Atomic spins on a surface make good quantum bits

Individual atoms on a surface can be used as quantum bits (qubits) for quantum computing applications. That is the claim of scientists at IBM Research who have shown that they can control the positions of each qubit with atomic precision by manipulating the atoms in a scanning tunnelling microscope (STM). Controlling the position of these qubits also allows the team to modify  interactions between pairs of atoms. Classical computers make use of bits that can have one of two values, “0” or “1”. As well as taking these distinct values, qubits can also exist in quantum states that that are superpositions of “0” and “1” at the same time. A quantum computer made from such qubits can solve certain problems faster and more efficiently than conventional classical computers. However, the quantum nature of qubits (their quantum coherence) is extremely fragile and can easily be destroyed by interactions with the surrounding environment. Now, IBM researchers led by Christopher Lutz have used the magnetic spin of a titanium atom to create a qubit that can point in either an up (0) or down (1) direction. They placed the atom on an ultrathin layer of magnesium oxide to protect the quantum nature of its spin and coaxed it into a chosen quantum superposition state. They did this by applying a time-varying electric field with a frequency in the microwave range to the titanium atom. These microwaves come from the tip of the STM and steer the atom’s magnetic direction. At the end of the process, the atom points either in a 0 or 1 direction or a superposition, depending on how long the researchers apply the microwaves.

This new work builds on a major breakthrough by the same group in 2015 in which it combined ESR with STM and used a voltage between the microscope tip and the sample as the driving field. This voltage oscillated at gigahertz frequencies and drove the spin resonance of individual iron atom placed on a magnesium oxide film. The story does not end there though. Since these single-atom qubits are highly sensitive to magnetic fields, they might also be used as quantum sensors to measure the weak magnetism or electric fields of nearby atoms, say the researchers.

Coherent spin manipulation of individual atoms on a surface
Kai Yang1,, William Paul1, Soo-Hyon Phark1,2,3, Philip Willke1,2,4, Yujeong Bae1,2,4, Taeyoung Choi2,3, Taner Esat1,2,4, Arzhang Ardavan5, Andreas J. Heinrich2,3,, Christopher P. Lutz1,*

Science 25 Oct 2019:
Vol. 366, Issue 6464, pp. 509-512
DOI: 10.1126/science.aay6779

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