 1995, a discovery for which Ketterle was awarded the Nobel Prize in Physics in 2001, along with Pritchard’s former graduate student, Eric Allin Cornell and Carl Wieman who was an informal Pritchard mentee as an undergraduate at MIT. Ketterle gave Pritchard his Nobel medal.

Ketterle and Pritchard then partnered to study atom optics and interferometry with Bose condensates, demonstrating coherent amplification of matter waves, superradiant Rayleigh scattering, the power of Bragg spectroscopy to probe the condensate, and even used laser light to establish coherence between two condensates that never touch.

Pritchard is a pioneer in the precise measurement of atomic and molecular masses using ion traps, an advance enabled by his group’s developing highly sensitive radio-frequency detectors based on SQUIDs (superconducting quantum interference devices) and techniques to coherently cross-couple the motion of different modes of an ion’s oscillation in the trap. These advances culminated in an ion balance in which one each of two different ions were simultaneously confined while their cyclotron frequencies were inter-compared to better than one part in 10¹¹.[12] This led to the discovery of a new type of systematic shift of the cyclotron frequency due to the polarizability of the ion, providing the most accurate measurement of ionic molecule polarizability. It also resulted in a fifty-fold improvement of experimental tests of Albert Einstein’s mass–energy equivalence that ${\displaystyle E=mc^{2}}$ (where E is the energy, m is the mass and c the speed of light)– now at ½ part per million.[13]

Precise measurements of the masses of rubidium and caesium atoms made with the MIT apparatus have been combined with others’ high-precision atom interferometric measurements of h/m (Planck’s constant divided by the atom mass) to give the most accurate value of the fine structure constant at 0.2 ppb (parts per billion), differing by ~ 2.5 combined errors from measurement based on quantum electrodynamics. This is the most precise comparison of measurements made using entirely different theoretical bases.

Throughout his career, Pritchard has excelled as a mentor and advisor. In addition to the three Nobel prize winners mentioned above, four of Pritchard’s students have won national thesis awards. A disproportionate number of his mentees and students have positions in top-20 physics departments or top laboratories like the National Institute of Standards and Technology (NIST). Every alumnus of his education group stepped into a job in a university as an assistant professor or education-related position or in an education company.

He has been a director of a failed precast concrete company as well as the Optical Society of America, now Optica, and was founder of Effective Educational Technologies (sold to Pearson Education in 2006).