Quantum Computing

All the computers used to allow me to create, and for you to read this article use bits to shuffle, manipulate and store information. A bit can be off or on – zero or one. Computers string bits together to represent data and perform operations on that data. Classical machines can deal with just a handful of those bit-strings at a time and though some can crunch through billions of strings very second some problems are so complex that even the latest computer cannot keep pace.

Finding the prime factors of a big number is one example: the difficulty of the problem increases exponentially as the number in question gets bigger. Each tick of Moore’s law, in other words, enables the factoring of only slightly larger numbers. And finding prime factors forms the mathematical backbone of much of the cryptography that protects data as they scoot around the internet, precisely because it is hard.

Quantum bits behave differently due to two counterintuitive quantum phenomena. The first is “superposition”, a state of inherent uncertainty that allows particles to exist in a mixture of states at the same time. For instance, a quantum particle, rather than having a specific location, merely has a certain chance of appearing in any one place.

For computing this mean that a qubit is not a 1 or 0 but exits as a mixture of both.

The second quantum phenomenon, “entanglement”, binds together the destiny of a quantity of different particles, so that what happens to one of them will immediately affect the others. That allows a quantum computer to manipulate all of its qubits at the same time.

This all sounds like a mess so why would anyone bother? A quantum machine can represent and process vast amounts of data at once. A 300-qubit machine could represent up to 2 to the power 300 different strings at the same time – a number roughly equivalent to the number of atoms in the visible universe.

OK, that sounds great when can I buy one? Building qubits is hard – superposition’s are delicate and the record for maintaining them without silicone in 2012 was 2 seconds. By last year that had risen to 6 hours.

Great progress, so I can buy one now then? No one is sure what to build qubits out of. Different groups are trying:

• Tickling tightly confined ions with laser beams
• Using quasi particles call anyons.
• Represent qubits as currents flowing through superconducting wires – This is an attractive option as the technology to build this is already familiar to the industry. Last year John Martinis (Google) published a paper describing a system of nine superconducting qubits in which four could be examined without collapsing the other five, allowing the researchers to check for, and correct, mistakes.

OK, so can I buy Google’s quantum computer? Maybe soon but the next problem is how to get the right answer. For a quantum algorithm to work the machine must be manipulated in such a way that the probability of obtaining the right answer is continually reinforced while the chances of getting a wrong answer are suppressed. One of the first useful algorithms for this purpose was published in 1994 by Peter Shor, a mathematician; it is designed to solve the prime-factorising problem. Dr Shor’s algorithm was one of the crucial advances which persuaded researchers that quantum computers were more than just a theoretical curiosity and since then more such algorithms have been discovered.

So once all that’s sorted what can I do on my quantum computer? Code breaking and AI. An AI face recognition application could be shown thousands of faces and objects and come up with its own rule that efficiently transforms the input into correct identification much faster than current computers.

Simulating quantum mechanics itself, specifically the complicated dance of electrons that is chemistry might be of interest. Being able to simulate the quantum processes accurately would revolutionise all sorts of industrial chemistry – better catalysts, improved engine design, improving the Haber process which produces the bulk of the worlds fertilisers.

Reference:

http://www.economist.com/technology-quarterly/2016-03-12/after-moores-law#section-4