At the Niels Bohr Institute, University of Copenhagen, researchers have discovered the exchange of electron spins between distant quantum points. The discovery brings us a step closer to future applications of quantum information, as the small dots must leave enough space on the microchip for delicate control electrodes. The distance between the dots has now become large enough for integration with traditional microelectronics and perhaps a future quantum computer. The result is achieved through a multinational collaboration with Purdue University and the University of Sydney, Australia, now published in nature Communications.
The size is important in quantitative information exchange even on the nanometer scale
Quantity information can be stored and exchanged using electron-spin modes. The charge of the electrons can be manipulated by gate voltage pulses which also control their spin. It was assumed that this method can only be practical if quantum points touch each other; If the pressure too close to each other spins react too violently if they are placed too far apart, the spins will interact too slowly. This creates a dilemma because if a quantum computer ever comes to see the light of day, we need both quick spin and enough space around quantum points to accommodate the pulsating gate electrodes.
Normally, the left and right dots in the linear array of quantum points are too far apart to exchange quantum information with each other. Frederico Martins, postdoc at UNSW, Sydney, Australia, explains: "We encode quantum information in the electron spin states, which have the desirable feature that they do not interact much with the noisy environment, making them useful as robust and long-lasting environments. But when you want to actively deal with quantum information, the lack of interaction is counterproductive – because now you want the spins to interact! "What to do? You can't have both long-lived information and information exchange – or it seems. "We discovered that by placing a large, elongated quantum point between the left dots and right dots, it can convey a coherent exchange of spin states within a billionth of a second without ever moving electrons out of their dots. In other words, we now have both quick interaction and the space needed for the pulsating gate electrodes, "says Ferdinand Kuemmeth, associate professor at the Niels Bohr Institute.
Cooperation is an absolute necessity, both internally and externally
The collaboration between researchers with diverse expertise was the key to success. Internal collaborations are constantly increasing the reliability of nanofabrication processes and the sophisticated low temperature technology. In fact, at the Center for Quantum Devices, major candidates for the implementation of solid state quantum computers have been intensively studied, namely semiconducting spin qubits, superconducting gatemon qubits and topological Majorana qubits.
All are voltage-controlled qubits, so researchers can share tricks and solve technical challenges together. But Kuemmeth is quick to add that "all this would be useless if we didn't have access to extremely clean semiconducting crystals in the first place." Michael Manfra, professor of material engineering, agrees: "Purdue has put a lot of effort into understanding the mechanisms that lead to quiet and stable quantum points. It is great to see these benefits for Copenhagen's new qubits."
The theoretical framework for the discovery is provided by the University of Sydney, Australia. Stephen Bartlett, professor of quantum physics at the University of Sydney, said: "What I find exciting about this result as a theorist is that it frees us from a quartz limiting geometry that depends only on its closest neighbors." His team performed detailed calculations that provide the quantum mechanical explanation for the conflicting discovery.
Overall, the rapid spin exchange demonstration not only represents a remarkable scientific and technical achievement, but can have profound implications for the architecture of solid state quantum computers. The reason is the distance: "If spins between non-nearby qubits can be exchanged regularly, this will allow the realization of networks where the increased qubit qubit connection changes to a significantly increased computational amount volume," Kuemmeth predicts.
University of Copenhagen. .