23 Oct 2012

Researchers edge closer to large quantum computer

In a key step towards creating a working quantum computer, US researchers have developed a method that may allow the quick and reliable transfer of quantum information throughout a computing device

Researchers from Princeton University have fabricated a circuit that uses microwaves to read the quantum state of an electron.

According to lead researcher Jason Petta, the new technique could eventually allow engineers to build quantum computers consisting of millions of quantum bits, or qubits.

To date, quantum researchers have only been able to manipulate small numbers of qubits, not enough for a practical machine. However, this could now change.

Petta's team first trapped a pair of electrons in a quantum dot, made from semiconductor nanowire, and then placed this inside a 1cm long microwave channel.

By passing a stream of microwave photons through the channel, the researchers could analyse the spin-state of the electron pair trapped within the quantum dot. This information serves as the qubit, the computer's basic unit of information.

"We created a cavity with mirrors on both ends, [1cm long]... to reflect microwave radiation," explains Petta. "We send microwaves in one end, and we look at the microwaves as they come out the other end. The microwaves are affected by the spin states of the electrons in the cavity, and we can read that change."

The next step is to increase the reliability of the setup for a single electron pair, and then the team will scale-up the operation, adding more quantum dots to create more qubits.

As Petta highlights, there appear to be no insurmountable problems at this point but, as with any system, increasing complexity could lead to unforeseen difficulties.

"The methods we are using here are scalable, and we would like to use them in a larger system," he says. "But to make use of the scaling, it needs to work a little better. The first step is to make better mirrors for the microwave cavity."

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