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Light is made up of small packets of energy called photons, so small, in fact, that they are almost impossible to measure directly. The amount of energy that is deposited when one photon of wavelength <math>\lambda</math> is absorbed is equal to <math>h/\lambda</math>, where <math>h</math> is Planck's constant. For example, a candle puts out <math>5 * 10^{16}</math> photons per second. Therefore, to detect single photons, physicists built a machine that magnifies the energy of the photon. This machine is called an avalanche photodiode. (why the low power is needed)

Light is made up of small packets of energy called photons, so small, in fact, that they are almost impossible to measure directly. The amount of energy that is deposited when one photon of wavelength <math>\lambda</math> is absorbed is equal to <math>h/\lambda</math>, where <math>h</math> is Planck's constant. For example, a candle puts out <math>5 * 10^{16}</math> photons per second. The human eye is not able to detect individual photons; even a light pulse of one million visible photons would not be bright enough to be seen.  Therefore, to detect single photons, physicists built a machine that magnifies the energy of the photon. This machine is called an avalanche photodiode. (why the low power is needed)

   

   

The normal diode is a device that allows electricity to flow one way, but not the other. It is used in a lot of very common electrical appliances, from computers to toasters. Photodiodes produce a single electron from each photon that hits the detector area. But since physicists are trying to detect single photons, that is not nearly enough. They had to create a device that releases many electrons for every single photon that hits the detector. (Discussion on PMT?) They could do just that with the avalanche photodiode. Diodes only allow electricity to flow one way. So if voltage is applied in the opposite direction of the way that the electrons were meant to flow, no electricity would cross. Yet every single diode has a breaking point. If enough voltage, or electrical force, is put across a diode, it could suddenly allow all the electricity through, like a dam breaking. The voltage that is applied in the reverse direction is called reverse bias voltage. Physicists take advantage of that effect by applying enough reverse bias voltage that the Avalanche photodiode that anything, even the energy from a single photon is sufficient to cause it to break down. This is called the breakdown voltage. If a photon were to hit this diode, it would cause a huge surge of electricity to go through the diode and therefore the entire curcuit, one that could be measured by the scientist.  

The normal diode is a device that allows electricity to flow one way, but not the other. It is used in a lot of very common electrical appliances, from computers to toasters. Photodiodes produce a single electron from each photon that hits the detector area. But since physicists are trying to detect single photons, that is not nearly enough. They had to create a device that releases many electrons for every single photon that hits the detector. (Discussion on PMT?) They could do just that with the avalanche photodiode. Diodes only allow electricity to flow one way. So if voltage is applied in the opposite direction of the way that the electrons were meant to flow, no electricity would cross. Yet every single diode has a breaking point. If enough voltage, or electrical force, is put across a diode, it could suddenly allow all the electricity through, like a dam breaking. The voltage that is applied in the reverse direction is called reverse bias voltage. Physicists take advantage of that effect by applying enough reverse bias voltage that the Avalanche photodiode that anything, even the energy from a single photon is sufficient to cause it to break down. This is called the breakdown voltage. If a photon were to hit this diode, it would cause a huge surge of electricity to go through the diode and therefore the entire curcuit, one that could be measured by the scientist.