Portland, Ore. -- Physicists have been predicting that interactions between quantum dots could prove just as dissipative as electronic communications on silicon chips, for the same reason: the randomness of multiple-electron behavior. But a team at Ohio University asserts that, given the appropriate environmental conditions, communications among arrays of semiconducting quantum dots can be coherent.
If the researchers' computer simulations are proved correct though subsequent experiments in the physical realm, the finding could open the door to complex quantum computers built from arrays of quantum dots.
"We have shown that . . . you can pass quantum states from one dot to another with no losses," said Ohio professor Sergio Ulloa. "This is not a way to communicate quantum information over long distances, but a method of doing local quantum computations."
Ulloa and Ohio doctoral candidate Ameenah Al-Ahmadi have worked exclusively with computer simulations, a condition that has let them ignore the extraneous physical factors that Ulloa believes have hidden the essentially coherent nature of quantum-dot interactions. Ulloa is leaving it to others to prove out the prediction with follow-up experiments. A group "here at Ohio University is planning to demonstrate our results experimentally," he said. He added that its results are expected within the next few months.
If the simulations prove out, Ulloa said, basic design principles could be disseminated to EEs looking to craft quantum-computer components.
Photonic coherence is a familiar property of optical networking. In laser communications, an emitter sends out a coherent stream of photons that travels directly to the receiver; losses stem only from the medium (air, waveguide or optical fiber) through which the stream must pass. Incoherent light, in which the photons are randomly distributed in phase and direction, suffers further signal losses, attributable to the random movement of the photons.
Quantum dots are tiny clumps of semiconducting material a few nanometers across that can trap a few electrons, much as an atom binds a number of electrons orbiting its nucleus. Quantum-dot pairs model molecules, at least on the electronic level, and thus are of interest to physicists hoping to enable molecular electronic components.
One interesting discovery about how coupled quantum dots interact has become known as the Forster mechanism--a resonant effect that involves electron/hole pairs called excitons, as do many of the photonic effects seen in semiconductors. While the resonant Forster interaction does not produce an actual photon, the energy transfer produces the same result as if a photon had been emitted by the sender and absorbed by the receiver.
Ulloa and Al-Ahmadi believe the exciton-mediated interaction between quantum dots is coherent, in the same sense that laser light is coherent. "This coherent communication takes place by virtue of what we call a virtual photon--one that appears, communicates the quantum state and then disappears, with no energy loss," said Ulloa.
The work sought to characterize the conditions under which coherent communications among quantum dots could be supported in architectures that are likely to be useful in building future quantum computers. For instance, arrays and lines of quantum dots are likely to be useful in quantum computers, so the researchers concentrated on characterizing the conditions for coherent communications in those topologies.
The team ran simulations using the known dynamics of the exciton state in arrays by using the time evolution of the density matrix of the system. The dipole moments of the excitons modeled the Forster coupling among quantum dots and were corrected for overestimates by calibrating against known experimental results. Ulloa is therefore confident that the Ohio group now attempting to confirm his results will find that the full quantum state is being coherently communicated among dots.
"With our approach, you can manipulate the quantum state of the dot and communicate it coherently to the next quantum dot nearby with full information--not just the energy, but the complete quantum state," said Ulloa. "Of course, the quantum states are still very sensitive to environmental conditions--that is the bugaboo of all quantum systems and, in particular, this one. Our point is that by carefully controlling the environment, you can keep errors to a minimum."
The next phase
Ulloa currently is crafting methods by which information can be processed with quantum dots. Next, he hopes to show that quantum states can be extracted from the polarization of light and processed while in a quantum state of superposition to yield usable information.
"So far we have only shown that the energy is conserved," he said, "but in the next month we plan to release results showing that we can preserve the quantum information encoded in the polarization of light--and that after communications [take place] from dot to dot, the information can be extracted."
Thus far, Ulloa has characterized gallium arsenide, cadmium selenium, cadmium silicon and indium phosphorus. He has concluded that the optimal pitch of quantum-dot arrays depends not only on the material but also on the radius of the excitons.
Next, he plans to characterize various architectural configurations of quantum dots, such as nearest-neighbor arrays.