Scientists at the School of Physics, Trinity College Dublin, with collaborators at Yale University, USA, CentraleSupelec, France, and Nanyang Technological University in Singapore, have developed a tiny chip-scale laser system to harvest quantum fluctuations in semiconductor lasers on ultrasmall scales at unprecedented speed. The new technology can be used to underpin modern technologies’ requirements for randomly generated digital information.
Their technique, published today in Science, uses a specially designed hour-glass-shaped semiconductor laser to generate hundreds of tiny, random light waves that when detected with a device called a photodetector, can be transformed into random strings of ‘1’s and ‘0’s, or binary code, the foundation of modern digital communications.
The quest for this level of randomness is not automatically visible – but is vital for day-to-day digital communications. Randomness, particularly truly random randomness, is a valuable resource. Our ever-increasing reliance on e-commerce, internet based financial services and virtual social contacts, depends on how well and how fast these processes and interactions can be protected from unwanted access or observation through random-keys and random-number based encryption.
The problem is that today’s random numbers are usually at best, ‘pseudo-random’ numbers, generated by a computer using special algorithms to calculate a sequence of numbers that appears ‘random’. There is a key limitation in this approach: the ‘random sequence’ is only random to a point because it is generated from a deterministic algorithm meaning this encryption can be ‘cracked’ by a fast computer, or other avenue.
Given the potential scale of the problem researchers have started to explore physical sources of randomness, such as randomly generated steams of light, while acknowledging that the key technical challenges for physical random number generation are speed and scalability. Looking at how the physical properties of light-matter interactions might be harnessed, scientists looked at lasers, as Professor Ortwin Hess, from the School of Physics and the CRANN Institute at Trinity, explains:
“Normal semiconductor lasers emit coherent light, light that is uniform and that can be focused to a tight beam. To produce and amplify this light inside a laser, it first travels around a cavity through semiconductor gain materials. In previous designs of large-area semiconductor lasers this bouncing back and forth of light creates optical filaments – sections of the light that swiftly begin to act chaotically. The optical filaments are a bit like optical tornadoes. Once they form, they move about chaotically leading to chaotic and unruly light. However, these ‘optical tornadoes’ have a characteristic size and speed, so that in current semiconductor lasers there is an upper limit on how much randomness can be generated in space and at any given period of time.”
To create more random and spatially unconnected ‘optical tornados’ the team designed a new cavity-shape for the laser. The shape of a lasers’ cavity is important; it does for light what the body of different string instruments does for sound. Very different sounds can be created from different ‘shaped’ string instruments from a violin to double-bass, as the body of the instrument interacts with acoustic waves generated from vibrating strings.
In the case of (edge-emitting) semiconductor lasers, most cavities are cuboid in shape, but by changing this to an hour-glass shaped cavity, the team were able to induce optical tornados on much smaller scales ‘harvesting’ the quantum noise allowing a massively parallel arrangement and essentially more randomness at much higher speeds.
Professor Hess, who contributed much of the theory and interpretation of the semiconductor laser dynamics in the study said: “By creating an optical landscape that supports significantly smaller optical ‘mini-tornados’ that all very efficiently and directly ‘harvest’ the endless supply of quantum fluctuations via spontaneous emission on ultrafast time scales, making them both fast, scalable and truly random”
The team gained insight into the processes and cavity shapes likely to create this kind of laser from theories and experiments in laser cavities and semiconductor laser dynamics, quantum chaos and quantum nanophotonics.
Professor Hui Cao, from Yale University, said: “Our laser cavity serves as a resonator for optical waves, and we have designed its shape to resonate with many spatio-temporal modes of light so that light in these modes will be amplified. The emission from all these modes creates a broad frequency spectrum for intensity fluctuations, which we utilize for massively parallel ultrafast random bit generation.
Professor Hess added: “I have been working on spatio-temporal and quantum dynamics in semiconductor lasers since my PhD, so it is gratifying to return to it now with the knowledge gained from metamaterials and nanophotonics. Physically generating randomness based on a quantum process is a key to many applications in security and data modelling but, in particular, for quantum simulation of new materials that, in turn, help us to design and enable practical quantum technologies at room-temperature.”
‘Massively parallel ultrafast random bit generation with a chip-scale laser’, Kyungduk Kim, Stefan Bittner, Yongquan Zeng, Stefano Guazzotti, Ortwin Hess, Qi Jie Wang and Hui Cao is published in Science [DOI: 10.1126/science.abc2666]