Cold Enough To Super Conduct
The exotic material in D-Wave’s quantum chip is niobium; cool it enough and it becomes a superconductor. When ordinary metal conducts electricity, the electrons carrying the electric current collide with the imperfections in the metal and you get resistance. When you cool superconducting metal like niobium, the metal’s electrons form Cooper pairs where the motion of one electron is matched by an equal and opposite motion of the paired electron, which stops the electrons hitting the imperfections and generating resistance, which means the electrons flow freely without you needing to pump in extra current. When the Cooper pairs enter the Josephson junctions in the chip – made up of two segments of superconducting niobium linked by a weak insulating barrier – they break up, creating electron-like quasi-particles that can tunnel through the insulator in the junction, effectively conducting the current through the junction.
The niobium is arranged in rings through which the current can flow clockwise, anti-clockwise or in a mixture of both directions – corresponding, according to Rose, to the 0, 1 or superposition of the two values in the quantum bit of information (qubit) that quantum computing is based on. “The chip is a series of metal traces on a silicon substrate; the substrate is the same as you’d use for any semiconductor process but on top are layers of metal interrupted by insulators. This is an entirely metal based magnetic thing where all the information is stored in the direction of the current flows around the metal loop and interruptions.”
The direction of the current converts into a value for the qubit based on whether that qubit has a bias towards one direction (0 or 1), whether neighbouring qubits are running the in same or opposite directions and the energy barrier between the different qubit states. The current chip, Leda, has 28 rings, giving 28 qubits, but they’re not all interconnected to each other, only to a number of ‘neighbours’. The Cooper pair in the niobium are technically bosons so they all exist in the same quantum state, Rose claims, which gives the entire superconductor quantum properties even without interconnecting every qubit. Reducing the number of interconnections simplifies manufacturing and has enabled D-wave to go from 2 qubits in 2002 to 16 in 2007, 28 today – and 512 and 1024 over the next year, if things go well.