BB84 Protocol - RPIQuantumComputing/QuantumCircuits GitHub Wiki

Background: BB84 is a quantum key distribution protocol proposed by Charles Bennett and Gilles Brassard in 1984. It enables two parties to establish a shared encryption key over a potentially insecure communication channel, even in the presence of an eavesdropper.

The protocol can be implemented by encoding bits into single photons or encoding bits into photon polarization. The general idea for encoding bits into single photons is to encode each bit of a secret key into the phase of a single photon. Since the polarization state of a single photon can't be measured without destroying the photon, this information is considered "fragile," and interactions from an eavesdropper will expose them.

For instance, an eavesdropper attempting to listen to the communication between Alice and Bob must intercept the photons sent between them. To gain information from the photons, the eavesdropper must interact with them, which will change the states of the photons. If the eavesdropper wants to conceal their actions, they must re-encode the photon and send it back to Bob. However, the eavesdropper will inevitably choose the wrong basis for re-encoding the photon, resulting in a photon with the wrong polarization state, thus revealing their presence to Alice and Bob.

How the protocol works: Suppose two individuals, Alice and Bob, wish to securely share information. Alice generates a random string of n bits that she wants to send to Bob securely. She also randomly chooses a set of bases for each bit. For each bit, Alice encodes it in one of the two bases she chose, creating a sequence of qubits using either the |0⟩ and |1⟩ states or the superposition states |+⟩ = $\frac{1}{\sqrt{2}}(|0⟩ + |1⟩$ and |-⟩ = $\frac{1}{\sqrt{2}}(|0⟩ - |1⟩$.

Alice then sends these qubits to Bob through the communication channel. Upon receiving the qubits, Bob randomly chooses a basis for each qubit to measure it. After the measurement, Alice and Bob publicly communicate which bases they used for each qubit, but not the values themselves. Alice and Bob discard the bits for which they used different bases, and the remaining bits form their shared secret key. To detect eavesdropping, Alice and Bob compare a subset of their key bits (for instance, 10% of their key). If the error rate is too high, it indicates the presence of an eavesdropper, and they discard the keys and repeat the protocol.

Image below illustrates the transmission of an 8-bit key between Alice and Bob:

The security of the BB84 protocol relies on the principles of quantum mechanics, particularly the properties of single photons and the no-cloning theorem, which states that it is impossible to create an exact copy of an unknown quantum state.

Applications: The BB84 protocol has applications in various fields where secure communication is crucial. Apart from its original intent of secure communication in telecommunications, BB84 finds applications in fields such as:

Quantum Cryptography: BB84 forms the basis for quantum cryptography protocols, providing a secure means of exchanging cryptographic keys.
Quantum Networks: BB84 facilitates the establishment of secure communication channels in quantum networks, enabling secure transmission of data between quantum nodes.
Quantum Computing: BB84 is relevant in the field of quantum computing, where secure communication between quantum computers or between a classical computer and a quantum computer is essential.
Government and Military Communications: Governments and military organizations employ BB84 for secure communication to protect sensitive information from eavesdropping and interception.
Finance and Banking: BB84 is used in financial transactions and banking systems to ensure the confidentiality and integrity of transactions and customer data.

Overall, the BB84 protocol serves as a cornerstone for secure communication in the quantum era, offering a robust solution to the challenges posed by eavesdropping and interception in traditional communication systems.