10 May 2013

Quantum device students - Alice Mahoney and James Colless

Image courtesy: ARC Centre of Excellence for Engineered Quantum Systems PhD students, Alice Mahoney and James Colless, School of Physics.


Quantum dots are nanoscale systems that can confine or trap single electrons. Temperatures close to absolute zero are required to study quantum behaviour.

Using a dilution refrigerator to conduct experiments on quantum dots, a team of physicists from the University of Sydney has discovered a new way of detecting changes in charges smaller than one electron.

“Our new method for detecting charge in quantum systems is exciting and has implications for a range of  nanotechnologies,” said Associate Professor David Reilly from the ARC Centre of Excellence for Engineered Quantum Systems.

Researchers have studied quantum dots in transistors, solar cells, LED’s and diode lasers. They have also investigated quantum dots as agents for medical imaging and hope to use them as qubits in quantum computing (A qubit or quantum bit is a unit of quantum information)

“We have been successful in finding a new, more convenient way to detect changes in charge of a single electron on quantum dots,” Professor Reilly said.

“Quantum devices will revolutionise computing, enabling huge calculations to be completed that traditional computers cannot do. Our goal is to scale up a large number of quantum dots and ultimately create a machine to process quantum information—a quantum computer.

“Previously, sensitive electrometers which measure minute charges were used to read-out the electron state on quantum dots. These work well, but they are somewhat separate devices built onto the ends of the quantum dot system. They are a bit like having microphones nearby that can pick up the sound of electrons.

“We've focused on quantum dots as their properties can be tuned in the laboratory—we can control their energy spectrum by turning a knob in the lab.

“Being able to detect single electron charges on the quantum dots is absolutely essential, as it's the way information is retrieved from such quantum mechanical systems. We call it ‘read-out’, it is similar to reading information from the memory or a hard drive in a conventional computer,” said Professor Reilly.

“Our new method makes the whole quantum system easier to build and use, as adding nanoscale electrometers for every quantum dot in a million-dot-array is a hard problem. By using the electrodes already in the system, we've found an efficient new way to measure charge in the big quantum systems of the future.”

Scientists have already built basic quantum computers that can perform certain calculations but agree that a practical quantum computer is still years away.

This research is supported by the ARC and the US Government Intelligence Advanced Research Projects Activity (IARPA).

For more information about the ARC Centre of Excellence for Engineered Quantum Systemsemail Associate Professor David Reilly.