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Posted: May 7, 2010

Artificial atoms light up

(Nanowerk News) 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 of a Single Artificial Atom").
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.
A schematic representation of resonance fluorescence
Figure 1: A schematic representation of resonance fluorescence. (a) In a natural atom, an incoming light beam (left arrow) is absorbed and light is re-emitted in all spatial directions. (b) An artificial atom made from a superconducting circuit can achieve the same function. The light coming in along a one-dimensional waveguide (I0) couples to the circuit. Light is then scattered (Isc) in both directions of the wire, so that the original transmitted light (tI0) is suppressed.
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.
This research is funded by the Japanese government through a Kakenhi Grant-in-Aid for Scientific Research on Quantum Cybernetics.
Source: RIKEN
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