Scientists read and write data using silicon qubit

The Australian-led breakthrough, using a single phosphorus atom embedded in a silicon chip, is a big step forward toward creating a quantum computer.

An international group of scientists, headed up by researchers at the University of New South Wales in Australia, has created the first-ever working quantum bit based on a silicon atom.

The work, which was published on Friday in the journal Nature, outlines how the team managed to read and write information using the spin of an electron bound to a single phosphorus atom embedded in a silicon chip.

The researchers refined their technique until they were able to implant a single phosphorous atom into the exact location required on their nanoscale device. The phosphorus atom sits next to a silicon transistor that is so small, electrons have to flow through it one after the other.

The transistor's circuitry is designed so that that current will only flow if the electron from the phosphorus atom moves to an island at the centre of the transistor, which can only happen if the electron has a particular spin state, or magnetic moment. This means that the flow of current through the transistor indicates a spin-up state, and no current flow indicates spin-down.

"For the first time, we have demonstrated the ability to represent and manipulate data on the spin to form a quantum bit, or 'qubit', the basic unit of data for a quantum computer," Scientia Professor Andrew Dzurak said in a statement. "This really is the key advance towards realising a silicon quantum computer based on single atoms."

Quantum computers — first proposed by Richard Feynman in 1982 — have the potential to tackle problems that would stymie even the most powerful of conventional computers. Instead of performing calculations sequentially, they are able to make millions of computations in parallel. In theory, this makes them ideally suited for code-cracking, or simulating hugely complex molecules, as in pharmaceutical or biological research. But the reality is that building one is extremely difficult.

The basic unit of a classical computer is the bit, which exists either as a zero or a one. The equivalent in a quantum computer is a qubit. Depending on your approach, this could be an electron, or a photon, for example. In this case, it is an electron, and the electron's intrinsic magnetic moment, or spin, is the one or the zero. The clever bit is that it can be both at the same time.

But it gets better. Collect multiple qubits and persuade them into a so-called entangled state, and you can have them be in many states simultaneously. This means your computational power grows exponentially, as the University of New South Wales (UNSW) researchers explained in their announcement: Using two qubits, the operation could be performed using four values, for three qubits on eight values, and so on.

As you add more qubits, the capacity of the computers to perform operations increases exponentially. In fact, with just 300 qubits it is possible to store as many different numbers as there are atoms in the universe.

This breakthrough doesn't get us all the way there — it is a single qubit, not entangled with anything at all. But the researchers report being able to set the spin state of their electron and the read it out again afterwards. This level of control the researchers have been able to exert over the electron is a big step forward.