Quantum dots (QDs) and the electrons that can be trapped in their discrete energy levels are of great interest for quantum information processing. The spin state of these trapped electrons could act as carriers of quantum information or ‘qubits’.

Researchers at ETH in Zürich, Switzerland have taken an important step toward quantum information processing by demonstrating conditional dynamics for two coupled quantum dots [Robledo et al., Science (2008) 320, 772].

“To process quantum information, an essential ingredient is a system of two qubits that are coupled to each other,” explains Atac Imamoglu of ETH. “More specifically, what is required is ‘conditional dynamics’, where the evolution on the state of one qubit is controlled by the state of the other.”

Using a GaAs device containing two layers of self-assembled InGaAs QDs separated by a 15 nm potential barrier, the researchers show that the probability of whether a quantum dot makes a transition to an optically excited state is determined by the presence (or absence) of optical excitation in a neighboring QD. The interaction between the neighboring QDs relies on quantum mechanical tunneling between the optically excited states. This enables the coupling to be tuned and also means that the effects are much larger than in other coupling approaches based on dipole-dipole interactions. Applying a laser field of a particular frequency, which can be done on a very fast timescale, can turn the interaction on or off.

“This is a huge step toward realizing a solid-state two qubit gate,” comments Richard J. Warburton of Heriot Watt University in the UK.

The researchers suggest that the next steps will be to demonstrate the general conditional interaction mechanism for two coupled spins. Eventually, they hope to show that their approach can be used to control the quantum state of two coupled QDs in a time-resolved way.