Superconducting qubit

A qubit made from a small superconducting circuit on a chip, operated about a hundredth of a degree above absolute zero.

• Aluminium circuits with a Josephson junction, a nanometre-thin insulating barrier, behave like artificial atoms with two usable energy levels.
• They sit in dilution refrigerators at around 10 millikelvin and are driven by microwave pulses; gates take tens of nanoseconds.
• Fast gates and chip-factory manufacturing are the strengths; Google and IBM lead this approach.
• Each qubit needs several control lines into the fridge, microscopic material defects cause errors, and states last roughly 0.1–1 millisecond.