Quantum computers exploit the weird laws of quantum mechanics to perform some calculations much faster than conventional computers – at least in principle. The main challenge facing physicists trying to build quantum computers is how to preserve fragile quantum bits (qubits) of information, which tend to deteriorate rapidly in real-world devices.

One approach is to use the spin of the electron – which can point up or down – as a qubit. Spin qubits have been made from tiny pieces of semiconductor called quantum dots, and quantum-logic devices have been made by coupling these qubits together. Unfortunately, the spin states in these devices rapidly deteriorate – or "decohere" – by interacting with nuclear spins in the compound-semiconductor materials normally used to make quantum dots.

Silicon spins

This source of decoherence can be greatly reduced by making the dots from silicon, the most common isotope of which (silicon-28) has zero nuclear spin. The new CNOT logic gate, which has been created by Andrew Dzurak, Menno Veldhorst and colleagues at the University of New South Wales and Keio University, has been made by coupling two silicon spin qubits for the first time.

The two quantum dots were made by placing an array of electrodes on top of a piece of silicon-28. By applying voltages to some of the electrodes, two electrons are trapped within the silicon, separated by about 100 nm. These electron spin states are set by generating a microwave pulse using one of the electrodes as an antenna – a technique known as electron spin resonance (ESR). The states of the spin qubits can be set individually by using the electrodes to apply an electric field to one of the spins, which changes how that spin responds to the microwave signal. The values of the qubits are also read out using ESR.

The spins are coupled via the exchange interaction, which is a purely quantum-mechanical effect that can be tuned to cause the spins to point in the same direction, or in opposite directions. This tuning is done by adjusting the voltages on some of the electrodes.

Improvements needed

The team verified that it had created a CNOT gate by first initializing the spins in a specific configuration, for example both spin down. A series of microwave pulses and voltages was then applied to the qubits to create a CNOT gate. When the team read out the values of the qubits, they were found to be in line with the expected output from a CNOT gate.

However, the researchers say that they were not able to show that the two qubits were quantum-mechanically entangled during the CNOT process, which they say was the result of errors in the read-out process. Entanglement is required for the operation of a quantum-logic device, and the team is now working towards improving the read-out process to confirm that the qubits are indeed entangled.

"All the physical building blocks for a silicon quantum computer have now been successfully constructed," says Veldhorst.

Dzurak adds that the team has "patented a design for a full-scale quantum computer chip that would allow for millions of our qubits, all doing the types of calculations that we've just experimentally demonstrated". He says the team is also looking for an industrial partner to manufacture a full-scale quantum-processor chip.

The CNOT gate is described in Nature.