Quantum communication is a process that utilizes the laws of quantum physics to protect data. It works by sending encrypted data as classical bits over networks, while the keys to decrypt the information are encoded and transmitted in a quantum state using qubits. By comparing measurements of the state of a fraction of these qubits—a process known as “key sifting”—Alice and Bob can establish that they hold the same key. As the qubits travel to their destination, the fragile quantum state of some of them will collapse because of decoherence. To account for this, Alice and Bob next run through a process known as “key distillation,” which involves calculating whether the error rate is high enough to suggest that a hacker has tried to intercept the key. If it is, they ditch the suspect key and keep generating new ones until they are confident that they share a secure key. Alice can then use hers to encrypt data and send it in classical bits to Bob, who uses his key to decode the information.

Key sifting is a process in quantum communication in which Alice and Bob compare measurements of the state of a fraction of the qubits they have sent, in order to establish that they are both holding the same key. For example, if Alice sends Bob a set of five qubits and measures the state of the first two, and Bob measures the state of the remaining three, they can compare the results of their measurements and determine whether the states match. If they don’t, it could mean that a hacker has intercepted the key, and the two parties would have to start the process over. If the results match, it means that Alice and Bob both have the same key, and they can then use it to encrypt and decrypt data.