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.

Rabi oscillations

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.

“When tuned to the right frequency, this field can rotate the spin of individual atoms to any angle, where the rotation angle depends on how long we apply the microwaves,” explains Lutz. “This Rabi oscillation takes only about 20 ns to switch the qubit between 0 and 1 and then back again. This technique is known as electron spin resonance (ESR) and it is widely used for measuring the properties of magnetic materials. Here we have applied it to individual atoms.”

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. “The technique can create any superposition state we want and we can control and observe these spin rotations using the STM’s extreme sensitivity,” says Lutz.

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. “We now show that we can coherently drive the spin of titanium atoms using microwave frequencies and perform several coherent (perfectly deterministic) spin rotations of the spins before their quantum coherence is lost,” says Heinrich.

Faster spin rotation

The larger the amplitude of the microwaves, the faster the spin rotates, he adds. “To quickly drive these oscillations from spin-up to spin down, we thus simply turned on the microwave and maintained it a high amplitude for 20 ns. The Rabi oscillation is a critical step to creating quantum superpositions and to show that we can use certain quantum systems as qubits.”

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. “Combined with our ability to move atoms with atomic-scale precision with the tip of an STM – a technique that was pioneered at IBM – we can now also probe the magnetic or electric fields of engineered nanostructures or unknown molecules with atomic-scale precision too,” says Lutz.

The team, which includes researchers from the Center for Quantum Nanoscience at the Institute for Basic Science (IBS) and Ewha Womans University, both in Seoul in Korea, and the Clarendon Laboratory at the University of Oxford in the UK, now plans to optimize the local environment of the atomic qubits to improve their quantum coherence time. “For example, we will try different surfaces and types of magnetic atoms,” says Lutz. “We would also like to design and build atomic structures containing more magnetic atoms to explore quantum entanglement for quantum simulations.”

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