Researchers have designed and fabricated a quantum processor capable of factoring a composite number – 15 – into its constituent prime factors, 3 and 5.

According to a press release, this “represents a milestone on the road map to building a quantum computer capable of factoring much larger numbers, with significant implications for cryptography and cybersecurity”. The results are published in advance online access at *Nature Physics*.

Erik Lucero, lead author, is quoted saying:

Fifteen is a small number, but what’s important is we’ve shown that we can run a version of Peter Shor’s prime factoring algorithm on a solid state quantum processor. This is really exciting and has never been done before.

Some information is given about the process:

The quantum processor was implemented using a quantum circuit composed of four superconducting phase qubits — the quantum equivalents of transistors — and five microwave resonators. The complexity of operating these nine quantum elements required building a control system that allows for precise operation and a significant degree of automation — a prototype that will facilitate scaling up to larger and more complex circuits.

And the result? Lucero again: “after repeating the experiment 150,000 times, we showed that our quantum processor got the right answer just under half the time”. But “the best we can expect from Shor’s algorithm is to get the right answer exactly 50 percent of the time, so our results were essentially what we’d expect theoretically.”

The next step is to scale up: “to increase the quantum coherence times and go from nine quantum elements to hundreds, then thousands, and on to millions”.

**Source**: UCSB researchers demonstrate that 15=3×5 about half of the time.

**Paper**: Computing prime factors with a Josephson phase qubit quantum processor. (*£22 to the unwashed. – CP*)