Binary makes way for five-way quantum computing

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Binary makes way for five-way quantum computing

Paul Marks

Developers of quantum computers should give up on the familiar 1s-and-0s binary system used in conventional computers. By switching to a novel five-state system, they will find it easier to build the staggeringly powerful machines.

So claim Matthew Neeley and colleagues at the University of California, Santa Barbara (UCSB).

So far, the development of quantum computers has followed the traditional binary computing model. This encodes all information using components that can be in two states, either 1 or 0.

But other possibilities exist, Neeley explains. "We could use a 'trinary' system with three digits - 0, 1 and 2 - and then the fundamental units would be trinary digits, or trits, that would essentially be three-position switches." A single "trit" would contain more information a conventional "bit".

Neeley's team have now built a quantum computer whose building blocks have five basic states.

Five-state system

Until now, quantum computers' basic components have been binary quantum bits - qubits - which encode two states in the quantum spin of atoms, electrons or photons. The ability of such particles to defy everyday logic, and exist in multiple quantum states at once, should one day enable quantum computers to perform vast numbers of calculations simultaneously.

Neeley's group used a superconducting aluminium and silicon circuit on a sapphire wafer to make five-state qubits, or "qudits", that operate at 0.025 kelvin.

"There's more information stored in a qudit than a qubit, so a given computation can be done with fewer qudits," Neeley told New Scientist.

By firing microwave photons of five different frequencies into the circuit, they were able to encourage it to jump between five discrete energy levels. "We also developed a quantum measuring technique that can distinguish between all of these levels," says Neeley.

Simultaneous existence

Because, in probabilistic terms, the qudit's five quantum states are able to exist simultaneously, the team had a working qudit on their hands.

One qudit alone is of little use, however.

Jonathan Home at the US National Institute of Standards and Technology in Boulder, Colorado, says Neeley's team needs to extend its basic system in such a way that two or more qudits can transport information between them, which would allow more complex computational operations to be undertaken.

"Designing the sort of system where two qudits interact, but still retain the interesting properties of a five-level system, will be a major challenge," Home says.

Quantum spies

The potential power of quantum computers means has attracted the interest of the US Intelligence Advanced Research Projects Agency (IARPA), which hopes to use them to break codes.

Home's team has received funding from the agency to work on a room-temperature quantum computer that allows binary qubits to interact and swap information.

Their latest results show that magnesium ions can be used to stop the qubits destabilising one another by transferring heat as well as their quantum states.

The trick, reported in this week's Science (DOI: 10.1126/science.1177077) is to use serried ranks of trapped beryllium ions as the qubits, while using neighbouring magnesium ions to absorb any heat. The heat would normally destroy quantum information as it is transported between them.

"This will pave the way to large-scale quantum computing, because it addresses the major task: information transport," says Home.

Journal reference: Science, DOI: 10.1126/science.1173440


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