Text 1. Quantum cryptography. Find in the text the description of the phenomena of quantum mechanics.

Quantum cryptography is another kind of cryptography in this world. With it, you can create a communication channel where it is impossible to eavesdrop without disturbing the transmission. The laws of physics secure this quantum channel: even if the eavesdropper can do whatever he wants, even if the eavesdropper has unlimited computing power. Charles Bennett, Gilles Brassard, Claude Crepeau and others have expanded on this idea, describing quantum key distribution.

According to quantum mechanics, particles don't actually exist in any single place. They exist in several places at once, with probabilities of being in different places if someone looks. However, it isn't until a scientist comes along and measures the particle that it "collapses" into a single location. But you can't measure every aspect (for example, position and speed) of a particle at the same time. If you measure one of those two quantities, the very act of measuring it destroys any possibility of measuring the other quantity. The quantum world has a fundamental uncertainty and there's no way to avoid it.

That uncertainty can be used to generate a secret key. As they travel, photons vibrate in some direction; up and down, left to right or more likely at some angle. Normal sunlight is unpolarized; the photons vibrate every which way. When a large group of photons vibrate in the same direction they are polarized. Polarization filters allow only photons that are polarized in a certain direction through; the rest are blocked. For example, a horizontal polarization filter only allows horizontally polarized photons through. Turn that filter 90 degrees, and only vertically polarized photons can come through.

Let's say you have a pulse of horizontally polarized photons. If they try to pass through a horizontally polarized filter, they all get through. Slowly turn that filter 90 degrees; the number of photons getting through gets smaller and smaller, until none gets through. You'd think that turning the filter just a little will block all the photons, since the photons are horizontally polarized. But in quantum mechanics, each particle has a probability of suddenly switching its polarization to match the filter. If the angle is a little bit off, it has a high probability. If the angle is 90 degrees off, it has zero probability. And if the angle is 45 degrees off, it has a 50 percent probability of passing through the filter.

Answer the questions

What laws of physics is quantum cryptography based on?

Why can't you measure every aspect of a particle at the same time?

How do polarization filters work?

How can the laws of quantum mechanics be used in cryptography?

Does quantum cryptography have practical application? What are the spheres?

Mark the following statements true or false. Correct the false statements.

1. The communication channel can be protected from the eavesdropper by the laws of physics.

2. No eavesdropper can intercept the transmission protected by quantum cryptography.

3. All characteristics of particles as studied by quantum mechanics can be easily measured.

4. Polarization filters let polarized photons pass through the communication channel in any direction.

5. If we turn the vertically polarized filter 90 degrees the horizontally polarized photons won’t get through.

6. You can learn the polarization only if you measure it in the basis it is polarized.

7. If we turn the horizontally polarized filter 90 degrees the vertically polarized photons will get through.

8. In quantum mechanics particles can change the polarization to get through the filter.

9. Quantum cryptography is still theoretical.

10. According to quantum mechanics particles can adapt to the polarization of the filter.

Vocabulary tasks