Superconducting circuits

Quantum devices often look very different from their classical counterparts. There is one exception – the central piece of some of the most advanced quantum computers is still a chip. It is not made from silicon but from materials that are superconducting. These are not unusual – Aluminium is an important one of them, a more rare metal, Niobium, is another one that is commonly used. Superconductivity is the property of some materials to conduct electricity and shield magnetic fields perfectly if they are cooled down to low temperatures. It has been known since 1911 and understood since 1957 – at least in these simple metals.

Why are superconductor good candidates for quantum technologies? Because superconductivity is itself a macroscopic quantum phenomenon: The carriers of electric charge in a superconductor first pair up and then condense into a single quantum state as if they were a large atom. Using small contacts between superconductors, Josephson junctions, researchers can engineer a variety of quantum circuits at their will and run quantum algorithms on them.

These circuits have gone a long way: First proposed in the 1980s as a way to address basic quantum physics such as the Schrödinger cat paradox, they were first used as qubits in 1999 and improved their properties – for example by reducing decoherence by five orders of magnitude – over time. They are now a core technology for quantum computers.

Fun fact: these are not the only applications of superconducting circuits with Josephson junctions. They are used for sensors, addressing a variety of challenges such as the measurement of electric currents in the human brain or detecting fatigue in steels. From these activities, the ways to make the very low temperatures needed – supercondcuting quantum computers operate one hundereth of a degree above absolute zero – has been mastered better and better.