Quantum Dots Create Distance Between Electron Spins for Quantum Computing

Scientists at the Niels Bohr Institute have realized the swap of electron spins between distant quantum dots. This is significant for future quantum communications because it will allow the distance between the quantum dots to be large enough for integration with traditional microelectronics and, perhaps, a future quantum computer. The Copenhagen team collaborated with researchers at Purdue University and the University of Sydney to make the discovery.

One method of storing and exchanging quantum information is through electron spin states, where the electrons’ charge and spin is manipulated by gate-voltage pulses. It was believed that this method can only work if quantum dots touch each other. If the dots are squeezed too close together, the spins will react too violently; if placed too far apart, the spins will interact too slowly. Both fast spin exchange and enough room around quantum dots to accommodate the pulsed gate electrodes are needed.

Normally, the left and right dots in a linear array of quantum dots are too far apart to exchange quantum information with each other. The researchers discovered that a large, elongated quantum dot placed between the left dots and right dots could mediate a coherent swap of spin states, within a billionth of a second, without ever moving electrons out of their dots. “In other words, we now have both fast interaction and the necessary space for the pulsed gate electrodes,” said professor Ferdinand Kuemmeth.

Researchers at the Niels Bohr Institute cooled a chip containing a large array of spin qubits below −273 °C. To manipulate individual electrons within the quantum-dot array, they applied fast voltage pulses to metallic gate electrodes located on the surface of the gallium-arsenide crystal (see scanning electron micrograph). Because each electron also carries a quantum spin, this allows quantum information processing based on the array’s spin states (the arrows on the graphic illustration). During the mediated spin exchange, which took only a billionth of a second, two correlated electron pairs were coherently superposed and entangled over five quantum dots, constituting a new world record within the community. Courtesy of Ferdinand Kuemmeth.

Professor Stephen Bartlett at the University of Sydney said, “What I find exciting about this result as a theorist is that it frees us from the constraining geometry of a qubit only relying on its nearest neighbors.” His team performed detailed calculations, providing the quantum mechanical explanation for the counterintuitive discovery, while the Purdue team provided access to extremely clean quantum dots.

“If spins between nonneighboring qubits can be controllably exchanged, this will allow the realization of networks in which the increased qubit-qubit connectivity translates into a significantly increased computational quantum volume,” Kuemmeth said.

The research was published in Nature Communications (https://www.nature.com/articles/s41467-019-09194-x).