June 14, 2010
By Robin Donovan

Entanglement has long intrigued physicists. Without physically touching, two entangled items react instantaneously and identically to stimuli applied to one member of the entangled pair. Scientists collaborating between Ohio University and the Eindhoven University of Technology in the Netherlands recently observed new excited states of matter through this phenomenon.

The study highlights the first observation of hybrid excitons and appeared as an Advance Online Publication in Nature Physics on May 23. The observance of hybrid excitons in particular brings physicists a step closer to quantum information processing.

Using lasers, scientists targeted electrons in quantum dots positioned near a metallic layer within a lattice-shaped atomic structure. When the laser struck the quantum dots, electrons leapt from low to high energy levels, creating quasiparticles called excitons.

Excitons consist of an excited electron and hole, or space left behind at the low energy of a semiconductor. As excited electrons relax, they release secondary photons that can be studied to reveal properties of quantum dots.

“Looking at the spectrum of emission, we understand more about the state of the electron. The spectrum gives us the information about the hybrid [entangled] state, the state of the exciton, and the state of the electron and how it’s entangled,” said Sasha Govorov, a theoretical physicist from Ohio University’s Nanoscale and Quantum Phenomena Institute who co-authored the paper.

Unlike previous research that examined this type of entanglement, this study used optics to more directly observe unusual electron states. As scientists examined the properties of photons given off by relaxing electrons, they were also able to identify hybrid excitons. These quasiparticles were created through entanglement of an electron from a quantum dot and electrons from the metallic film.

Quantum dots are groups of atoms five to 50 nanometers wide that function as “artificial atoms.” Controlled by voltage, they can be arranged to form designer crystal structures and have become a popular research topic.

In a 2003 study, Govorov and his co-authors predicted that hybrid exciton states would exist and be observable under certain conditions. Before this project, however, this type of electron entanglement had not been observed in the optical spectra.

Previous research demonstrated electron entanglement and a related Kondo effect on a microscale, but the effect could only be observed indirectly using measurements of electric current through a sample. The Kondo effect describes the electrical resistance of conduction electrons that become entangled with localized impurities. By conducting research with optics on a nanoscale, scientists observed electron interactions and entanglements with some properties of the Kondo state. They not yet able to observe Kondo excitons, but this milestone is the subject of active experimental research.

The Ohio University Biomimetic Nanoscale and Nanotechnology group and Condensed Matter and Surface Science Program provided funding for this research. Experimental work was led by Professor Paul M. Koenraad and performed at the Eindhoven University of Technology.  Koenraad’s team included Joost van Bree, Niek Kleemans, Andrei Silov, Joris Keizer, Rian Hamhuis and Richard Nötzel. Theory was developed by Govorov at Ohio University.