Quantum computers and higher resolution sensors are a step closer after scientists develop a way to control a single atom with an electric field for the first time.
Described by the engineering team from UNSW as a ‘happy accident’, the discovery could replace the tried and tested magnetic resonance imaging used in medicine.
It also has ‘major implications’ for quantum computing as being able to control a single atom with an electric field gives engineers ‘much more precision’.
Generating magnetic fields, the previous solution, required large coils and high currents but electric fields can be produced on the top of a tiny electrode.
Lead author Andrea Morello says this will make it easier to control single atoms and place them in nanoelectric devices such as quantum computers and sensors.
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An artist’s impression of how a nanometer-scale electrode is used to locally control the quantum state of a single nucleus inside a silicon chip
‘This discovery means that we now have a pathway to build quantum computers using single-atom spins without the need for any oscillating magnetic field for their operation,’ said Morello.
This means that instead of having bulky magnets spinning fast to generate the field required to place the single atoms, they can use a simple electric field.
‘Moreover, we can use these nuclei as exquisitely precise sensors of electric and magnetic fields, or to answer fundamental questions in quantum science.’
The discovery shakes up the paradigm of nuclear magnetic resonance, according to Morello, who said it could cover everything from medicine to mining.
The researchers had originally set out to perform nuclear magnetic resonance on a single atom of antimony – an element that possesses a large nuclear spin.
‘Our original goal was to explore the boundary between the quantum world and the classical world, set by the chaotic behaviour of the nuclear spin,’ said one of the lead authors of the work, Dr Serwan Asaad.
‘This was purely a curiosity-driven project, with no application in mind,’ he said.
A dilution refrigerator (shown) is used to cool quantum processors. A number of companies and institutions are working on creating a quantum computer and this research could speed that process up
Once they started the experiment they realised ‘something was wrong’ and that the nuclear behaved very strangely and refused to respond to certain frequencies but show a strong response to others.
They discovered they had fabricated a device containing an antimony atom and a special antenna, optimised to create a high-frequency magnetic field to control the nucleus of the atom.
Their experiment demands this magnetic field to be quite strong, so they applied a lot of power to the antenna, and we blew it up.
‘This puzzled us for a while, until we had a ‘eureka moment’ and realised that we were doing electric resonance instead of magnetic resonance,’ said Dr Vincent Mourik, also a lead author on the paper.
‘With the antimony nucleus, the experiment continued to work. It turns out that after the damage, the antenna was creating a strong electric field instead of a magnetic field. So we ‘rediscovered’ nuclear electric resonance.’
‘Nuclear Magnetic Resonance is one of the most widespread techniques in modern physics, chemistry, and even medicine or mining,’ says Morello.
‘Doctors use it to see inside a patient’s body in great detail while mining companies use it to analyse rock samples.
‘This all works extremely well, but for certain applications, the need to use magnetic fields to control and detect the nuclei can be a disadvantage.’
He used the analogy of a billiard table to explain the difference between controlling nuclear spins with magnetic and electric fields.
‘Performing magnetic resonance is like trying to move a particular ball on a billiard table by lifting and shaking the whole table,’ he says.
‘We’ll move the intended ball, but we’ll also move all the others.’
‘The breakthrough of electric resonance is like being handed an actual billiards stick to hit the ball exactly where you want it.’
Magnetic Resonance Imaging scans used in hospitals are one of the uses that the new electric field system could be put to, according to researchers
The idea of controlling a single atom with an electric field isn’t a new one – it was first posed as a thought experiment in 1961 by Nobel Laureate Nicolaas Bloembergen.
The puzzle stood unsolved until Morello and his team ‘accidentally’ realised what they had achieved in their lab.
‘I have worked on spin resonance for 20 years of my life, but honestly, I had never heard of this idea of nuclear electric resonance,’ he said.
‘We ‘rediscovered’ this effect by complete accident – it would never have occurred to me to look for it.
‘The field of nuclear electric resonance has been almost dormant for more than half a century, after the first attempts to demonstrate it proved too challenging.’
After demonstrating the ability to control the nucleus with electric fields, the researchers used sophisticated computer modelling to understand how exactly the electric field influences the spin of the nucleus.
This effort highlighted that nuclear electric resonance is a truly local, microscopic phenomenon: the electric field distorts the atomic bonds around the nucleus, causing it to reorient itself.
‘This result will open up a treasure trove of discoveries,’ says Morello.
‘The system we created has enough complexity to study how the classical world we experience every day emerges from the quantum realm.
‘Moreover, we can use its quantum complexity to build sensors of electromagnetic fields with vastly improved sensitivity.
‘All this, in a simple electronic device made in silicon, controlled with small voltages applied to a metal electrode!’
The research has been published in the journal Nature.
WHAT IS A QUANTUM COMPUTER AND HOW DOES IT WORK?
The key to a quantum computer is its ability to operate on the basis of a circuit not only being ‘on’ or ‘off’, but occupying a state that is both ‘on’ and ‘off’ at the same time.
While this may seem strange, it’s down to the laws of quantum mechanics, which govern the behaviour of the particles which make up an atom.
At this micro scale, matter acts in ways that would be impossible at the macro scale of the universe we live in.
Quantum mechanics allows these extremely small particles to exist in multiple states, known as ‘superposition’, until they are either seen or interfered with.
A scanning tunneling microscope shows a quantum bit from a phosphorus atom precisely positioned in silicon. Scientists have discovered how to make the qubits ‘talk to one another
A good analogy is that of a coin spinning in the air. It cannot be said to be either a ‘heads’ or ‘tails’ until it lands.
The heart of modern computing is binary code, which has served computers for decades.
While a classical computer has ‘bits’ made up of zeros and ones, a quantum computer has ‘qubits’ which can take on the value of zero or one, or even both simultaneously.
One of the major stumbling blocks for the development of quantum computers has been demonstrating they can beat classical computers.
Google, IBM, and Intel are among companies competing to achieve this.