One potential approach to quantum information processing involves the use of semiconductor quantum dots, tiny islands of semiconducting material capable of containing just one or two electrons. These islands can act as solid-state quantum bits, or qubits, in quantum computing schemes.
Writing in the journal Science, a team of researchers in Switzerland reports that it has found a way to get two quantum dots to interact conditionally, a required condition for the successful implementation of two-qubit quantum gates. “The probability that one quantum dot makes a transition to an optically excited state is controlled by the presence or absence of an optical excitation in the neighbouring dot,” explained Atac Imamoglu, one of the researchers on the project.
The system consisted of a gallium arsenide device, containing two layers of indium gallium arsenide quantum dots grown one on top of each other. The two dots were coupled together by adjusting the gate voltage in a surrounding field-effect structure. Once coupled, the interaction between the dots can be optically gated by applying a laser beam at the right frequency. Imamoglu said that the work ‘represents substantial progress’ toward an optically controlled gate between two solid-state qubits.
An alternate approach to quantum information processing is aimed more at the interaction of photons as quantum bits. A group of researchers in California reports in the same edition of Science that they have found that manipulating a single quantum dot coupled to a photonic crystal nanocavity can serve to shift the phase of the photons emitted by the cavity.
“The accumulated phase of one beam is dependent on the total number of photons interacting with the atom,” said Jelena Vuckovic, one of the authors of the report.
With improvements in efficiency and better control of the output, the work has great potential for compact, scalable quantum devices, she said. She added that the two approaches are not exclusive: “Photonic qubits can be used to interconnect atom-like qubits realised in various systems.”
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