Synopsis: Targeting Single Qubits

A scheme based on a combination of lasers and microwaves can fully control a single atomic qubit sitting within a large multiqubit array.
Synopsis figure
T. Xia et al., Phys. Rev. Lett. (2015)

Quantum computation is still in its early stages. But as more and more qubits are assembled together in a quantum processor, the challenge becomes that of controlling each individual qubit, while not disturbing the nearby ones. In a new experiment with a two-dimensional array of 49 atomic qubits, researchers have demonstrated full control over single qubits using a combination of two electromagnetic fields—one in the microwave and the other in the visible range.

Atomic systems are considered for future quantum computers because they offer long-term stability. Certain atoms have electronic energy states that can act as the 0 and 1 of a qubit. Light fields can perform basic logic operations, such as turning a 0 into a 1. The problem is that this light often requires a long wavelength, making it hard to focus on just one qubit.

For their qubit system, Mark Saffman and his colleagues from the University of Wisconsin, Madison, loaded cesium atoms into a two-dimensional optical lattice with 3.8-micrometer site-to-site spacing. The cesium atoms have qubit states that respond to 9.2-gigahertz microwaves (whose wavelength is much larger than the qubit spacing). To select a single qubit, the team detuned their microwave emitter away from 9.2 gigahertz and then focused a laser beam (wavelength 459 nanometers) on a single site of their choosing. The laser light induces a Stark shift that alters the atomic energy levels, so that only the chosen qubit responds to the detuned microwaves. This scheme, based on a previously developed targeting technique, offers, for the first time, full control over all the possible logic operations on a single qubit in a two-dimensional array.

This research is published in Physical Review Letters.

–Michael Schirber


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