Multi-Dimension Measurement Tool 'Central' to Quantum Communication Security

Scientists have developed a tool that allows them to measure 27 dimensional states, which they say is key to high security quantum communications systems.

Previously, researchers at the University of Rochester had used a multistage process involving quantum tomography, which is similar to creating a 3D image from 2D ones.

However, the team have now applied a method called direct measurement to measure dimensions, a recently developed system that can be carried out in a single experiment with no post-processing.

As well as use in security industries, the researchers also say direct measurement can be used to further develop our understanding of quantum mechanics.

Published in Nature communications, the team shows direct measurements of the quantum state associated with orbital-angular momentum.

Robert Boyd, from the University of Rochester, said: "Our work shows that direct measurement offers an exciting alternative to quantum tomography. As the field of quantum information continues to advance, we expect direct measurement to play an increasingly important role in this."

Direct measurement was first developed in 2011 by scientists in Canada, who used it to work out the position and momentum of photons.

The latest research paper is the first to show that direct measurement can be used to measure the polarisation states of light. Previously, it was understood that direct measurement could not be used in wavefunction.

However, direct measurements involve two types of measurement performed consecutively. First a weak measurement is taken, then a stronger one. The act of measuring a quantum state disturbs it irreversibly, which is a phenomenon known as the collapse of the wavefunction.

In direct measurement, the first measurement is so gentle that it only slightly disturbs the system and does not cause the wavefunction to collapse.

Because it does not destroy the wavefunction, it allows for subsequent measurement. The sequence of weak and strong measurements can be repeated for identically prepared quantum systems.

"It is sort of like peeking into the box to see if Schrodinger's cat is alive, without fully opening the box," said lead author Mehul Malik.

Schrödinger's cat is a paradoxical experiment devised by Erwin Schrödinger in 1935. It presents the problem that a cat may be alive or dead, depending on an earlier event. It shows problems of quantum mechanics when applied to everyday objects.

He pointed out that the state of an unstable keg of gunpowder will eventually have a superposition of both exploded and unexploded states. Furthering this, Schrödinger said that according to one theory of quantum physics, the cat remains both alive and dead to the universe outside the box, until the box is opened.

"The weak measurement is essentially a bad measurement, which leaves you mostly uncertain about whether the cat is alive or dead. It does, however, give partial information on the health of the cat, which when repeated many times can lead to near certain information as to whether the cat is alive or dead."