Long-Range Quantum Information Exchange - Success At Nanoscale

Researchers at the Niels Bohr Institute at the University of Copenhagen have succeeded in obtaining exchanges of quantum information on a large array of quantum dots. The discovery brings us a step closer to the use of quantum information as it allows incorporation into traditional microchips and perhaps into a future quantum computer. The result was achieved in an international collaboration between the Niels Bohr Institute, Purdue University, the United States, and the University of Sydney, Australia.

Size matters in the exchange of quantum information - even on the nanometer scale
Quantum information can be stored and exchanged using electron spin mode and electron spin can be controlled by an electrical pulse. It was previously believed that the method was only applicable if the individual quantum dots touched each other. The thing is, if the individual quantum dots are too close together, the spin interacts too violently and too slowly if they are too far apart. It creates a dilemma, because if we ever have to hope that a quantum computer sees the light of day, we need both. Both allow for fast spin exchanges and space for electrodes to be connected to control the spin.

Generally, the right and left sides of the array of quantum dots seen in the illustration would not at all be able to exchange spins at the given distance. (Illustration 1). Frederico Martins, PhD student, explains: "We encode quantum information into the spin state of the electrons, which has the highly desirable feature that they do not interact with the environment. The information is thus both robust and has a long storage time. But, conversely, when we want to start a quantum information process, the lack of interaction seems to be the opposite - now we would like tohave our electron spin to interact! ”What to do - one cannot have both sailing quantum information and exchange at one time… or can you? "We discovered that by placing a large, elongated quantum dot in the middle between the left and right sides of our matrix, it can mediate a coherent exchange of spin states, in a billion-second second, without the electrons leaving their quantum dots at all. In other words, we now have both lightning fast exchanges and the necessary space for the pulse electrodes ”, says Ferdinand Kuemmeth, associate professor at the Niels Bohr Institute.

Cooperation is crucial, both internally and externally
Cooperation between different organizations and expertise is the key to success. Internal collaboration at the Niels Bohr Institute promotes the reliability of the processes in nanofabrication and the refinement of low-temperature techniques (below -273 degrees Celsius) that are required. At the Center for Quantum Devices, three solid techniques are studied, all of which are used in a quantum computer, namely semiconductor spin qubits, superconducting gatemon qubits and topological Majorana qubits.

All three are what are called electronically controlled qubits, and it lets researchers share tricks and solve technical problems together. But Kuemmeth quickly adds: "All this would be useless if we didn't have access to extremely clean semiconductor crystals to build our matrix. Michael Manfra, Professor of Materials Engineering at Purdue University, USA, agrees: "Purdue has worked hard to understand the mechanisms that create quiet and stable quantum dots. It is great to see this work bear fruit in the innovative qubits of the Copenhagen [Niels Bohr Institute].

The theoretical background for the discovery was provided by the University of Sydney in Australia. Stephen Bartlett, professor of quantum physics at the University of Sydney, says: "What I find most exciting about this result as a theorist is that it frees us from the constraining geometry that requires qubits to exchange information only with its closest neighbors." His team prepared the detailed calculations that underpin the quantum mechanical explanation of the no-further intuitive discovery.

On the whole, fast spin exchange not only demonstrates remarkable scientific and technical accomplishment, but may have fundamental implications for the development of solid-state quantum computers. The reason is the distance: "If the electron spin between qubits that are not close to each other can be exchanged and controlled, there is a sudden potential for networks where the increased qubit qubit connections can be translated directly into increased quantum computing power," predicts Ferdinand Kuemmeth.