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Using two specific exploits, Lu’s team was


able to reduce the effective complexity of the key to a level that allowed a rainbow table to be constructed in 55 days using consumer computer hardware, making possible a successful online attack, in most cases within just nine seconds.


“GSM is still widely used in telecommu-


nications, but its A5/1 encryption system is now very insecure,” says Lu. “Our results show that GSM’s 64-bit key encryption is no longer sufficient and should be upgraded to a stronger scheme as a matter of urgency.”


1. Lu, J., Li, Z. & Henricksen, M. Time–memory trade-off attack on the GSM A5/1 stream cipher using commodity GPGPU. Applied Cryptography and Network Security (2015). 13th International Conference, ACNS 2015, New York, NY, USA, June 2–5, 2015, in Lecture Notes in Computer Science 9092, 350–369.


Plasmonics:


LIGHTING THE WAY TO MINIATURE DEVICES


ELECTROMAGNETIC WAVES CREATED ON A LAYER OF ORGANIC MOLECULES COULD PROVIDE THE PERFECT ON-CHIP LIGHT SOURCE FOR FUTURE QUANTUM COMMUNICATION SYSTEMS


A team of scientists including A*STAR researchers has captured tiny flashes of light from an ultrathin layer of organic molecules sandwiched between two electrodes that could replace lasers and LEDs as signal sources for future miniature, ultrafast quantum computing and light-based communication systems1. To investigate electromagnetic waves called


plasmons, which skim along the interface between two materials, Nikodem Tomczak from the A*STAR Institute of Materials Research and Engineering and colleagues collaborated with Christian A. Nijhuis from the National University of Singapore to construct a junction consisting of a layer of thiol molecules on a metal electrode and liquid gallium-indium alloy as a top electrode. The team created plasmons by applying a


voltage across the thiol layer. Although thiol is an insulator, the layer was thin enough for electrons to quantum tunnel between the electrodes, exciting plasmons on the thiol layer’s surface in the process. The plasmons then decayed into photons, tiny pulses of light that Tomczak and his colleagues were able to detect. “We were surprised that the light did not


come from the whole junction, but instead just from very small spots that blink at different frequencies,” said Tomczak.


www.astar-research.com The team found that the light generated


by the plasmons was polarized, and that both the polarization and the wavelength of the light varied with the voltage applied across the junction and the molecules used to form the organic layer. “The spots are diffraction-limited, polar-


ized and their blinking follows power-law statistics,” said Tomczak. “We need further experiments to confirm, but it is very similar to emission from other single photon sources, such as quantum dots or nanodiamonds.” Further evidence that the light is from


plasmons decaying into a single photon came from Chu Hong Son and his team at the A*STAR Institute of High Performance Com- puting who modeled the spots as the product of the smallest possible source, a single dipole emitter, and achieved results consistent with the experimental observations. Tomczak believes the layers can be scaled


down to junctions built from a single mol- ecule, opening up the potential to integrate plasmonic light sources on to silicon-based circuits, replacing large external light sources such as a laser. The team also explored one-molecule


thick layers of carbon chains terminated with a metallic ferrocene group developed by the group of Christian A. Nijhuis. Because this


Electron tunneling (pink) through a single organic molecule gives rise to plasmons (blue ripples) at the interface between the organic layer and the electrode below.


compound is asymmetric, it allows tunneling in one direction, effectively acting as a diode. “By keeping the same architecture and


tuning the chemistry of the monolayer you can create a range of different devices,” said Tomczak.


1. Du, W., Wang, T., Chu, H.-S., Wu, L., Liu, R. et al. On-chip molecular electronic plasmon sources based on self-assembled monolayer tunnel junctions. Nature Photonics 10, 274–280 (2016).


A*STAR RESEARCH 41


© 2016 Tao Wang, National University of Singapore


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