Study: New step towards silicon-based quantum computer

Holger Büch and Professor Michelle Simmons
Tuesday, 18 June, 2013

UNSW researchers have proposed a new way to distinguish between quantum bits that are placed only a few nanometres apart in a silicon chip.

This takes them a step closer to the construction of a large-scale quantum computer.

Quantum bits, or qubits, are the basic building blocks of quantum computers - ultra-powerful devices that will offer enormous advantages for solving complex problems.

Professor Michelle Simmons, leader of the research team, said a qubit based on the spin of an individual electron bound to a phosphorus atom within a silicon chip is one of the most promising systems for building a practical quantum computer, due to silicon’s widespread use in the microelectronics industry.

“However, to be able to couple electron-spins on single atom qubits,  the qubits need to be placed with atomic precision, within just a few tens of nanometres of each other,”  she says.

“This poses a technical problem in how to make them, and an operational problem in how to control them independently when they are so close together.”

The UNSW team, in collaboration with theorists at Sandia National Laboratories in New Mexico, has found a solution to both these problems.  Their study is published in the journal Nature Communications.

In a significant feat of atomic engineering, they were able to read-out the spins of individual electrons on a cluster of phosphorus atoms that had been placed precisely in silicon. They also propose a new method for distinguishing between neighbouring qubits that are only a few nanometres apart.

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

“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.

The UNSW team is part of the Australian Centre of Excellence for Quantum Computation and Communication Technology, a world-leading research centre headquartered in Sydney, Australia.

“This is an elegant and satisfying piece of work,” says Professor Simmons, centre director and Mr Büch’s PhD supervisor.

“This first demonstration that we can maintain long spin lifetimes of electrons on multi-donor systems is very powerful. It offers a new method for addressing individual qubits, putting us one step closer to realising a practical, large-scale quantum computer.”

To make the tiny device, the researchers deposited a layer of hydrogen on a silicon wafer and used a scanning tunnelling microscope to create a pattern on the surface in an ultra-high vacuum.

This was then exposed to phosphine gas and annealed at 350 degrees so phosphorus atoms became incorporated precisely into the silicon. The device was then buried in another layer of silicon.

In a quantum computer information is stored in the spin, or magnetic orientation, of an electron. This spin can not only be in the two “classical” states – up and down – but also in a combination of both states at the same time, allowing exponentially larger amounts of information to be stored and processed in parallel.

Media Contacts:

Professor Michelle Simmons:, + 61 (2) 9385 6313

UNSW Science media: Deborah Smith, +61 (2) 9385 7307, + 61 (0) 478 492 060,

Timeline of the development of silicon quantum computing in Australia

1998:     Dr Bruce Kane, a research fellow at UNSW (now at the US Laboratory for Physical Sciences in Maryland), publishes a paper in Nature describing how you could build a quantum computer

He suggests encoding information in either the “spin” of the electron or the phosphorus  nucleus in a silicon transistor. The paper attracts great interest as silicon is industrially relevant and the electron and nuclear spins have very long coherence times, hence low error rates.

Critiquing the article, a leading researcher at IBM confirms that Kane’s proposal provides a vision of how to make large-scale quantum computation a real possibility, but that “the fundamental and engineering obstacles to implementing the scheme are vast”.

2000:     A group of researchers establish the ARC Special Research Centre for Quantum Computer Technology, headquartered at UNSW with Professor Bob Clark as director, to develop the technology to build both a silicon-based quantum computer and an optical quantum computer. Two parallel approaches in single atom engineering are pursued in the silicon program: one using ion implantation and the other using scanning probe microscopy. Over the next decade these two programs publish hundreds of papers demonstrating the ability to engineer single atoms in silicon, by developing, and patenting complete new fabrication strategies.

2011       New centre director, Professor Michelle Simmons, heads a successful bid for the ARC Centre of Excellence in Quantum Computation and Communication Technology, expanding the centre’s research to include quantum communication to become one of the largest combined and focused efforts in quantum information in the world. Over the past 2.5 years the centre has published about 25 articles in Science or the Nature suite of journals. This includes the ability to control and read-out the electron and nuclear spin states of phosphorus atoms in silicon.

Timeline for the precision single-atom qubit built by scanning probe microscopy

2003:     PhD student Stephen Schofield in Professor Simmons’ team demonstrates that single phosphorus (dopant) atoms can be positioned into silicon with atomic precision.

[Physical Review Letters 91, 136104 (2003)]

2004:     Theoretical chemist Dr Oliver Warschkow and his team develop a complete chemical understanding of how this process occurs. [Physical Review Letters 93, 226102 (2004)]

2004:     Following the development of a radical new strategy to build atomic-scale devices using scanning probe microscopy and single crystal growth, PhD student Frank Ruess develops the world’s first atomic precision device. [Nano Letters 4, 1969 (2004)]

2009:     A unique single-crystal transistor architecture is developed by research fellow Dr Andreas Fuhrer, where all the active device elements are fabricated on one atomic plane, demonstrating that single-crystal devices are inherently stable [Nano Letters 9, 707 (2009)].

2010:     The world’s smallest transistor is created by PhD student Martin Füchsle and is featured in the Guinness Book of Records. [Nature Nanotechnology 5, 502 (2010)]

2012:     PhD student Bent Weber creates the narrowest conducting wires in silicon, just one atom tall and four atoms wide, with the same current-carrying capacity as copper, showing that Ohm’s Law continues to the atomic scale. [Science 335, 6064 (2012)]

2012:     A team of researchers from UNSW, Sydney and Purdue demonstrate of the world’s first single-atom transistor, where a single dopant atom has been placed in the device with atomic precision, achieving a technological milestone ten years ahead of industry predictions. [Nature Nanotechnology 7, 242 (2012)]