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Quantum computer address bus just nanometres wide

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The University of New South Wales, working with Sandia National Laboratories in New Mexico, is celebrating what it hopes will be another step towards large-scale quantum computing: a technique that can address single electron qubits separated by mere nanometres.

Quantum bits - qubits - are the quantum-physics counterpart to the binary bits that make up data stored in today's digital computers - except that whereas your traditional binary bit is either 1 or 0, a qubit can be 1, 0 or what's known as a superposition of both. A quantum computer should be able to use this property to process huge numbers of calculations in parallel.

But at some point classical binary data will need to be written to a line of qubits, which is where today's research comes in.

“It is a daunting challenge to rotate the spin of each qubit individually,” said Holger Büch, lead author of the new study.

“If each electron spin-qubit is hosted by a single phosphorus atom, every time you try to rotate one qubit, all the neighbouring qubits will rotate at the same time – and quantum computation will not work. But if each electron is hosted by a different number of phosphorus atoms, then the qubits will respond to different electromagnetic fields – and each qubit can be distinguished from the others around it,” he says.

So there's the engineering challenge: create a material in which you've controlled the number of individual phosphorus atoms.

Professor Michelle Simmons, Holger Büch's supervisor, described the engineering involved to The Register.

The UNSW trick is in its use of a scanning tunnelling electron microscope, which can image single atoms on a surface – in this case, a surface of silicon, with a layer of hydrogen on top.

The researchers applied a voltage to the tip of the microscope, which removes the hydrogen under the tip. The surface is then bathed in phosphene gas, which deposits phosphorus atoms on the exposed silicon surface. When the surface is heated, the phosphorus atoms will displace the silicon.

Finally, thin layers of silicon are grown over the top, one at a time.

As a result, the technique is able to place very precise numbers of phosphorus atoms on the surface to act as qubits.

To be more accurate, it's the outermost electron in the phosphorus atom's shell that acts as the qubit (more on this in a minute) – and to sense something so small, one more thing is needed. Single electron transistors – a specialty of UNSW – are fabricated onto the structure to sense the spin of the phosphorus electrons, providing the reading mechanism.

In case you hadn't noticed, we haven't yet reached a point at which the atoms can act as a quantum computer – we're still back at “manipulating single electrons” like IBM demonstrated with its “boy and his atom” video. The spin is controlled with magnetic fields, with the whole system cryogenically cooled.

The crucial non-classical “spookiness” arrives when entanglement is created between spin states on different atoms. They need to be very close together to enable entanglement, but the system needs to resolve on a sufficiently fine scale to address single atoms.

Professor Simmons told The Register the research team is considering the work a double breakthrough: in addition to the fabrication success, she said, the material produces qubits with a long lifetime. Once the electron is put into its spin-up state, that can last for seconds.

That's why researchers favour silicon in solid-state quantum computation, she said: it can produce long-lived states. With the lifetime lasting in the seconds, “you can do a large number of operations without losing the state of the electron.”

The UNSW group is part of the Australian Centre of Excellence for Quantum Computation and Communication Technology. ®

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