Jun 3, 2005
Quantum dot creates single photon source
Experiments in quantum communications and computing could be about to get much easier thanks to the development of a semiconductor source of single photons at the telecoms window of 1.3 µm.
The quantum-dot based device was announced at last week’s CLEO/QELS conference in Baltimore, US. It has been developed by scientists from Toshiba Research Europe and the University of Cambridge, UK.
To date, single photon sources are notoriously difficult to build and rely on either heavily attenuating a laser or exciting single atoms. The drawback is that these schemes are often complex and it can be hard to prevent multiple photons being emitted.
In contrast, Toshiba’s quantum-dot emitter reliably generates single photons on demand when excited by short optical pulses. In addition, the semiconductor approach is potentially compatible with electrical pumping and should be much easier to package and commercialize.
"In terms of suppressing multiple photon generation, we’ve achieved an order of magnitude below what you get from a laser," said Martin Ward, a member of the research team from Toshiba Research Europe. "There are other ways of generating single photons, like down-conversion, but this is the first time that strong [multiple photon] suppression from a quantum-dot type source has been demonstrated at telecom wavelengths."
In order to ensure that single photons could be isolated and directed into an optical fibre, the team had to learn how to fabricate sparsely populated fields of InAs/GaAs quantum dots, each 45 nm in diameter and 10 nm high. The dots were grown by molecular beam epitaxy on a GaAs substrate at a temperature of about 500 °C.
After fabrication, a long-wavelength dot is embedded inside a pillar microcavity consisting of two mirrors (distributed Bragg reflectors) and an optical filter is applied to block emission from any surrounding dots of a smaller size.
At the moment, the source operates at cryogenic temperatures but the Cambridge team is confident that this can be raised to more practical levels. "The results in the paper are taken at 5 K and 30 K, but the long wavelength dots should also emit at higher temperatures - we’ve seen photoluminescence up to 200 K," Ward told Optics.org. "We certainly don’t envisage using cryrogenic liquids to cool any future commercial devices."
About the author
Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.