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SCIENCE

By Colin Jeffrey

March 22, 2015

3 Pictures

First-ever quantum devicethat detects and corrects itsown errors

Physicists working at UC Berkeley claim to have created breakthrough

quantum circuitry that checks and corrects its own errors (Photo: Julian

Kelly/UC Berkeley)

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Before the dream of quantum computing is realized, a number of inherent

problems must first be solved. One of these is the ability to maintain a

stable memory system that overcomes the intrinsic instability of the basic

unit of information in quantum computing – the quantum bit or "qubit". To

address this problem, Physicists working at the University of California

Berkeley (UC Berkeley) claim to have created breakthrough circuitry that

continuously self-checks for inaccuracies to consistently maintain the error-

free status of the quantum memory.

Vulnerability to environmentally-induced error – such as cosmic ray events

or simply an unknown collapse of quantum coherence, for example –

means that the information contained in a qubit is easily lost. And because

of the nature of of quantum entanglement required to encode the qubit in

the first place, any attempt to replicate the information will also immediately

destabilize it.

"One of the biggest challenges in quantum computing is that qubits are

inherently faulty," said Julian Kelly, graduate student researcher at the John

Martini physics lab at UC Berkeley. "So if you store some information in

them, they’ll forget it." Rather than attempt to maintain the integrity of a

qubit by, say, trapping it in an isotope of silicon, the UC Berkeley team has

instead opted for an algorithm-based approach.

Unlike conventional computers, quantum computers do not use binary data

storage (ones and zeroes), where a bit can be one of two states. Instead,

quantum computers use what is known as "superpositioning," where the

data contained in a qubit can also be either 0 or 1 (or even both

simultaneously if superdense coding is used), and may exist at any and all

possible positions simultaneously, and in various dimensions.

However, whilst this property of qubits is distinctly advantageous in terms

of computational power, it is also this trait which renders qubits prone to

"flipping" (randomly changing state), especially when in unstable

environments, and thus difficult to work with.

"It’s hard to process information if it disappears," said Kelly.

To help solve this problem, the new error detection and correction process

uses a system where several qubits are made to operate together to

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preserve the information. To achieve this, information is simultaneously

stored across a number of qubits.

"… the idea is that we build this system of nine qubits, which can then look

for errors," said Kelly. "Qubits in the grid are responsible for safeguarding

the information contained in their neighbors in a repetitive error detection

and correction system that can protect the appropriate information and

store it longer than any individual qubit can."

This is necessary because qubits exist in a quantum state where you can

either know the position of a particle or you can measure its momentum,

but not both. To do so will result in the decoherence of the qubit to a

random state.

"You can’t measure a quantum state, and expect it to still be quantum," said

UC Berkeley postdoctoral researcher Rami Barends. "The very act of

measurement locks the qubit into a single state and it then loses its

superpositioning power."

To do this, UC Berkeley staff scientist, Austin Fowler, used what is termed a

"surface code" to provide information about errors. Obtained by repeatedly

measuring each qubit after interaction with its nearest neighbor data qubits

on a matrix, changes in the measurement value indicate the presence of

chains of errors in space and time.

In other words, this code utilizes parity information to detect any variation

from the original data. In this case, if the polarization state applied to a set

number of qubits is "even" and these qubits are then transmitted elsewhere

in the system, any change to that polarization will be seen by comparing

that state between the original and transmitted qubits.

This is different to the standard way of checking data in a computer that

involves duplication of the original data to look for errors – an impossible

task in quantum computing, because the qubits must remain unobserved to

maintain their integrity.

"So you pull out just enough information to detect errors, but not enough to

peek under the hood and destroy the quantum-ness," said Kelly.

So far, research has proven that it is capable of negating a "bit-flip" qubit

error, but the team is hoping to next confront other qubit decoherence

problems, such as the complementary "phase-flip," error.

Senior researchers from the Martinis group have now also partnered with

Google to further explore this technology and research in quantum

computing applications.

The team's paper appears in the journal Nature.

Source: UC Berkeley

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About the Author

Colin discovered technology at an early age, pulling apart

clocks, radios, and the family TV. Despite his father's

remonstrations that he never put anything back together,

Colin went on to become an electronics engineer. Later he

decided to get a degree in anthropology, and used that to

do all manner of interesting things masquerading as work.

Even later he took up sculpting, moved to the coast, and never learned to

surf.   All articles by Colin Jeffrey

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