Researchers at the University of Rochester and the University of Ottawa have applied a recently developed technique to directly measure for the first time the polarization states of light. Their work both overcomes some important challenges of Heisenberg's famous Uncertainty Principle and also is applicable to qubits, the building blocks of quantum information theory.
The direct measurement technique was first developed in 2011 by scientists at the National Research Council, Canada, to measure the wavefunction -- a way of determining the state of a quantum system. Such direct measurements of the wavefunction had long seemed impossible because of a key tenet of the uncertainty principle -- the idea that certain properties of a quantum system could be known only poorly if certain other related properties were known with precision. The ability to make these measurements directly challenges the idea that full understanding of a quantum system could never come from direct observation.
The Rochester/Ottawa researchers, led by Robert Boyd, who has appointments at both universities, measured the polarization states of light -- the directions in which the electric and magnetic fields of the light oscillate. Their key result, like that of the team that pioneered direct measurement, is that it is possible to measure key related variables, known as "conjugate" variables, of a quantum particle or state directly. The polarization states of light can be used to encode information, which is why they can be the basis of qubits in quantum information applications.
"The ability to perform direct measurement of the quantum wavefunction has important future implications for quantum information science," explained Boyd, Canada Excellence Research Chair in Quantum Nonlinear Optics at the University of Ottawa and Professor of Optics and Physics at the University of Rochester. "Ongoing work in our group involves applying this technique to other systems, for example, measuring the form of a "mixed" (as opposed to a pure) quantum state."