Schrödinger's Cat could be visible after all

Posted by Emma Woollacott

Schrödinger's Cat could be (almost) as easy to observe as the internet's millions of LOLcats, with confirmation that there may be a way round Heisenberg's famous Uncertainty Principle after all.

Researchers at the University of Rochester and the University of Ottawa have used a comparatively new technique to directly measure for the first time the polarization states of light. Their work has implications for the weird Uncertainty Principle, which states that certain properties of a quantum system can be known only poorly if other related properties are known precisely.

The direct measurement technique was first developed in 2011 by scientists at Canada's National Research Council to measure the wavefunction - a way of determining the state of a quantum system. Such direct measurement had long been believed to be impossible on the basis that you could never fully understand a quantum system through direct observation.

Now, the Canadian researchers have come up with a parallel result - that it is possible to measure key related variables, known as 'conjugate' variables, of a quantum particle or state directly. The discovery is applicable to qubits, the building blocks of quantum information theory, as polarization states of light can be used to encode information.

"The ability to perform direct measurement of the quantum wavefunction has important future implications for quantum information science," says Robert Boyd, of both Rochester and Ottawa. "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."

The direct measurement technique employs a clever trick to measure the first property in such a 'weak' way that the system isn't disturbed significantly, and information about the second property can still be obtained.

Boyd and his team did this by passing polarized light through two crystals of differing thicknesses: the first, a very thin crystal that 'weakly' measured the horizontal and vertical polarization state; the second, a much thicker crystal that 'strongly' measured the diagonal and anti-diagonal polarization state.

As the first measurement was performed weakly, the system wasn't significantly disturbed, meaning that information gained from the second measurement was still valid. Repeating the process several times allows accurate statistics to be built up, giving a full, direct characterization of the polarization states of the light.