In conventional electronics, a NOT gate inverts the value of a bit of information from 1 to 0 or from 0 to 1. Such binary information could also be stored in two-state quantum systems, such as the horizontal and vertical polarization states of photons, or the spin-up and spin-down states of electrons.

One of the new two qubit logic gates was made using a pair of trapped beryllium ions, by David Winefield from the National Institute of Standards and Technology in Colorado and colleagues from Universities of Colorado, Oxford and Belgrade. The hyperfine structure of their spin-up and spin-down electronic energy states allows them to store extremely stable qubits.

The researchers applied an oscillating force that only acts when the two ions are in a mixture of spin-up and spin-down. The forces on the ions are then unbalanced and the ions oscillate slightly towards and away from each other. This changes the Coulomb forces between the charged ions and, while the force is acting, makes the energy of the mixed states different from that of half the energy of a completely spin-up or spin-down state. This is called a “phase gate” and is the logic equivalent to the controlled NOT operation in conventional electronics.

Meanwhile, Rainer Blatt and colleagues at the University of Innsbruck have created a similar logic gate between two trapped calcium ions. The added advantage of this system is that the ions can be addressed individually by focused laser beams.

In both experiments the ions are “entangled”, which means that a measurement of the internal state of one ion reveals the internal state of the other. The results show that considerable progress has been made in the controlled manipulation of entanglement, which is essential for quantum computing.