Quantum computing: First two-qubit logic gate in silicon
An Australian team of engineers has built a quantum logic gate in silicon for the first time, making calculations between two qubits of information possible – and thereby clearing the final hurdle to making silicon quantum computers a reality. Their work was published online in the international scientific journal, Nature, on 5 October 2015 (London time).
It’s the first time calculations between silicon quantum bits has been demonstrated. To achieve this, the University of New South Wales (UNSW) team constructed a device, known as a ‘quantum logic gate’, that allows calculations to be performed between two quantum bits, or ‘qubits’. The advance completes the physical components needed to realise super powerful silicon quantum computers.
Any conceivable application, or software program, that would run on a quantum computer is made up of a series of basic one-qubit and two-qubit calculations.
Until now it had not been possible to make two silicon quantum bits “talk” to each other, to perform such “two-qubit” calculations, or “logic gates”. The UNSW result means that all of the physical building locks have now been constructed, and so computer engineers can finally begin the task of building a functioning quantum computer in silicon.
Industrial manufacture now possible
A key advantage of the UNSW approach is that they have reconfigured the ‘transistors’ that are used to define the bits in existing silicon chips, and turned them in qubits.
“Because we use essentially the same device technology as existing computer chips, we believe it will be much easier to manufacture a full-scale processor chip than for any of the leading designs, which rely on more exotic technologies,” says Professor Dzurak.
“This makes the building of a quantum computer much more feasible, since it is based on the same manufacturing technology as today’s computer industry,” he adds.
Dzurak noted that that the team had recently “patented a design for a full-scale quantum computer chip that would allow for millions of our qubits, all doing the types of calculations that we’ve just experimentally demonstrated.”
He said that a key next step for the project is to identify the right industry partners to work with to manufacture the full-scale quantum processor chip.
The benefits of quantum computing
A functional quantum computer will provide much faster computation in a number of key areas, including: searching large databases, solving complicated sets of equations, and modelling atomic systems such as biological molecules and drugs. This means they’ll be enormously useful for finance and healthcare industries, and for government, security and defence organisations.
For example, they could be used to identify and develop new medicines by greatly accelerating the computer-aided design of pharmaceutical compounds (and minimizing lengthy trial and error testing); develop new, lighter and stronger materials spanning consumer electronics to aircraft; and achieve much faster information searching through large databases.
Functional quantum computers will also open the door for new types of computational applications and solutions that are probably too premature to even conceive.
How silicon quantum computers work
In current computing, information is represented by classical bits, which are always either a zero or a one. Physically, each bit is typically stored on a pair of transistors, one of which is switched on while the other is off.
An electron with spin ‘up’ can represent a 0, while a counter-clockwise (or downward) spin can represent a 1. But in the quantum realm, particles have a unique ability to exist in two different states at the same time, an effect known as ‘quantum superposition’, which gives rise to the unique ability envisioned for quantum computers to rapidly solve complex, data-intensive problems. For quantum computing you need an equivalent: and in the UNSW design, the data is encoded on the ‘spin’ – or magnetic orientation – of individual electrons, stored in devices that are almost identical to the transistors on existing silicon chips. These single electron spin devices are known as quantum bits, or ‘qubits’.
A quantum silicon approach: UNSW leading the way
In recent years, scientists around the world have been developing completely new systems based on exotic materials and devices or even light to build a quantum computer. At UNSW, however, the approach has been to use silicon – the material currently used in all modern day microprocessors, or computer chips. Silicon offers several advantages: the material is cheap, is already used in almost all commercial electronics, and its properties are very well understood – the result of trillions of dollars of investment into R&D by the computer and electronics industry.
Silicon electron ‘spins’ also have very long ‘coherence times’: this means data encoded on the spin can remain there for longer periods than it would in most materials, before it is scrambled and lost. This is important for performing successful calculations.
In 1998, former UNSW researcher Bruce Kane first proposed the idea of using silicon as a base material for quantum computing. In a paper in Nature, he outlined the concept for a silicon-based quantum computer, in which single phosphorus atoms in an otherwise ultra-pure silicon chip define the qubits. His visionary work spawned an international effort to develop a quantum computer in silicon.
In 2012 Dzurak also jointly led a team with Andrea Morello at UNSW that was the first in the world to demonstrate a spin qubit in silicon, as reported in Nature, but this used a single atom, rather than a modified silicon transistor to realise the qubit.
Last year Dzurak’s team discovered a way to create a qubit with a device remarkably similar to the silicon transistors used in consumer electronics, known as MOSFETs.
The latest result from UNSW show for the first time that quantum calculations can be performed between two qubits in silicon, meaning that all of the physical building blocks are now in place to realise a large-scale quantum processor chip.
Timeline of the development of silicon quantum computing
1994: Peter Shor from Bell Labs (USA) 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.
1998: Bruce Kane, then a postdoctoral researcher at UNSW (now a researcher at the U.S. Laboratory for Physical Sciences in Maryland), publishes a paper in Nature 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. This is the first such scheme in silicon – the material used for all modern day microprocessors. Kane’s paper attracts great interest because: (a) Silicon is ‘industrially relevant’; (b) Silicon electron ‘spins’ have very long ‘coherence times’ (hence, low error rates). This paper has now generated over 2,000 citations.
2000: Bob Clarkestablishes the ARC Special Research Centre for Quantum Computer Technology (CQCT), headquartered at UNSW, to attempt to build a quantum computer. The centre has now expanded to become an ARC Centre of Excellence – with more than 150 researchers in Australia, and major collaborations world-wide. Andrew Dzurak (UNSW), who was a founding investigator in CQCT, begins development of silicon device technologies for building a silicon quantum computer.
2007: Dzurak’s group develops a variation of a silicon MOSFET transistor that can be reduced to the level of a single electron. This is ultimately used for the two-qubit logic calculations by Dzurak’s group in 2015.
2000-2009: Various researchers at CQCT, including Dzurak, David Jamieson and Michelle Simmons develop ground-breakingsingle atom nanotechnologies in silicon, generating hundreds of papers.
2010: Andrea Morello and Dzurak publish in Nature a paper describing the “Single shot readout of an electron spin in silicon” – the step of measuring a silicon qubit.
2012: Paper in Nature by Morello and Dzurak groups: “A single-atom electron spin qubit in silicon” – the crucial step of writing information on an electron to operate the first silicon qubit.
2014: Dzurak, Veldhorst and Yang patent the concept of a gate-addressable quantum bit based on the modified silicon MOSFET transistor.
2014: Paper in Nature Nanotechnology by Dzurak’s group, with lead author Menno Veldhorst, describing “An addressable quantum dot qubit with fault-tolerant control-fidelity”, which is a high accuracy qubit based on the modified CMOS transistor developed by Dzurak in 2007.
2015: Dzurak’s group (with lead author Veldhorst) publishes in Nature the paper “A two-qubit logic gate in silicon”. This work completes the physical building blocks necessary to realise a silicon quantum computer chip, opening the pathway to industrial manufacture.
Key research team members and their roles in the paper
Scientia Professor Andrew Dzurak (UNSW) – Research team leader. Developed over 2007-2015 the concept, design and fabrication technologies for silicon spin qubits based on modified silicon transistors. Director of Australian National Fabrication Facility at UNSW and co-leader of silicon quantum computing programs at CQC2T.
Research Fellow Dr Menno Veldhorst (UNSW) – Post-doctoral researcher in Dzurak’s team at the UNSW School of Electrical Engineering & Telecommunications, and lead-author on the Nature paper. Jointly developed (with Dzurak and Yang) the design of the SiMOS qubits.
Dr Henry Yang (UNSW) – Post-doctoral researcher in Dzurak’s team at the UNSW School of Electrical Engineering & Telecommunications, and key contributor to the experimental work. Jointly developed (with Dzurak and Veldhorst) the design of the SiMOS qubits.
Associate Professor Andrea Morello (UNSW) – Expert on silicon spin qubit measurement and control, and long-time collaborator with Dzurak.
Professor Kohei Itoh (Keio University, Japan) – Collaborates with Dzurak, and provides isotopically purified silicon wafers for device production at UNSW.
Key stakeholders & funding bodies
Centre of Excellence for Quantum Computation and Computer Technology (CQC2T): Australian centre of research excellence, headquartered at UNSW, in which Dzurak jointly leads (with Michelle Simmons) the effort in Silicon Quantum Computing. Founded in January 2000.
Australian National Fabrication Facility (ANFF): Founded in 2006 under the Australian Government’s National Collaborative Research Infrastructure Scheme (NCRIS). Provides infrastructure and technical support at UNSW for fabrication of the qubit devices.
Australian Research Council (ARC): Major funder of CQC2T via the ARC Centres of Excellence Scheme (a funder since 2000).
U.S. Army Research Office: Funder of the Silicon Quantum Computer Program at UNSW and the University of Melbourne since 1999.
Australian Government Department of Education: Major funder of ANFF through the National Collaborative Research Infrastructure Scheme (NCRIS).
State Government of New South Wales: Through the Office of Scientific Research at the NSW Department of Trade & Investment, provides significant co-funding to CQC2T (since 2003) and also to ANFF (since 2006).
Commonwealth Bank of Australia: Has provided research funding to CQC2T, including Dzurak’s group, since 2013.
University of New South Wales: Has provided core financial and infrastructure support to both CQC2Tand ANFF since their establishment.