Australian quantum computing researchers have developed a new technique for reading the quantum spin of an atom, paving the way for immensely powerful computers connected by a super-fast quantum internet.
Qubits, however, will only be useful for storing data if scientists can find a way to control the spin state – either up or down – of tiny atomic components like electrons or the nucleus.
Then, other methods must be invented to “read” the spin state of the nucleus or electron, which means being able to detect whether it is in the up or down position.
Scientists had previously been able to use electrical or optical methods to read the spin of an atom.
In the latest development, scientists from the ARC Centre of Excellence for Quantum Computation and Communication Technology based at UNSW, the Australian National University and the University of Melbourne, were able to combine both optical and electrical methods to detect the spin on an atom of the chemical element erbium.
Optical methods involve shining a special laser beam on the erbium atom to knock an electron off it.
The researchers’ work is published today in the journal Nature.
“We combined optical detection of an atom’s quantum state with electrical detection, which is done in another sub-field. These two sub-fields basically never talk to each other,” said Professor Sven Rogge from UNSW’s Centre for Quantum Computation and Communication Technology.
“Previously, it was possible to use electrical detection to read out the quantum spin state of a phosphorous atom. But the disadvantage is that the resolution you have is considerably less than you have optically. Optical detection is better because it’s well isolated.”
Prefossor Rogge said the next step is developing new ways to deliberately position the quantum spin in an up or a down position.
“Now we can read it, we want to be able to control the quantum state,” he said, adding that such complicated quantum computing breakthroughs were made possible by experts in various sub-fields working together.
“This combined effort is what makes it possible to explore new routes.”
Professor Howard Wiseman, a quantum computing and physics expert from Griffith University said the new finding was very significant.
“It’s significant both in terms of being a new read-out technique and also because it opens up the possibility of getting solid state qubits talking to optical qubits,” said Professor Wiseman, who was not involved in the research.
“If you want to have a quantum internet, which is the long-term goal of this research, you need some way to have your optical qubits to talk to your solid state qubits.”
The new finding was more efficient because it combined the best of both the solid state techniques and optical techniques, he said.
“Most people think that to do quantum computing, you want to do that with solid state qubits, the same way we have solid state chips in conventional computers. But for communication, these days we use optical fibres because it’s much faster and efficient,” he said.
Professor Mike Thewalt, a physicist with the Silicon-based Quantum Information Group at Simon Fraser University in Canada, said the latest result was “very exciting, and for me, unexpected.”
“It will greatly expand the range of potential qubit systems that can be studied in silicon, and bring even more attention to the rapidly progressing field of silicon-based quantum information,” said Professor Thewalt, who was not involved in the Australian research.
“The most important result is that single atoms of erbium in a silicon device can be studied by selective optical ionization. We have been studying phosphorus in silicon using a similar approach of selective optical ionization and electrical detection, but so far this has been limited to the study of billions of almost-identical phosphorus atoms.”
Professor Thewalt said that efforts are underway to try to apply this optical method to a single phosphorus atom, but to date all of the successful single-phosphorus studies have been based on all-electrical methods.
The importance of single-atom detection is two fold, he said.
“First, it will likely be a prerequisite for any scalable quantum computing architecture. Second, it greatly expands our ability to study the details of potential qubits in silicon by eliminating what is known as inhomogeneous broadening, which broadens transitions when one is studying a large number of atoms since each atom is in a slightly different environment,” Prfoessor Thewalt said.
“This effect can actually be advantageous when one is able to study a single atom, since it would allow one to distinguish between different atoms of the same impurity present in the device.”