The researchers say that the device requires little power and could enable stand-alone random number generators or could be incorporated into laptops and smart phones to offer real-time encryption.
"While part of the control electronics is not integrated yet, the device we designed integrates all the required optical components on one chip," says first author Francesco Raffaelli, University of Bristol.
The chip was enabled by developments in silicon photonics technology, which uses the same semiconductor fabrication techniques used to make computer chips to fabricate optical components in silicon. It is now possible to fabricate waveguides into silicon that can guide light through the chip without losing the light energy along the way. These waveguides can be integrated onto a chip with electronics and integrated detectors that operate at very high speeds to convert the light signals into information.
The new chip-based random number generator takes advantage of the fact that under certain conditions a laser will emit photons randomly. The device converts these photons into optical power using a tiny device called an interferometer. Very small photodetectors integrated into the same chip then detect the optical power and convert it into a voltage that can be turned into random numbers.
"Despite the advancements in silicon photonics, there is still light lost inside the chip, which leads to very little light reaching the detectors," says Raffaelli. "This required us to optimise all the parameters very precisely and design low noise electronics to detect the optical signal inside the chip."
According to the researchers, their device not only brings portability advantages, but is also more stable than the same device made using bulk optics. This is because interferometers are very sensitive to environmental conditions such as temperature and it is easier to control the temperature of a small chip. It is also far easier to precisely reproduce thousands of identical chips using semiconductor fabrication, whereas reproducing the necessary precision with bulk optics is more difficult.
To experimentally test their design, the researchers had a foundry fabricate the random number generator chip. After characterising the optical and electronic performance, they used it for random number generation. They estimate a potential randomness generation rate of nearly 2.8gigabits per second for their device.
"We demonstrated random number generation using about a tenth of the power used in other chip-based quantum random number generator devices," explains Raffaelli. "Our work shows the feasibility of this type of integrated platform."
Although the chip containing the optical components is only one millimetre square, the researchers used an external laser which provides the source of randomness and electronics and measurement tools that required an optical table. They are now working to create a portable device about the size of a mobile phone that contains both the chip and the necessary electronics.
