We have developed the world’s first device able to generate and transmit a stream of single photons along a semiconductor chip. A single photon, the “elementary particle” of light, is considered as one of the most suitable candidates for the transmission of information in future information networks that take advantage the laws of quantum physics. A wide range of applications can be possible, from long-distance Quantum Key Distribution through optical fibre networks to logic operations at the nanoscale on a semiconductor chip.
The generation of single photons on a chip can be achieved by the use of semiconductor quantum dots, objects with dimensions of a few nanometres. We can easily excite one electron to a high energy level inside the quantum dot, which emits one single photon every time it relaxes to the lower energy level. Since there is no preference to the direction they are emitted, photons tend to escape from the chip via random pathways. In order to collect them efficiently and route them along the chip, we have developed light-guiding structures making use of state-of-the-art nano-fabrication technology.
Electromagnetic waves can be confined and transmitted by waveguides. On a semiconductor chip, we can achieve light confinement in regions surrounded by a “photonic crystal”, an array of holes that create a “forbidden” region for light at certain frequencies to enter. A thin material strip sandwiched between two photonic crystal regions can act as a waveguide structure. By placing a quantum dot in the waveguide region, we can efficiently generate and transmit single photons along the chip. Such a device contains most of the essential tools – generation and transmission of single photons – to perform quantum logic operations on a semiconductor chip in a scalable way.
1) On-chip single photon emission from an integrated semiconductor quantum dot into a photonic crystal waveguide.
A. Schwagmann, et al. Appl. Phys. Lett., 99, 261108 (2011).
2) Slow-light-enhanced single quantum dot emission in a unidirectional photonic crystal waveguide.
S. J. Dewhurst, et al. Appl. Phys. Lett., 96, 031109 (2010).