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Physicists Introduce Quantum Information to the Classical Computing World

A pair of Australian teams mimic a transistor and beat the world record for keeping a qubit intact.
This is an artist impression of an electron wave function (blue), confined in a crystal of nuclear-spin-free 28-silicon atoms (black), controlled by a nanofabricated metal gate (silver). The spin of the electron encodes a long-lived, high fidelity quantum bit. Image: Dr Stephanie Simmons, UNSW Australia.

A key feature in the historical development of computing technology is the idea of backwards compatibility. The future should build off the past, but stay connected to it; old code, for example, should be able to run on new machines. While this might seem antithetical to our present world of forced iPhone upgrades and Windows 8, as an engineering concept the idea persists, however imperfectly.

In this light, quantum computing becomes all the more daunting. The shift to multicore processors was jarring enough to the idea of backwards compatibility, but quantum computing involves a technological rewrite at the most basic level of, "What is information?" Is it a 1 or 0, a yes or no? Hardly. A quantum unit of information, the qubit, is instead not just a 1 and a 0, but also every possible state in between.

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How do we bridge a gap like that? A pair of research teams based at UNSW Australia has inched just a bit closer to an answer, developing two parallel pathways for quantum information that are able to exist on regular old silicon. One of the teams, led by electrical engineering professor Andrew Dzurak, has even succeed at building the quantum equivalent of classical computing's most fundamental component, the MOSFET transistor.

"It is really amazing that we can make such an accurate qubit using pretty much the same devices as we have in our laptops and phones," remarked post-doctoral researcher Menno Veldhorst in a statement.

Quantum computing rests on the notion of superposition. This is when a particle or system of particles manages to occupy every possible state (or configuration) at one time. If an electron can have a certain spin, either up or down, its superposition will be a combination of all possible states between up and down.

Imagine flipping a coin that never lands. Instead, it just hangs there in the air spinning, always somewhere in between heads and tails. It is a very real coin—we can see it and even touch it—but it behaves as a summation of possibilities, as a qubit. As such, it's able to contain immense amounts of information, at least compared to the simple heads or tails of classical computing, e.g. the bit.

The always-spinning, always-suspended coin represents a whole list of challenges, however. This superposition is fragile; think of how you'd handle a suspended spinning coin, with the slightest wrong touch threatening to break the spin and force the coin to choose a final state of heads or tails. This is the precariousness of quantum information: If a system is disturbed, either by forcing a measurement on it or just some natural perturbance, this superposition of possibilities and information collapses.

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Quantum computing researchers are slowly getting a handle on this fragility, handling more and more qubits for longer and longer periods of time. Information is always going to be lost or distorted, classical or quantum, and so the challenge is to build a device that can not just hold quantum information, but do so with a margin of error small enough that can be corrected. In quantum computing, error correction schemes can handle just a single percentage point of error.

Both Australian teams were able to achieve this. In the case of Dzurak's group, this was done using a confinement scheme based on quantum dots, super-tiny cages that are able to imprison pairs of particles in a sort of "hole." In this case, that hole is engineered within super-thin sheets of silicon. A qubit is able to rest in this hole with a minimal threat of being disturbed, achieving a "control fidelity" of 99.6 percent.

Developing qubits in semiconductors would be a promising route to realize scalable quantum information devices.

The second group took a different approach, using the "natural" atom phosphorus as an information vehicle. "The phosphorus atom contains in fact two qubits: the electron, and the nucleus," said the second team's leader, Andrea Morello. "With the nucleus in particular, we have achieved accuracy close to 99.99 percent. That means only one error for every 10,000 quantum operations."

"The spin of an electron or a nucleus in a semiconductor naturally implements the unit of quantum information—the qubit," the second team explained in their study. "In addition, because semiconductors are currently used in the electronics industry, developing qubits in semiconductors would be a promising route to realize scalable quantum information devices."

What's more, the Morello team achieved a world record "coherence time" for maintaining a qubit: 30 seconds. "Half a minute is an eternity in the quantum world," Morello said. "Preserving a 'quantum superposition' for such a long time, and inside what is basically a modified version of a normal transistor, is something that almost nobody believed possible until today."