Yakir Aharonov

Yakir Aharonov was born in Haifa in 1932, during the British Mandate for Palestine. He studied physics at the Technion in Haifa, took his BSc in 1956, and went to England for graduate work, where he became one of David Bohm's first doctoral students at Bristol. The match was decisive: Bohm had recently arrived in Bristol after his McCarthy-era exile from US academia, and his focus on the foundations of quantum mechanics — the meaning of the wavefunction, the role of the potential versus the field, the structure of measurement — was unusual at the time and matched Aharonov's interests precisely. Aharonov took his PhD under Bohm in 1960 with the thesis that contained the result for which both men are now remembered.

The 1959 paper *Significance of Electromagnetic Potentials in the Quantum Theory*, written while Aharonov was at Brandeis and Bohm at Bristol, predicted the effect now universally called the Aharonov-Bohm effect. The setup is simple: send an electron beam through a two-slit apparatus with a thin solenoid centred between the slits, with the magnetic field tightly confined to the solenoid's interior so that the electron paths through both slits pass through regions where B is identically zero. Classically nothing should happen — no force acts on the electrons. Quantum-mechanically, however, the electrons accumulate a phase shift Φ = (q/ℏ) ∮ A·dℓ around the loop, which by Stokes's theorem equals (q/ℏ)Φ_B where Φ_B is the magnetic flux enclosed by the solenoid. The interference fringes shift laterally on the detection screen. Robert Chambers observed the effect at Bristol in 1960, the year after the prediction; Akira Tonomura's electron-holography experiment at Hitachi in 1986 closed every conceivable loophole and is now the standard demonstration. The effect is among the most-cited results in twentieth-century physics — it forced the gauge potential A_μ, not just the field tensor F^{μν}, into the foundational vocabulary of quantum theory.

Aharonov's later work has continued to centre on the foundations of quantum mechanics. With Lev Vaidman and Daniel Albert he developed the *two-state vector formalism* (a time-symmetric reformulation of quantum measurement), and with Albert and Vaidman in 1988 introduced *weak measurements* — the technique behind the now-celebrated experimental result that a measurement of the spin component of a spin-½ particle can yield a value of 100 (the so-called "amplification by post-selection"). The Aharonov-Casher effect (1984), an electric dual of the AB effect, predicts a phase shift for a neutral particle with magnetic moment moving past a line charge. Aharonov has held positions at Tel Aviv, the University of South Carolina, and currently James J. Farley Professorship at Chapman University. He won the Wolf Prize in 1998 (shared with Michael Berry for the Berry phase, which generalised the AB phase to arbitrary parameter spaces) and the US National Medal of Science in 2010.