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Arpil 24 Colloquium

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Jeff Lundeen, University of Ottawa

Seeing is Believing: Direct Observation of a General Quantum State

Host: Michael Raymer

Abstract: Central to quantum theory, the wavefunction is a complex distribution associated with a quantum system. Despite its fundamental role, it is typically introduced as an abstract element of the theory with no explicit definition. Rather, physicists come to a working understanding of it through its use to calculate measurement outcome probabilities through the Born Rule. Tomographic methods can reconstruct the wavefunction from measured probabilities. In contrast, I present a method to directly measure the wavefunction so that its real and imaginary components appear straight on our measurement apparatus. At the heart of the method is a joint measurement of position and momentum that is made possible by weak measurement (a concept that I will attempt to demystify). I will describe an experimental example of the method in which we directly measured the transverse spatial wavefunction of a single photon. New experimental work extending this to mixed states will be presented as well. Our direct measurement method gives the wavefunction a plain and general meaning in terms of a specific set of operations in the lab.

Related Papers:

Nature, 474, 188 (2011).

Phys. Rev. Lett. 108, 070402 (2012).

Physical Review Letters, 112, 070405 (2014).