Although quantum mechanics was invented to describe atoms more than a century ago, a physical object does not have to be small to behave quantum mechanically, and superconductors are perhaps the best know example of this. Superconducting devices used as qubits for storage and processing of quantum information are made of millions of atoms, and most of the circuitry is so large that it is almost visible to the naked eye. Consequently, these devices are relatively easy to manufacture, and we can use centimeter radio waves, i.e. microwaves very similar to those used in cooking and mobile telephones, to programme and control the evolution of these devices.
Our quantum circuits will be cast on-site in a ‘quantum foundry’, by depositing thin layers of aluminum on an insulating substrate of silicon or sapphire. At very low temperature aluminum is superconducting, and by inserting Josephson junctions in the circuit a non-linear analog of a conventional electronic LC oscillator is obtained. This circuit acts like an artificial atom, whose energy levels (wave-functions) can be used to encode quantum information. Any two levels can serve as a qubit, and because the potential is not harmonic (neither is the Coulomb potential in an atom) these levels can be individually addressed using microwaves (‘talking to qubits by radio’). Even at the extremely low temperature inside our dilution refrigerators (10 milli-Kelvin) these circuits can only remain in a coherent superconducting state for some microseconds, before unavoidable interactions with the environment destroys the coherence. However, since it only takes a few nanoseconds to manipulate the state of the quantum oscillator this is sufficient time to execute hundreds of controlled quantum operations.