Photons, atoms or trapped ions can be used as qubits but it should be easier to build working devices using solid-state qubits. Quantum computing works on two basic quantum mechanical principles. The first is the superposition of states, which is a one-particle phenomenon. The second is entanglement, which involves two or more particles.

The spin of a particle can point in two opposite directions, “up” and “down”, but the particle can also exist in a superposition of these states. This superposition also holds true for two-particle states including entangled states. When two particles are entangled they behave as one, regardless of how far apart they are.

For quantum computing to work, however, these entangled states must be made to interact in a controlled manner.

Tsai and co-workers used micron-sized “boxes” of superconducting material that were connected to a Josephson junction - a type of superconducting “reservoir” - via a capacitor. A Cooper pair of electrons can tunnel from the junction onto the box. The box is the qubit, which can exist in two states: one state has an excess of Cooper pairs while the other has no excess Cooper pairs. The qubits are made to interact using the capacitor, which leads to a mixing of two-particle states and thus entanglement of the qubit pair.

Although the team has not yet measured a specific entangled state, they have shown that the qubit pair is strongly entangled. “This result shows that it is indeed possible to construct a quantum logic gate using such a solid-state device,” Tsai told *PhysicsWeb*. “A quantum computer could be made using such gates as the basic units.”

The superconducting flux qubit developed by the Delft-NEC team, on the other hand, comprises three Josephson junctions in a loop. The two quantum states in this system are macroscopic currents consisting of billions of Cooper pairs travelling around the loop in opposite directions. The qubit can undergo hundreds of oscillations between these two states, and can be read with a superconducting quantum interference device.