Free-Space Quantum Key Distribution

Quantum key distribution is the most advanced technology to arise from the emerging field of quantum information. In fact, a few companies (MaqiQ Technologies and id Quantique) already offer commercial quantum key distribution products. But why so much excitement over QKD when our standard techniques serve just fine? Well the thing is that our standard techniques are safe only under certain assumptions. Even worse, we already know that these assumptions are false if we have a quantum computer at our disposal. To see this, let’s take the classic example that anyone working in QKD will give you. One of the most popular and common encryption algorithms is the RSA encryption algorithm. The beauty of the RSA algorithm is that it’s extremely simple (searching for RSA on Google returns a plethora of tutorials on the algorithm) and relies on one basic fact: factoring large numbers seems to be a hard task on a classical computer. Here, the word hard is used to mean that as the size of the number grows linearly in bits, the time to factor it grows exponentially. The RSA algorithm works by encrypting data with a public key, comprised of the product of two large prime numbers. The message can then only be decrypted efficiently (on a classical computer) if the two prime factors are known. The size of the numbers are chosen so that it would take roughly twice the lifetime of the universe to factor their product on a classical computer. However, this rosy outlook all changed in 1994 when Peter Shor came up with his famous factoring algorithm [1] implementable on a quantum computer that’s able to factor large numbers in a polynomial amount of time.