Computing with a single electron – background

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Background information about the ‘single electron reader’ invention as published in Nature

Australian engineers and physicists have developed a ‘single electron reader’, one of the key building blocks needed to make a quantum computer. Their work was published online by Nature on Monday 27 September.

Contents

Computing with a single electron
The spooky world of quantum physics
A Christmas  present
Further information
Timeline of the development of silicon quantum computing in Australia
Key research team members and their roles in the paper
Key Funding Bodies/Stakeholders
Photographs and captions

Computing with a single electron


Quantum computers will use the spin, or magnetic orientation, of individual electrons for their calculations. And, because of the quantum nature of electrons, quantum computers could be exponentially faster at certain tasks than traditional computers.

In order to employ electron spin, a quantum computer needs both a way of changing the spin state (writing information) and of measuring that change (reading information). Together these two form a quantum bit or qubit – the equivalent of the bit in a conventional computer.

The Australian invention is a reader. The team, led by engineers from the University of New South Wales (UNSW), is also working on a writer. Then they will combine pairs of these devices to create a 2-bit logic gate – the basic processing unit of a quantum computer.

“Our device detects the spin state of a single electron in a single phosphorus atom implanted in a block of silicon. The spin state of the electron controls the flow of electrons in a nearby single electron transistor circuit,” says the lead author, Dr Andrea Morello from the UNSW School of Electrical Engineering and Telecommunications.

The new device was made at UNSW with components from The University of Melbourne, and with assistance from researchers at Aalto University in Finland. It is significant that it uses silicon—the foundation material of our present computers—rather than light or the esoteric materials and approaches being pursued by other researchers. This opens the way to constructing a simpler quantum computer, scalable and amenable to mass-production.

“Quantum computers won’t speed up all day-to-day computing,” says co-author Prof Andrew Dzurak from UNSW. “But there are three areas where we know they will be much faster: cracking most modern forms of encryption; searching databases; and modelling atomic systems such as biological molecules and drugs.”

The spooky world of quantum physics


The work depends on quantum physics, the field of science that describes what happens at the level of sub-atomic particles. At that tiny scale, materials show famously strange properties. And in the mid-1980s, scientists realised that some of those spooky properties could have a huge positive impact on computing power.

The characteristic that has the most relevance to computing is known as ‘superposition’. It means that any quantum element can be in several different states at the same time—and resolves into one particular state only when someone examines it.

Because this provides a way of carrying out a whole lot of calculations simultaneously, as opposed to one after another, scientists realised more than 20 years ago that, if you could find a way to harness superposition, you could build a computer of enormous power.

In normal computers the basic unit of information, the bit, exists either as a 1 or a 0, power on or power off, but qubits can effectively be in both states at once. This means that a computer operation on a single qubit can give results for both of the values at the same time. By performing an operation on one qubit, the computer has actually performed the operation on two different values simultaneously.

Using two qubits, the operation could be performed using four values, for three qubits on eight values, and so on. As you add qubits, the capacity of computers to perform operations, hence their power, increases exponentially. Checking which one of a thousand of different variants of a drug blocks the action of a virus could be done all at once, for instance.

In recent years, scientists around the world have been trying to work out how to turn that vision into reality. They have been developing completely new systems based on exotic materials or light. But there are great advantages to sticking with silicon. Not only is it cheap and forms the basis of almost all commercial electronics today, but also we already know a great deal about its properties and how to handle it.

The team has built on a body of research that has put Australia at the forefront of the race to construct a working quantum computer. In 1998 Bruce Kane, then at UNSW, outlined in Nature the concept for a silicon-based quantum computer, in which the qubits are defined by single phosphorus atoms in an otherwise ultra-pure silicon chip. The new device brings his vision closer.

The spin of individual electrons has been measured before, but in materials other than silicon, where it is much easier to control and manipulate single charges. Doing so in silicon is significant, however, because silicon is a much better suited material for quantum computers, in addition to being the base for all classical computers.

The device the researchers have developed uses the ‘spin state’ of a single electron as a qubit. The spin state, a quantum property of electrons, can be either up ( ) or down (¯). In the Australian device, this solitary electron is associated with a phosphorus atom implanted in a silicon chip. Next to this phosphorus atom is an electronic circuit so small that electrons have to travel along it one after the other. “We call it a single-electron transistor,” Morello says.

The engineers designed their circuit so that the current would only flow if the electron from the phosphorus atom moved to an ‘island’ at the centre of the transistor. They also set up their device so that the electron could make this jump only if it had a particular spin state. If the electron spin was up, then it could tunnel into the single-electron transistor, but if it was down then it couldn’t move.

This means that the researchers can tell whether the electron’s spin is up or down simply by measuring the current through the transistor. “When the electron is sitting on the phosphorus atom, the transistor is off. If and when the electron tunnels into the transistor, then it turns on.”

This system is a smart modification of a technique other researchers have used. But, until now, the best that scientists have been able to do in silicon is take an average reading of many different electrons.

A Christmas  present


Morello and Dzurak first realised their experiments had been successful on a Saturday morning just before Christmas in 2008.

Morello had left the experiment to run automatically overnight on the Friday night, and went into the lab on that Saturday to check the results. The graphs on the screen in front of him told him the long-awaited triumph had been achieved. He jumped into his car and drove to a friend’s house, where he and Dzurak had both been invited for a barbeque.

“He came in and said ‘Look at this, your Christmas present!’” Dzurak remembers. One glance at the printouts and Dzurak recognized the significance of the result too. “We just jumped up and down and hugged each other immediately.”

Now that the researchers have created the single electron reader, they have moved onto ways to manipulate the spin state – to make a writer. “What we have demonstrated in this Nature paper is the ability to read out when the electron is in some random state. The next thing is to prepare it in exactly the state we want it,” Morello said.

“We then have to demonstrate the controlled coupling of two electron spins, to make a 2-bit logic gate.

“At the same time we will work on creating chains of atoms that can carry electrons from one logic gate to the next. Finally we will put all this together to create a collection of 2-bit logic gates linked by atom chains,” he says.

Curiously the approach taken by the team uses the same properties of silicon and phosphorus that were proposed for the first lasers 50 years ago. At the time, the laser was an esoteric device, today it is in every modern home, car and office.

The research team is part of the Australian Research Council (ARC) Centre of Excellence for Quantum Computer Technology, which is headquartered at UNSW. The team is led by Professor Dzurak and Dr Morello, with Mr Jarryd Pla and Dr Floris Zwanenburg as key supporting experimentalists. The paper’s co-authors include Prof David Jamieson from the University of Melbourne; Dr Bob Clark, Australia’s Chief Defence Scientist, and 10 other researchers from UNSW, The University of Melbourne, and Finland’s Aalto University.

The research was funded by UNSW, the University of Melbourne, and the Australian, US, and NSW governments.

Further information


Timeline of the development of silicon quantum computing in Australia
1994: Peter Shor from Bell Labs (US) shows that a quantum computer would be able to decrypt Public Key Encrypted codes (at the heart of modern secure communications) exponentially faster than today’s supercomputers. This triggers massive interest in quantum computing worldwide.

1994-1998: Various schemes proposed for making a quantum computer using different systems including ion traps, organic molecules, superconductors, semiconductor quantum dots

1998: Dr Bruce Kane, then a postdoctoral researcher at UNSW (now a researcher with the US National Security Agency’s Laboratory for Physical Sciences), publishes a paper in Nature [Nature 393: p 133 (1998)] outlining the concept for a silicon-based quantum computer, in which the qubits are defined by single phosphorus atoms in an otherwise ultra-pure silicon chip.

The quantum information is encoded in either the spin of the electron or the spin of the P nucleus. This is the first such scheme in silicon – the basis of all modern day microprocessors.

Kane’s paper attracts a lot of interest because: (i) Silicon is “industrially relevant”; (ii) Silicon electron “spins” have very long “coherence times” (hence, low error rates). This paper has now generated more than 1700 citations.

Despite the great potential for a silicon quantum computer, the task of building devices at the single atom level is considered almost science fiction back in 1998.

2000: Prof Bob Clark establishes the ARC Special Research Centre for Quantum Computer Technology (CQCT), headquartered at UNSW, to attempt to build a quantum computer.

CQCT has now expanded to become an ARC Centre of Excellence – with more than 150 researchers in Australia, and major collaborations world-wide.

The Centre now generates about 100 research publications a year.

Bob Clark retired from CQCT in October 2008 to take up the role of Australian Chief Defence Scientist.

Prof Andrew Dzurak (UNSW) and Prof David Jamieson (University of Melbourne) were founding chief investigators in CQCT and were charged with using ion implantation to build a silicon quantum computer.

Ion implantation is a technology for injecting “dopant” atoms (such as phosphorus) into modern silicon integrated circuits. As a“mass production” technology ion implantation is very attractive if we are ever to build a commercially viable large-scale silicon quantum computer processor chip.

2000-2009: Various researchers at CQCT contribute to ground-breaking single atom nanotechnologies – generating hundreds of papers.

Two main approaches:

  • “Top-down” approach – using ion-implantation [led by Andrew Dzurak and David Jamieson]
  • “Bottom-up” approach – using scanning-probe lithography [led by Michelle Simmons]

Despite the amazing progress in single atom nanotechnologies over the past decade, until now there has been no demonstration of a single electron spin device based on phosphorus donors, as first envisaged by Kane back in 1998.

2006: Dr Andrea Morello joins the team at UNSW, in the quest to build silicon qubits using the “top-down” approach. Morello brings a background in quantum spin physics, which is essential to the team.

Morello and Dzurak form a close partnership – aimed at realising the dream of a silicon quantum bit.

2008: The “Top-Down” team at UNSW conceives a new scheme to read out the spin of an electron on an implanted phosphorus donor.

Paper published in Physical Review B in 2009 [A. Morello et al.].

December 2008: Morello and Dzurak, and their experimental teams, see the first experimental evidence of the read out of the electron spin on a phosphorus atom in a qubit device.

September 2010: Paper published in Nature, describing the “Single shot spin readout in Si” – the crucial step in realizing a silicon qubit.

Key research team members and their roles in the paper:


Dr Andrea Morello (UNSW) – Project Co-leader (with Dzurak). Lead experimentalist. Lead author. Joint conceptual design of experiment.

Prof Andrew Dzurak (UNSW) – Project Co-leader (with Morello). Joint conceptual design of experiment. Co-leader of “top-down” single-atom device engineering (with Jamieson).

Mr Jarryd Pla (UNSW) – Key supporting PhD student in experiment.

Dr Floris Zwanenburg (UNSW) – Key supporting postdoctoral researcher in experiment.

Prof David Jamieson (University of Melbourne) – joint conceptual design of experiment. Co-leader of “top-down” single-atom device engineering (with Dzurak).

Dr Bob Clark (Chief Defence Scientist) – joint conceptual design of experiment (while at UNSW in 2008).

Other authors

Kok Wai Chan, UNSW

Kuan Yen Tan, UNSW

Hans Huebl, UNSW

Mikko Möttönen, Aalto University, Finland

Christopher D. Nugroho, UNSW (now at the University of Illinois, US)

Changyi Yang, University of Melbourne

Jessica A. van Donkelaar, University of Melbourne

Andrew D. C. Alves, University of Melbourne

Christopher C. Escott, UNSW (now at Sapphicon Pty Ltd in Sydney)

Lloyd C. L. Hollenberg, University of Melbourne

Key Funding Bodies/Stakeholders


1. Australian Research Council (ARC): major funder of the research via the ARC Centre of Excellence Scheme (funder since 2000).

2. US National Security Agency and the US Army Research Office: funder of the Silicon Quantum Computer Program at UNSW & University of Melbourne since 1999.

3. Australian National Fabrication Facility (ANFF): Founded in 2006 under the National Collaborative Research Infrastructure Scheme of the Australian Department of Innovation Industry Science & Research. Provides infrastructure and technical support at UNSW for fabrication of qubit devices.

4. NSW Government – Department of Industry & Investment: provides significant co-funding to CQCT (since 2003) and also to ANFF (since 2006).

Photographs and captions


Project leaders Andrew Dzurak (left) and Andrea Morello
Credit: UNSW

Project leaders Andrea Morello (third from left) and Andrew Dzurak (fourth from left) with their team at UNSW
Credit: UNSW

Core UNSW experimental team: (from left) Jarryd Pla, Andrew Dzurak, Andrea Morello and Floris Zwanenburg
Credit: UNSW

Artist’s impression of a phosphorus atom (red sphere surrounded by a blue electron cloud, with spin) coupled to a silicon single-electron transistor, to achieve single-shot readout of the phosphorus electron spin.

Credit: William Algar-Chuklin, College of Fine Arts, UNSW.

Written by Stephen Pincock and Niall Byrne for the ARC Centre for Quantum Computer Technology. This article and images may be reproduced in part or in full. We welcome, but do not require, acknowledgement of the authors.