The quantum computer has come a step closer: researchers at the University of Santa Barbara have developed a relatively cheap technique for entangling qubits. Qubit entanglement is one of the strengths of quantum computers because every measurement of one entangled qubit by definition yields information about the state of the other. However, the entanglement is very difficult to implement: the qubit pairing record currently stands at eight. The American researchers used superconducting circuits to control the qubits. An advantage of this technique is that the electronics are cheaper to produce than, for example, ion traps that are manipulated with lasers, because traditional chip lithography can be used to make the necessary circuits.
An equally important advantage of the new technique is the fact that the qubit circuits can be connected with a soldering iron, so to speak: previous solutions required that the quantum particles were in close proximity to each other before they became entangled. Evidence for entanglement is also not easy to provide. The researchers used a technology known as ‘quantum state tomography’ to show that the particles behave according to the so-called DiVincenzo criteria. The better a set of qubits conforms to these criteria, the more useful the technology is. ‘The test set-up now behaves 87 percent as the theory prescribes,’ said researcher Matthias Steffen. According to him, the materials used have a major influence on the results, and the reliability percentage can easily be increased even further with material research.