Physical Phenomena With Quantum-random Properties
There are two fundamental sources of practical quantum mechanical physical randomness: quantum mechanics at the atomic or sub-atomic level and thermal noise (some of which is quantum mechanical in origin). Quantum mechanics predicts that certain physical phenomena, such as the nuclear decay of atoms, are fundamentally random and cannot, in principle, be predicted (for a discussion of empirical verification of quantum unpredictability, see Bell test experiments.) And, because we live at a finite, non-zero temperature, every system has some random variation in its state; for instance, molecules of gases composing air are constantly bouncing off each other in a random way (see statistical mechanics.) This randomness is a quantum phenomenon as well (see phonon).
Because the outcome of quantum-mechanical events cannot in principle be predicted, they are the ‘gold standard’ for random number generation. Some quantum phenomena used for random number generation include:
- Shot noise, a quantum mechanical noise source in electronic circuits. The term is a clipping of the term "Schottky noise," referring to the scientist who first published regarding this phenomenon. A simple example is a lamp shining on a photodiode. Due to the uncertainty principle, arriving photons create noise in the circuit. Collecting the noise for use poses some problems, but this is an especially simple random noise source. However, shot noise energy is not always well distributed throughout the bandwidth of interest. Gas diode and thyratron electron tubes in a crosswise magnetic field can generate substantial noise energy (10 volts or more into high impedance loads) but have a very peaked energy distribution and require careful filtering to achieve flatness across a broad spectrum
- A nuclear decay radiation source (as, for instance, from some kinds of commercial smoke detectors), detected by a Geiger counter attached to a PC.
- Photons travelling through a semi-transparent mirror. The mutually exclusive events (reflection — transmission) are detected and associated to ‘0’ or ‘1’ bit values respectively.
- Amplification of the signal produced on the base of a reverse-biased transistor. The emitter is saturated with electrons and occasionally they will tunnel through the band gap and exit via the base. This signal is then amplified through a few more transistors and the result fed into a Schmitt trigger.
- Spontaneous parametric down-conversion leading to binary phase state selection in a degenerate optical parametric oscillator.
Read more about this topic: Hardware Random Number Generator
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