Doped nanotubes open a new path toward quantum information technologies

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In optical communication, critical information ranging from a credit card number to national security data is transmitted in streams of laser pulses. However, one can steal the information transmitted in this manner by splitting out a few photons of the laser pulse. This type of eavesdropping could be prevented by encoding bits of information on quantum mechanical states (e.g. polarization state) of single photons. Realization of such a quantum communication scheme would be a lot easier if the stream of single photons could be generated by simply making a reading lamp dimmer. However, photons emitted from lamps and lasers are distributed randomly in time; therefore “simultaneous” emission of two or more photons is always possible no matter how much they are attenuated.

True single photon generation requires an isolated quantum mechanical two-level system that can emit only one photon in one excitation-emission cycle. In the past decade, atomic-like quantum states of individual atoms, ions and molecules as well as artificial nanoscale materials such as quantum dots, quantum wires, and nitrogen vacancy centers in diamonds have been explored for their potential in single photon generation. However, none of these systems have emerged as the ideal candidate meeting all technological requirements critical for implementation of quantum communication. These requirements include the ability to generate single photons in the 1.3 - 1.5 μm telecommunication wavelength range at room temperature, and compatibility with silicon microfabrication technology to enable electrical stimulation and integration of other electronic and photonic network components. Carbon nanotubes have the potential to meet all these needs. However, earlier studies revealed that nanotubes were capable of single photon emission only at cryogenic temperature, with inefficient emission also showing strong fluctuations and degradation. Therefore, researchers considered nanotubes to be less promising as materials for single photon generation.

In contrast to this belief, researchers led by Han Htoon and Stephen Doorn (Center for Integrated Nanotechnologies, MPA-CINT) reported in Nature Nanotechnology that incorporation of pristine carbon nanotubes into a silicon dioxide (SiO2) matrix could lead to incorporation of solitary oxygen dopant states capable of fluctuation-free, room temperature single photon emission in the 1100 - 1300 nm wavelength range. The team investigated the effects of temperature on photoluminescence emission efficiencies, fluctuations, and decay dynamics of the dopant states to determine the conditions most suitable for the observation of single-photon emission. In principle, the emission could be tuned to 1500 nm via doping of smaller band-gap single- walled carbon nanotubes. This is a distinct advantage compared with diamond nitrogen vacancy centers, in which single photon emission is possible for a few discrete wavelengths shorter than 1 μm.

The oxygen-doped nanotubes can be encapsulated in a SiO2 layer deposited on a Si wafer, presenting an opportunity to apply well-established micro-electronic fabrication technologies for the development of electrically driven single photon sources and integration of these sources into quantum photonic devices and networks. The team has demonstrated oxygen doped carbon nanotubes as a viable system for development of single photon sources. Beyond implementation of quantum communication technologies, nanotube-based single photon sources could enable transformative quantum technologies including ultra- sensitive absorption measurements, sub-diffraction imaging, and linear quantum computing. The work has potential for photonic, plasmonic, optoelectronic and quantum information science applications. It is a significant advance for carbon nanotube optics and might motivate experimental and theoretical studies of new types of covalent dopants with quantum optical and spin properties that enable spintronic and quantum information processing functionalities.

Reference: “Room-temperature Single-photon Generation from Solitary Dopants of Carbon Nanotubes,” Nature Nanotechnology (2015) published online ahead of print; doi: 10.1038/nnano.2015.136. Researchers include: Xuedan Ma, Nicolai F. Hartmann, Jon K. S. Baldwin, Stephen K. Doorn, and Han Htoon (Center for Integrated Nanotechnologies, MPA-CINT).

The Laboratory Directed Research and Development (LDRD) program funded the work, which was performed at the Center for Integrated Nanotechnologies (CINT), a DOE Office of Basic Energy Sciences user facility. The research supports the Lab’s Global Security mission area and the Materials for the Future science pillar via the development of materials for secure communication transmission. Technical contacts: Han Htoon and Stephen Doorn

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Figure 6. A solitary oxygen dopant (red sphere) covalently attached to the sidewall of the carbon nanotube (gray) can generate single photons (red) at room temperature when excited by laser pulses (green).

 

Figure 7. (Top) Left: Optical spectroscopic data of individual dopant states shows 1.27 μm photoluminescence emission. Right: Single exponential photoluminescence decay with 573 ps long lifetime (white and red curves) and fluctuation- free photoluninescence emission (gray trace). (Middle): Second order photon correlation function provides evidence of single photon generation. (Bottom): Schematic illustration of carbon nanotube and oxygen dopant states encapsulated in SiO2 matrix.