STM creates first single-atom qubit


Rebecca Pool

Thursday, October 31, 2019 - 15:00
Image: Christopher Lutz of IBM Research - Almaden in San Jose, California, stands with IBM’s STM, used to achieve the first single-atom qubit. [Stan Olszewski]
Researchers at IBM Research have used scanning tunnelling microscopy to control the quantum behaviour of individual atoms, demonstrating a versatile new building block for quantum computation.
By modulating magnetic interactions between the STM tip and surface atoms, IBM Research staff scientist, Dr Christopher Lutz, and colleagues were able to drive quantum Rabi oscillations between spin-up and spin-down states in as little as 20 nanoseconds.
In this way, they showed how single atoms on a surface can be used as qubits, so very crucial for quantum computing processing.
“This is the first time a single-atom qubit has been achieved using a scanning tunnelling microscope,” says Lutz. “This is an important breakthrough because the STM can image and position each atomic qubit to precisely control the arrangement of nearby qubit atoms.” 
An artist’s view of a single titanium atom (yellow ball) sitting on top of a prepared surface of magnesium oxide. The top of the image shows the STM’s sharp needle tip, which is used to perform coherent control.
To demonstrate qubit control, the researchers first placed a titanium atom on a ultrathin magnesium oxide layer.
Then, using the STM tip to apply a time-varying electric field to the atom, they were able to rotate the magnetic spin of that atom to any angle.
By controlling this Rabi oscillation, the researchers could switch the qubit between 0 and 1, and back again, in just 20 nanoseconds. 
As Lutz states: “The technical term of this key technique is pulsed electron spin resonance, and it can create any superposition state we want. We control and observe these spin rotations using the STM’s extreme sensitivity.”
The researchers also build a two-qubit device, using the STM to nudge two titanium atoms precisely into the desired atomic positions.
Due to the quantum interaction between these atoms, the qubits also form an entangled state, which holds the key to the power of quantum computing.
As Lutz says: “We are able to control the properties of this entanglement by adjusting the distance between the atoms, and by choosing the duration and the frequency of the radio waves that control them.”
The researchers would now like to build structures containing more magnetic atoms to further explore quantum entanglement.
Research is published in Science.
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