*Originally posted by humy*

**This is still far from making a useful practical quantum computer but:
**

http://physicsworld.com/cws/article/news/2015/mar/09/how-to-make-a-tougher-quantum-computer

it uses "parity" to do quantum error correction allowing 9 quantum bit processing. To make a 'useful' quantum computer, need to do it for a much greater number than just 9 quantum bits while somehow maintaining adequate error correction.

Each qubit is a small circuit consisting of a capacitor and a Josephson junction, and is made from an aluminium film evaporated onto a sapphire substrate.

Sounds expensive! That's really clever. They do the error detection using an observable that commutes with the Hamiltonian, and claim to be able to correct the error. Error correction codes can get quite complex. This is a big step forwards as it means that the machines don't have to be quite as robust. I wonder how many qubits are available for computation.

They say they can cope with errors in two qubits. For an error in one classical bit you can use:

abc def

d = P(a, b);

e = P(a, c)

f = P(b, c)

if a flips then d and e change but f doesn't - so you can tell which bit's flipped. The catch is that if two data bits flip that can't be distinguished from one bit flipping in the error bits. You need as many error correction bits as data bits, and a low probability of one bit flipping, so that the chances of two flipping are very small. They seem to have used a different system, but it leaves me wondering how many data bits they have.

I need to read the full article, which Nature will let me do, one doesn't need a subsciption for this one, but I can't download a copy to my machine.

Bear in mind, Intel's first microprocessor had 4 bits, the 4004. It was a commercial success used in calculators. To do a 16 bit integer add you needed to do four 4 bit adds and cope with the carry. Maybe someone has a use for this thing - well me for one - it's a hobbyists dream.