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This article defines a "quantum random number generator" (QRNG) as "a device that exploits the probabilistic nature of quantum mechanics to produce a sequence o

This article defines a "quantum random number generator" (QRNG) as "a device that exploits the probabilistic nature of quantum mechanics to produce a sequence of entirely random digits" and claims that QRNGs are "complex, expensive devices". Firstly, people have been building RNGs from web cams and radioactive sources cannibalized from smoke detectors for years, at a total cost of less than $100. Radioactive decay is a quantum process so it seems fair to describe these as QRNGs. Secondly, the smartphone camera-based random number generator (SCRNG) described in the article/paper does not appear to generate its random numbers from quantum-level events (i.e. the emission of individual photons). The classic model for quantum cryptography uses a half-silvered mirror to create a quantum state by introducing uncertainty over whether an individual photon has been deflected or not. The SCRNG is not observing individual photons. Instead, it's using the variation between the observed number of electrons that are generated as a result of exposing the camera to a light source, and the number of electrons that the light source is expected to generate. Because the variation is random (i.e. the entropy is high enough), this generates random numbers but I question whether it's really a QRNG. Disclaimer: I am not a physicist so I could be wrong. In fact, the question of whether I'm right or wrong is currently in a quantum superposition, pending "observation" by someone who is suitably qualified to determine whether I'm right or wrong. ;-)

nabla9 12y ago
For the purpose of this discussion there are two types of noise in electronics: shot noise and thermal noise. Shot noise is discrete noise that happens when single electrons are emitted and adsorbed. Shot noise is quantum noise and can't be reduced by cooling and don't vary with frequency. Thermal noise is noise that depends on the temperature of the circuit. Temperature increases the energy of material and this energy allows electrons to bounce around more creating noise. For the purpose of QRNG, you don't have to separate shot noise from thermal noise if you know the shot noise to thermal noise ratio. We are not interested of knowing actual bits generated by shot noise. We just want to collect enough random bits so that we can guarantee that there is enough quantum randomness to make the quality equal to just quantum bits. For example, if the ratio is 1/100, we collect more than 100 bits and mix them together to get one bit of quantum randomness. Shot noise is amplified and has easily detectable effects in in p-n junctions (diodes, photodectectors).
- analog31 12y ago
  That sounds reasonable. For a camera sensor, the noise budget is made up of four primary terms: 1. So called "fixed pattern noise," i.e., a pixel by pixel pattern in the measured value that isn't reduced by averaging. This pattern would be a "fingerprint" of the camera, probably long term if not forever. You can make a well averaged measurement of the fixed pattern (somewhat temperature dependent) and subtract it from subsequent exposures. 2. Thermal noise of the amplifier that converts charge into a signal that can be read by the analog-to-digital converter. This is referred to as "readout noise." 3. Shot noise in "good" photocurrent, i.e., photons from the scene being converted into charge. This becomes dominant at higher light levels, as it's proportional to the square root of photocurrent. 4. Shot noise in "bad" photocurrent, i.e., a constant leakage current in the light sensitive elements, whose magnitude is temperature dependent.
jpmattia 12y ago
> but I question whether it's really a QRNG. It really is a QRNG, but the only "quantum" thing you need to know is that light arrives in lumps. As nabla9 points out, they are really measuring the shot noise. The fundamental assumptions of shot noise are that you have distinct items arriving at an average rate, and that the probability of a single item arriving in an interval dt is proportional to the length of the interval dt. Since light arrives in quantized lumps, and you expect an avg number of lumps that increases linearly with collection time, there is shot noise associated with it. It could describe the arrival of buses, which also happen to arrive in quantized lumps, if you could somehow convince the drivers to increase your probability of being picked up proportionally to how long you wait at the bus stop. It's a nice enough hack, but the medium article is overselling "probabilistic nature of quantum mechanics to produce a sequence of entirely random digits." The arxiv paper is clearer and is much less of a sales job. FWIW, the tough thing here is the nature of shot noise: In some interval of time, the expectation value from a constant power of light is N photons in a single pixel. The RMS deviation over a series of trials is sqrt(N). So the phone needs to measure sqrt(N)/N accurately enough to pull out random digits. The ratio sqrt(N)/N implies that you wish to measure low N, or low numbers of photons. That a cell phone has such sensitivity is the key observation. I expected the paper to have a plot of RMS(photons) vs LED output power to show definitively that the noise is photon-induced shot noise (there are other sources), but it is a nice paper nonetheless.
pontifier 12y ago
Last night I found this video about webcam and Americium random number generators: https://www.youtube.com/watch?v=A2WgaVwMHo8 https://www.youtube.com/watch?v=A2WgaVwMHo8 The appeal to me is that the particle detections look so crisp. The camera image is easy to inspect visually. See the video at 46:17. With this smartphone camera method, only the algorithm seems to be able to pull the "quantum-nes" out. This ability to inspect is the root of my trust of the system.

nabla9 12y ago

For the purpose of this discussion there are two types of noise in electronics: shot noise and thermal noise. Shot noise is discrete noise that happens when single electrons are emitted and adsorbed. Shot noise is quantum noise and can't be reduced by cooling and don't vary with frequency. Thermal noise is noise that depends on the temperature of the circuit. Temperature increases the energy of material and this energy allows electrons to bounce around more creating noise. For the purpose of QRNG, you don't have to separate shot noise from thermal noise if you know the shot noise to thermal noise ratio. We are not interested of knowing actual bits generated by shot noise. We just want to collect enough random bits so that we can guarantee that there is enough quantum randomness to make the quality equal to just quantum bits. For example, if the ratio is 1/100, we collect more than 100 bits and mix them together to get one bit of quantum randomness. Shot noise is amplified and has easily detectable effects in in p-n junctions (diodes, photodectectors).

analog31 12y ago
That sounds reasonable. For a camera sensor, the noise budget is made up of four primary terms: 1. So called "fixed pattern noise," i.e., a pixel by pixel pattern in the measured value that isn't reduced by averaging. This pattern would be a "fingerprint" of the camera, probably long term if not forever. You can make a well averaged measurement of the fixed pattern (somewhat temperature dependent) and subtract it from subsequent exposures. 2. Thermal noise of the amplifier that converts charge into a signal that can be read by the analog-to-digital converter. This is referred to as "readout noise." 3. Shot noise in "good" photocurrent, i.e., photons from the scene being converted into charge. This becomes dominant at higher light levels, as it's proportional to the square root of photocurrent. 4. Shot noise in "bad" photocurrent, i.e., a constant leakage current in the light sensitive elements, whose magnitude is temperature dependent.

jpmattia 12y ago

> but I question whether it's really a QRNG. It really is a QRNG, but the only "quantum" thing you need to know is that light arrives in lumps. As nabla9 points out, they are really measuring the shot noise. The fundamental assumptions of shot noise are that you have distinct items arriving at an average rate, and that the probability of a single item arriving in an interval dt is proportional to the length of the interval dt. Since light arrives in quantized lumps, and you expect an avg number of lumps that increases linearly with collection time, there is shot noise associated with it. It could describe the arrival of buses, which also happen to arrive in quantized lumps, if you could somehow convince the drivers to increase your probability of being picked up proportionally to how long you wait at the bus stop. It's a nice enough hack, but the medium article is overselling "probabilistic nature of quantum mechanics to produce a sequence of entirely random digits." The arxiv paper is clearer and is much less of a sales job. FWIW, the tough thing here is the nature of shot noise: In some interval of time, the expectation value from a constant power of light is N photons in a single pixel. The RMS deviation over a series of trials is sqrt(N). So the phone needs to measure sqrt(N)/N accurately enough to pull out random digits. The ratio sqrt(N)/N implies that you wish to measure low N, or low numbers of photons. That a cell phone has such sensitivity is the key observation. I expected the paper to have a plot of RMS(photons) vs LED output power to show definitively that the noise is photon-induced shot noise (there are other sources), but it is a nice paper nonetheless.

pontifier 12y ago

Last night I found this video about webcam and Americium random number generators: https://www.youtube.com/watch?v=A2WgaVwMHo8 https://www.youtube.com/watch?v=A2WgaVwMHo8 The appeal to me is that the particle detections look so crisp. The camera image is easy to inspect visually. See the video at 46:17. With this smartphone camera method, only the algorithm seems to be able to pull the "quantum-nes" out. This ability to inspect is the root of my trust of the system.