A beam splitter, as used in quantum key distribution (QKD), is a mechanism that divides photons and polarizes them. Beam splitters can divide beams of light linearly or diagonally. In quantum cryptography, this polarization process allows the photons (known as bits in cryptography) to appear in two different states to the interacting parties. The two parties (Alice and Bob) will see bits as 0 or 1.
The following example of a coin flipping protocol one of the methods of performing quantum key distribution between two parties illustrates how a beam splitter works in QKD. To begin the coin flipping protocol, a method used between two mistrustful parties, Alice uses a beam splitter to polarize photons, per quantum physics principles. The beam splitter divides the photons. The generated quantum bits, or qubits, she then sends to Bob. Bob also uses a beam splitter to polarize the photons and records the resulting bit measurements, often measured as either 0 or 1 like traditional bits. He sends those to Alice to compare results. If Alice has the same results, she will then confirm that. Alice and Bob will then use the bits that had the same polarization to create a secret encryption key. The process is largely invisible to users and handled by encryption software.
Beam splitters can also be used by an attacker to interrupt or eavesdrop on the key creation process. The attacker (Eve) can use a beam splitter to intercept photons as Alice sends them to Bob, which will not cause a disturbance in quantum mechanics as most eavesdropping in quantum cryptography does. Eve may then be able to control what she sends to Bob, interfering with their secret quantum key distribution process. Alice and Bob no longer have a secure channel in which they can create a secret key. Security issues such as beam splitting attacks and other interferences like light injection must be addressed in quantum encryption systems.