Privacy is one of society’s most valued qualities. The ability to send private messages and to carry out financial transactions without fear of being monitored lies at the heart of many government, military, and commercial activities.
One technology that allows this to be done perfectly is quantum cryptography, and it requires a powerful source of random numbers.
But there’s a problem. Random numbers are surprisingly hard to generate in large quantities. One of the best sources is the quantum world which is fundamentally random. But the best commercially available quantum random number generators produce them only at a rate of a million per second, far short of the many tens of billions per second that many applications require.
Today, that looks to have changed thanks to the work of You-Qi Nie at the Hefei National Laboratory for Physical Sciences in China and a few pals who say they have built a quantum random number generator capable of producing 68 billion of them per second. They say the technique should remove an important barrier preventing governments, the military, and the rest of us from benefiting from perfect security.
Random numbers have to be unpredictable and irreproducible, and this rules out generating them using ordinary algorithmic processes, which tend to be both predictable and reproducible. Computer scientists have long known that programs claiming to produce random numbers usually turn out to do nothing of the sort.
Instead, physicists have turned to natural processes to produce random numbers. For example, turbulence is thought to be entirely random so measuring that turbulent effects that the atmosphere has on a laser beam is one method of producing random numbers, albeit a rather slow one and one that could easily be biased by environmental factors.
That’s why physicists prefer to use quantum processes to generate random numbers. These are thought to be random in principle and fundamental in nature which is important because it means there cannot be some underlying physical process that might introduce predictability.
Physicists have tried lots of ways to produce quantum random numbers. One of the most popular is to send a stream of photons through a beam splitter, which transmits or reflects them with a 50 percent probability. Simply counting the photons that are reflected or transmitted produces a random sequence of 0s and 1s.
That’s exactly how the world’s only commercially available quantum random number generator works. But its speed is limited to about one megabit per second. That’s because single photon detectors cannot count any faster than this.
Recently, physicists have begun to mess about with a new technique. This arises from the two different ways photons are generated inside lasers. The first is by stimulated emission, which is a predictable process producing photons that all have the same phase. The second is spontaneous emission, an entirely random quantum process. These photons are usually treated as noise and are in any case swamped when the laser is operating at full tilt.
However, spontaneous emission becomes significant when the laser operates at its threshold level, before stimulated emission really takes hold. If it is possible to measure these photons, then it may be possible to exploit their random nature.
You-Qi and co have done exactly that. These guys have created a highly sensitive interferometer that converts fluctuations in the phase of photons into intensity changes. That’s important because intensity changes can be easily measured using conventional photodetectors that work at much higher rates than single photon detectors.
That has allowed the team to measure these random changes and digitize them at a rate of 80 Gbps. This data stream then has to be cleaned up in various ways to remove any biases introduced by the measurement process.
But after this, the team is still able to produce random numbers at the rate of 68 Gbps.
There’s no way of guaranteeing that any sequence of numbers is perfectly random but there are a set of standard tests that can spot certain kinds of patterns, if they are present. You-Qi and co say their random number sequences pass all these tests with flying colors.
The end result is the fastest quantum random number generator ever produced by some margin.
That’s impressive work that should prepare the ground for some mainstream applications using quantum cryptography. “Our demonstration shows that high-speed quantum random number generators are ready for practical usage, say You-Qi and co. “Our quantum random number generator could be a practical approach for some specific applications such as QKD systems with a clock rate of over 10 GHz.”
In other words, many organizations that need a practical system that offers secrecy guaranteed by the laws of quantum physics may not have much longer to wait.
Ref: arxiv.org/abs/1506.00720 : 68 Gbps Quantum Random Number Generation by Measuring Laser Phase Fluctuations