Sensing single spins in silicon

An electron’s spin can exist in either an up or down state, relative to an external magnetic field. Such states are analogous to the binary 0 and 1 states, familiar from classical computing. As an electron is a quantum mechanical object, it can be regarded as a potential qubit; the fundamental unit of quantum computing. The ability to read out the state of a qubit, in a material such as silicon, thus represents a major stride forward in the field.

 

Readout from single spins has previously been achieved using optical techniques, but it is only recently that electronic methods of achieving the same goal have become feasible. Initial electronic techniques obtained the spin state by measuring the change in local charge associated with the movement of the electron. As the electron’s movement is dependant on its spin state, the change in charge directly correlates to its initial state.

 

Now, Morello and coworkers have combined this concept with a single-electron transistor (SET). In their device, only an electron with a specific spin state can move from a phosphorous atom imbedded in the silicon onto an island at the centre of the SET. A current can only flow through the SET when the island possesses specific values of charge. Thus the spin state can be deduced.

 

By using this novel method to read the spin state, the result was obtained quickly, with a fidelity of over 90%.

 

The concept behind the device dates back to 1998 from former UNSW researcher, Bruce Kane. Prof. Dzurak commented “After a decade of work trying to build this type of device, this is a very special moment”. The team is now working on a way to manipulate the electron’s spin state, such that they can both read and write to the qubit. According to Dr Morello, the final goal is to “create a collection of 2-bit logic gates linked by atom chains”.