Four-Qubit Entanglement

I stumbled across this news item this morning on a new methodology being implemented to try to solve a very difficult problem, the higher order entanglement that underlies qubit/qubit states. (Qubits, a term which arises from Information theory and commonly used by folks working in the field of quantum computing is a simply entangled quantum state consisting of two “pure” states. It’s effectively the basic unit of entanglement.)

From the article about the new technique (which opens with a bit of helpful background):

“‘Quantum entanglement leads to the most counterintuitive effects in quantum mechanics, and it is of great relevance in advanced quantum information methods and also opens a number of questions about the nature of entanglement itself,’ Bourennane told ‘Until now, there is, in general, no known measure for entanglement for a system of more than two particles; therefore, no one is able to say that a state is more or less entangled than the other. At the same time, it is still quite difficult to observe multi-particle entanglement.’

As Bourennane explained, inevitable interactions with the environment can cause quantum entanglement to become noisy during the information processing. An important and crucial question is to know which of the noisy states can be distilled to maximally entangled states, with the help of local operations and classical communication, and then be useful again for further information processing. For this reason, the theoretical discovery of bound entanglement (by Ryszard Horodecki, Michal Horodecki and Pawel Horodecki) is very important, being a class of quantum entangled states where no entanglement can be distilled.

‘Our paper reports for the first time on the experimental evidence of the existence of the bound entangled state, the so-called Smolin state, and fully characterizes it using quantum state tomography,’ Bourennane said. ‘We also study its entanglement properties, using the separability criterion, the Bell inequality, and the witness method. As can be seen, the paper contains new achievements and new insight regarding the general understanding of entanglement.’ “

Read the full article here.

It looks to me like the upshot of the article is that by entangling a pair of entangled photon states, the experiment allows the researchers to study a more intense entanglement than would normally be available in a simple qubit. It appears that the same quantum behavior is still present in this more complicated system. That’s not unexpected, but nice to have verified.

The real-world application of this idea will initially by in cryptography and will allow for the creation of a solution to the man-in-the-middle attack scheme. (The basic methodology of which underlies the now trivially defeated WEP encoding and the more difficult but still possible partial decoding of WPA-TKIP messaging.)

Author: Nick Knisely

Episcopal bishop, dad, astronomer, erstwhile dancer...