**Difficulty**

*by Filip Rozpedek *

## Entanglement, is it always pure?

You have probably already heard about entanglement. Entanglement is this fascinating phenomenon, in which two distant objects can manifest correlations, even if they are far far away from each other. You may have also heard that remote entanglement is a necessary ingredient for many quantum information processing tasks. For example, in quantum cryptography, two people who hold entangled particles can use those correlations to obtain shared secret keys, whose security is guaranteed by the laws of quantum mechanics. Today, we will not discuss how to use remote entanglement, but rather, what to do if our entanglement is too weak.

Unfortunately, fully entangled states which are perfectly correlated are a great idealization and from an experimental perspective almost impossible to create. In general, there can be many reasons for this, e.g. our experimental equipment isn’t perfect or we cannot maintain our quantum system long enough. All those things combined lead to various forms of contamination of the entanglement. That is, the correlations become weaker and completely diluted in a mixture of various other quantum states.

So what do we do with those so-called “partially entangled states”? Let us say that two parties working at QuTech, whom we call Alice and Bob, share those partially entangled states and would like to use them to generate shared secret keys. Let us also say that their experimental setup allows them to produce partially entangled states very fast, but the amount of entanglement in each of them is insufficient to generate shared secret keys. It is known from Quantum theory that it is not possible to increase the amount of entanglement in a given quantum state by only performing operations on the entangled particles locally and exchanging classical messages. It seems that there is no choice for Alice and Bob, but to go home without a key.