Quantum information: Computing with a single nuclear spin in silicon

A research team, including members from the London Centre for Nanotechnology (LCN), has created the first working quantum bit based on the nuclear spin of a single phosphorus atom in silicon, opening the door for dramatically improved data processing in ultra-powerful quantum computers of the future.

A landmark paper published today in the journal Nature, describes how to write and read quantum information with record-setting accuracy using the nuclear spin, or magnetic orientation, of a phosphorus atom in a silicon transistor – similar to silicon chips used in modern electronics.

The nucleus of a phosphorus atom is a very, very weak magnet, and can be imagined as a compass needle that can point north or south. These north or south positions are equivalent to the zero and one of binary code, which governs classical computing. In this experiment, the researchers controlled the direction of the nucleus, in effect “writing” an arbitrary value onto its spin, and were then able to “read” the value out. They observed quantum oscillations of the spin between north and south, and all the quantum superpositions of those two directions – where the spin exists in both states simultaneously.

“We achieved a read-out fidelity of 99.8 per cent, which sets a new benchmark for qubit accuracy in solid-state devices,” says Scientia Professor Andrew Dzurak, from the University of New South Wales (UNSW), Australia.

The primary author on the paper is Jarryd Pla, a Postdoctoral Research Assistant at the LCN, who at the time of writing the paper was a PhD student at the Quantum Spin Control group led by Associate Professor Andrea Morello, UNSW.

Nuclear qubit rivals ion trap for top qubit

The accuracy of this nuclear spin qubit means it’s near the level of what many people consider to be the most “perfect” quantum bit yet realised – a single atom isolated in an electromagnetic trap inside a vacuum chamber. The pioneers of this “Ion Trap” technology were awarded the 2012 Nobel Prize in physics.

“Our nuclear spin qubit is essentially at that level, but it’s not held in a vacuum chamber – it’s in a silicon chip that can be wired up and operated electrically like normal integrated circuits,” says Associate Professor Andrea Morello.

This is a huge advantage, says Morello. Silicon is the dominant material in the microelectronics industry, and by working with this material, the technology will be easier to scale up and incorporate within existing industry standards.

The Nuclear Qubit vs. Electron Qubit

In September 2012, the same team reported in Nature the first functional quantum bit based on an electron bound to a phosphorus atom embedded in silicon, “writing” information onto its spin and then “reading” the spin state back out.

Electron spin qubits will likely act as the main “processor” bit for quantum computers of the future. These are the key focus moving forward, and will be coupled with other electron qubits to perform operations.

But the nucleus also provides intriguing possibilities. The nucleus is the core of an atom, containing most of its mass, and is roughly one million times smaller than the overall size of the atom, determined by the orbits of the electrons . It is also 2000 times less magnetic than the electron. This has two consequences: it’s very challenging to measure, but it is also nearly immune to magnetic noise or electrical interference from the outside world. As a result, the nuclear spin has an excellent coherence time – this is what determines the time during which delicate quantum operations can be performed with minimal errors.

Pioneering studies on nitrogen-vacancies in diamond have previously demonstrated the potential of nuclear spins as quantum bits, but this is the first time that one has been successfully realised in silicon.

“The key benefit of using the nuclear spin as a quantum bit is that information stored on it can last for a long time in comparison with the time required to do calculations, meaning that very few errors occur during computation,” says Dzurak.

The nuclear spin qubit could also be integrated with electron qubits, serving a vital memory function, or assist the quantum logic operations between pairs of electrons.

The benefits of quantum computing

A functional quantum computer will provide much faster computation in three key areas: searching large databases, cracking most forms of modern encryption, 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. Functional quantum computers will also open the door for new types of computational applications and solutions that are, at this stage, difficult to predict.

How quantum computers work

In current computing, information is represented by classical bits. These are always either a zero or a one – physically represented by a transistor device being switched on or off. For quantum computing you need a physical system that has two distinguishable quantum “states”. In the UNSW design, the quantum data is encoded on the spin orientation of individual electrons, bound to single phosphorus atoms. With this new result, the team has shown that quantum data can be encoded on a nuclear spin as well, thus obtaining two fully functional quantum bits out of a single atom.

A spin pointing “north” would represent a one and a spin pointing “south” would represent a zero – 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. This is one of the properties that gives rise to the unique ability envisioned for quantum computers to rapidly solve complex, data-intensive problems.

Multiple, coupled qubits can exist in states that have no classical analog, and they can be in many of such states at the same time. These special states are called “entangled states” because the information they contain tells you something about the correlations between the particles, but not the individual state of each particle. “The entangled states represent extra ‘codes’ in a quantum computer, which classical computers do not possess. Their number grows exponentially with the number of qubits. As a consequence, with just 300 qubits it is possible to store as much information as there are atoms in the universe,” says Morello.

A functional quantum bit – or qubit

In order to employ an electron or nuclear spin qubit, a quantum computer needs both a way of setting the spin state (writing information) and of measuring the result (reading information). These two capabilities together form a quantum bit or qubit – the equivalent of the bit in a conventional computer.

In 2012, the research team completed both stages for a single electron bound to a phosphorus atom in silicon. In their latest result, published in Nature, they have achieved the same result – writing and reading information – but now using the spin of the atom’s nucleus, which is much weaker than that of the electron.

To write information on the nuclear spin the team used a technique known as “nuclear magnetic resonance”, which is the same phenomenon used in magnetic resonance imaging for brain scans – but in this case they were controlling and measuring the nuclear spin of just one atom, rather than many trillions of atoms.

The magnetic field generated by the phosphorus nuclear spin is one thousand times smaller than that of the electron spin, so this new type of quantum bit is, in principle, much harder to measure than in their previous work. However, the team used a new type of readout process, which involved using the electron as an intermediary to measure the nuclear spin, leading to a one billion-fold amplification. This allowed the team to read out the nuclear spin in real time with extremely high accuracy.

Journal link: http://www.nature.com/nature/journal/v496/n7445/full/nature12011.html

Figure: Scanning electron micrograph of the active area of the qubit device, showing an implanted donor (donor as blue arrow), the single electron transistor (SET) and the short-circuit termination of the microwave line. The device is mounted in a dilution refrigerator with an electron temperature of,300 mK, and is subjected to static magnetic fields B0 between 1.0T and 1.8 T. B0 is oriented perpendicular to the short-circuit termination of the microwave line (solid orange single-ended arrow), which carries a current (solid double-ended arrow) and produces an oscillating magnetic field B1 (represented by the solid and dashed circles) perpendicular to the surface of the device. TG, top gate; PL, plunger gate; LB, left barrier; RB, right barrier.

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Kuan Y. Tan, Juan P. Dehollain, Wee H. Lim, Floris A. Zwanenburg, David N. Jamieson, Andrew S. Dzurak & Andrea Morello
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