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InAs quantum dots offer a new route to large-scale quantum computing

Oct 25, 2012
Indium arsenide QDs offer a better alternative to gallium arsenide and may allow quantum researchers to manipulate a large number of qubits, enough for a practical machine
In a step towards creating a working quantum computer, Princeton researchers have developed a method that may allow the quick and reliable transfer of quantum information throughout a computing device.

The finding, by a team led by Princeton physicist Jason Petta, could eventually allow engineers to build quantum computers consisting of millions of quantum bits, or qubits. So far, quantum researchers have only been able to manipulate small numbers of qubits, not enough for a practical machine.



Jason Petta

"The whole game at this point in quantum computing is trying to build a larger system," says Andrew Houck, an assistant professor of electrical engineering who is part of the research team.



Andrew Houck

To make the transfer, Petta's team used a stream of microwave photons to analyse a pair of electrons trapped in a tiny cage called a quantum dot (QD). The "spin state" of the electrons - information about how they are spinning - serves as the qubit, a basic unit of information. The microwave stream allows the scientists to read that information.

The researchers used InAs for their QDs and developed an architecture allowing them to achieve a charge-cavity coupling rate of about 30 megahertz. This, they say, is consistent with coupling rates obtained in GaAs quantum dots.

"We create a cavity with mirrors on both ends – but they don't reflect visible light, they reflect microwave radiation," Petta explains. "Then 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 distances involved are very small with the entire apparatus operating over a little more than a centimetre. But on the subatomic scale, they are vast. It is like coordinating the motion of a top spinning on the moon with another on the surface of the earth.

"It's the most amazing thing," says Jake Taylor, a physicist at the National Institute of Standards and Technology and the Joint Quantum Institute at the University of Maryland, who worked on the project with the Princeton team. "You have a single electron almost completely changing the properties of an inch-long electrical system."



Hybrid quantum dot-superconducting resonator device. (a) Circuit schematic and micrograph of the hybrid device design. Scanning electron micrograph (b) and cross-sectional schematic view (c) of the InAs nanowire double quantum dot (DQD). The left and right barrier gates (BL and BR), left and right plunger gates (L and R), and middle gate (M) are biased to create a double-well potential within the nanowire. The drain contact of the nanowire, D, is grounded, and the source contact, S, is connected to an antinode of the resonator, oscillating at a voltage VCavity. (Credit Petersson et. al.)

For years, teams of scientists have pursued the idea of using quantum mechanics to build a new machine that would revolutionise computing. The goal is not to build a faster or more powerful computer, but to create one that approaches problems in a completely different way.

Standard computers store information as classical "bits", which can take on a value of either 0 or 1. These bits allow programmers to create the complex instructions that are the basis for modern computing power. Since Alan Turing took the first steps toward creating a computer at Princeton in 1936, engineers have created vastly more powerful and complex machines, but this basic binary system has remained unchanged.

The power of a quantum computer comes from the complex rules of quantum mechanics, which describe the universe of subatomic particles. Quantum mechanics says that an electron can spin in one direction, representing a 1, or in another direction, a 0.

But it can also be in something called "superposition" representing all states between 1 and 0. If scientists and engineers can build a working machine that takes advantage of this, they would open up entirely new fields of computing.

"The point of a quantum computer is not that they can do what a normal computer can do but faster; that's not what they are," says Houck. "The quantum computer would allow us to approach problems differently. It would allow us to solve problems that cannot be solved with a normal computer."

Mathematicians are still working on possible uses for a quantum system, but the machines could allow them to accomplish tasks such as factoring currently unfactorable numbers, breaking codes or predicting the behaviour of molecules.

One challenge facing scientists is that the spins of electrons, or any other quantum particles, are incredibly delicate. Any outside influences, whether a wisp of magnetism or glimpse of light, destabilises the electrons' spins and introduces errors.

Over the years, scientists have developed techniques to observe spin states without disturbing them. (This year's Nobel Prize in physics honoured two scientists who first demonstrated the direct observation of quantum particles.) But analysing small numbers of spins is not enough; millions will be required to make a real quantum processor.

To approach the problem, Petta's team combined techniques from two branches of science: from materials science, they used an InAs quantum dot to hold and analyse electrons' spins; and from optics, they adopted a microwave channel to transfer the spin information from the dot.

To make the quantum dots, the team isolated a pair of electrons on a small section of material called a "semiconductor nanowire." Basically, that means a wire that is so thin that it can hold electrons like soda bubbles in a straw. They then created small "cages" along the wire. The cages are set up so that electrons will settle into a particular cage depending on their energy level.

This is how the team reads the spin state; electrons of similar spin will repel, while those of different spins will attract. So the team manipulates the electrons to a certain energy level and then reads their position. If they are in the same cage, they are spinning differently; if they are in different cages, the spins are the same.

The second step is to place this quantum dot inside the microwave channel. This allows the team to transfer the information about the pair's spin state - the qubit.

Petta said the next step is to increase the reliability of the setup for a single electron pair. After that, the team plans to add more quantum dots to create more qubits. Team members are cautiously optimistic. There appear to be no overwhelming 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," Petta 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."

The research was reported in the journal Nature on Oct. 18

This research was supported by the National Science Foundation, the Alfred P. Sloan Foundation, the Packard Foundation, the Army Research Office, and the Defence Advanced Research Projects Agency Quantum Entanglement Science and Technology Program.

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