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Scientists demonstrate new way of manipulating quantum circuits

by CIFAR Feb 26 / 13

In the quantum world, the intrinsic spinning motion of individual electrons, called the electron spin, can be used to store and manipulate quantum information. This concept is being investigated by countless research labs around the world, with the ultimate goal of building a quantum computer.

An image of the device used in the experiment, showing an electron in a quantum superposition that involves a finite probability of the electron existing in the two edge quantum dots but zero probability in the center dot. Image: Andrew Sachrajda

Recently, a team of researchers, including CIFAR Senior Fellow Andrew Sachrajda (NRC) and Fellow Michel Pioro-Ladrière (Université de Sherbrooke), demonstrated a novel and non-intuitive phenomenon that may lead to new ways of transporting spins around a quantum circuit. Their findings were published in Nature Nanotechnology.

The researchers were first able to create a state-of-the-art quantum circuit by arranging a series of three ‘quantum dots’ – tiny boxes that can hold one or two electrons – in a line and then transmitting electrons through the line of dots, one at a time. Under certain conditions, however, it became impossible to transmit the electrons through the circuit.

“Although the physics of this part of the experiment was well understood, this was the first time ever that scientists have been able to physically turn a conductor into an insulator – a material that resists electrical current – just by playing with the spin of electrons,” says Sachrajda. “Two electrons in one quantum dot cannot have the same spin; if you try to put a second electron that has the same spin as the first electron into a dot, it simply won’t go. So, by configuring the spin of the electrons in the circuit, we were able to block their movement through the circuit and create a new kind of insulator, which one of our students dubbed ‘spinsulator.’”

The big surprise, however, came when the team was able to overcome this blockade and allow the current to flow again. By carefully tuning their circuit, the scientists showed that under certain conditions the electrons could transfer between the first and last quantum dots in the line without ever being present in the center dot. This magic-like effect was due to a fundamental property of quantum mechanics called ‘quantum superposition,’ in which a particle like an electron can exist in multiple states – or in this case, in any of the quantum dots – until it is measured. The specific superposition that the researchers were able to create enabled the electrons to have a probability of existing in the quantum dots at either edge of the circuit but with zero probability of being in the middle dot. Since the middle dot was instrumental for the blockade process, the superposition was able to avoid the block and let the current run again.

These achievements in the lab have enormous implications for quantum computing and information processing. For instance, being able to transport electrons to other parts of a circuit in this way will improve the efficiency of quantum processors. This is called a ‘spin bus’ and an important component of a quantum computer’s architecture.

“Working with the devices that we have designed and the principles we were able to harness, we will now be able to incorporate technology developed by Dr. Pioro-Ladrière into our devices,” says Sachrajda. “His technology will allow us to additionally manipulate the spins by enabling us to reverse the spin direction of individual electron spins, a critical task for our processors.”

This study was generously funded by the Canadian Institute for Advanced Research, National Research Council Canada, and Natural Sciences and Engineering Research Council of Canada.