Redundant qubit construction may prove useful in quantum computing

Ars Technica : Measuring the state of entangled particles is destructiveâmdash;it severs the entanglement and causes the superposition of the particles to vanish. Hence, the measurement can’t be repeated. However, quantum computing requires the same measurement be taken multiple times in order to determine the probability for any given particle to be in a particular state at a particular time. A team of researchers from the University of Innsbruck in Austria appears to have developed a way to work around the destructiveness of entangled-state measurements. Instead of using electrons, which have just a single spin state, the team used calcium ions, which have a large number of possible spin states. Only four of the calcium ion’s total possible states are used, which the researchers defined as representing logical 1, logical 0, a measurement state, and a hidden state. By entangling three separate ions and tying separate states to each other via lasers, the researchers established a network of redundancy in the system. An initial measurement via laser pulse of one of the ions takes it out of superposition and determines whether it is in the logical-1 or logical-0 state. A second laser pulse is used to redefine the superposition of the logical-0 and hidden states as the superposition of the logical-1 and logical-0 states. That allowed the researchers to repeatedly measure the superposition of the logical-1 and logical-0 states in order to determine the probability of the ion being in a given state upon measurement.