Mar 12 2020
An accidental and groundbreaking discovery during an experiment in the laboratory has allowed researchers to not only resolve an issue that existed for over 50 years but also has important implications for developing sensors and quantum computers.
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. Image Credit: University of New South Wales/Tony Melov.
In a research recently published in the Nature journal, a group of engineers at the University of New South Wales (UNSW Sydney) has made a major breakthrough by using only electric fields to control the nucleus of a single atom. This concept was initially suggested by a renowned researcher in 1961 but has evaded everyone since that time.
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. Moreover, we can use these nuclei as exquisitely precise sensors of electric and magnetic fields, or to answer fundamental questions in quantum science.
Andrea Morello, Scientia Professor of Quantum Engineering, University of New South Wales
The fact that a nuclear spin can be regulated with electric fields rather than magnetic fields has major implications. High currents and large coils are required to produce magnetic fields, but according to the laws of physics, magnetic fields cannot be easily confined to extremely small spaces because they tend to have a broad area of influence.
On the other hand, electric fields can be generated at the tip of a small electrode, and these fields fall off quite sharply away from the tip. As a result, individual atoms positioned in nanoelectronic devices can be controlled more easily.
A New Paradigm
According to Professor Morello, this latest finding redefines the paradigm of nuclear magnetic resonance—an extensively used method in diverse fields such as mining, chemistry, or medicine.
“Nuclear magnetic resonance is one of the most widespread techniques in modern physics, chemistry, and even medicine or mining,” Professor Morello added. “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.”
The analogy of a billiard table was used by Professor Morello to describe the variation between regulating nuclear spins with the electric field and magnetic field.
Performing magnetic resonance is like trying to move a particular ball on a billiard table by lifting and shaking the whole table. 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.
Andrea Morello, Scientia Professor of Quantum Engineering, University of New South Wales
Fascinatingly, Professor Morello was completely oblivious of the fact that his group had solved the longstanding issue of identifying a method to regulate nuclear spins using electric fields. This concept was initially proposed by Nicolaas Bloembergen, a Nobel Laureate and pioneer of magnetic resonance Nicolaas Bloembergen, in 1961.
“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,” added Professor Morello. “We ‘rediscovered’ this effect by complete accident—it would never have occurred to me to look for it. The whole 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.”
Out of Curiosity
The scientists had originally set out to carry out nuclear magnetic resonance on one atom of antimony, an element with 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. This was purely a curiosity-driven project, with no application in mind.
Dr Serwan Asaad, Study Lead Author, University of New South Wales
“However, once we started the experiment, we realised that something was wrong. The nucleus behaved very strangely, refusing to respond at certain frequencies, but showing a strong response at others,” recalled another lead author of the study, Dr Vincent Mourik.
“This puzzled us for a while, until we had a ‘eureka moment’ and realised that we were doing electric resonance instead of magnetic resonance,” Dr Mourik added.
Dr Asaad added, “What happened is that we fabricated a device containing an antimony atom and a special antenna, optimized to create a high-frequency magnetic field to control the nucleus of the atom. Our experiment demands this magnetic field to be quite strong, so we applied a lot of power to the antenna, and we blew it up!”
Game On
“Normally, with smaller nuclei like phosphorus, when you blow up the antenna it’s ‘game over’ and you have to throw away the device,” added Dr Mourik. “But 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.”
Once the researchers demonstrated the potential to regulate the nucleus with electric fields, they turned to advanced computer modeling to figure out how the electric field precisely impacts the nuclear spin. This effort emphasized the fact that nuclear electric resonance is indeed a local and microscopic phenomenon—the atomic bonds surrounding the nucleus are distorted by the electric field and this causes the electric field to reorient itself.
“This landmark result will open up a treasure trove of discoveries and applications,” added Professor Morello. “The system we created has enough complexity to study how the classical world we experience every day emerges from the quantum realm.
Professor Morello continued, “Moreover, we can use its quantum complexity to build sensors of electromagnetic fields with vastly improved sensitivity. And all this, in a simple electronic device made in silicon, controlled with small voltages applied to a metal electrode.”
Key investigators include Scientia Professor Andrea Morello, Dr Serwan Asaad, and Dr Vincent Mourik, all from UNSW Sydney; Associate Professor Jeffrey McCallum from the University of Melbourne, and Dr Andrew Baczewski from Sandia National Laboratories.
Australian Research Council Discovery Project, Next Generation Technologies Fund of the Department of Defence, is the funding partner.
Nuclear Electric Resonance - the key to controlling single atoms in electronic devices
Video Credit: University of New South Wales.