Artificial atoms light up


A superconducting circuit that strongly interacts with light paves the way
for optical computing schemes


Before a quantum effect such as resonance fluorescence—resulting
from the interaction of light with atoms—can be applied to quantum
computing schemes, scientists need to replicate it in the laboratory.
Thus far, however, efforts using artificial atoms made from superconducting
circuits have been unsuccessful. Now, resonance fluorescence of a single
artificial atom has been demonstrated by researchers from the NEC Nano
Electronics Laboratory in Tsukuba and the RIKEN Advanced Science
Institute in Wako.
Resonance fluorescence occurs when a light beam with an energy
that matches an atom’s resonance energy gets absorbed by the atom
and then re-emitted in random directions. As resonance fluorescence can
be used to couple two photons, or light particles, scientists are keen to
exploit this effect in quantum computing operations. However, this effect
in atoms is too small to be useful for practical applications since photons
and atoms interact very weakly due to their small size, according to Jaw-Shen
Tsai, who led the research team.

To circumvent this problem, researchers created artificial atoms on computer
chips (Fig. 1), where the interaction between light and the artificial atom can
be optimized. “With a solid-state device such as ours, made from superconducting
circuits, the coupling can be very strong,” says Tsai.

Earlier attempts by researchers in the field to observe resonant fluorescence
in artificial atoms resulted in low efficiencies of around 12%, owing to poor re-emission
of the absorbed light by these atoms. To enhance the re-emission process, the
researchers used a one-dimensional waveguide coupled to the artificial atom. This
resulted in an efficient re-emission of light from the artificial atom because in the
waveguide the light is channelled in only two directions. Tsai and colleagues
demonstrated that about 94% of the incoming light at the resonance frequency
of the superconducting circuit was absorbed and re-emitted.

By building on this strong interaction between incoming light and the artificial atom
a number of potential applications are now possible, according to Tsai. “There
are a whole series of experiments one can do, for example towards photon-based
quantum computing,” he says. The absorption of a photon by an artificial atom, for
example, could be used to control the propagation of a second photon along the
waveguide, owing to the non-linear nature of the interaction of light with the artificial
atom, Tsai explains.