Quantum key distribution (QKD) is a method of secure communication that utilizes principles from quantum mechanics to establish a shared secret key between two parties, typically referred to as Alice and Bob, while detecting any potential eavesdropping attempts by a third party, commonly known as Eve.
The fundamental principle behind QKD is the use of quantum properties, such as the superposition principle and the no-cloning theorem, to enable the distribution of cryptographic keys in a manner that is theoretically immune to interception or decryption by an adversary. The most widely used protocol in QKD is the BB84 protocol, proposed by Charles Bennett and Gilles Brassard in 1984.
In the BB84 protocol, Alice prepares a sequence of quantum bits (qubits), which can be in one of two possible states (e.g., polarization states of photons) randomly chosen from a set of four states (basis states). Alice then sends these qubits to Bob through a quantum channel, typically a fiber optic cable or free space transmission. Bob randomly selects a measurement basis for each received qubit and measures it accordingly.
After the transmission of qubits, Alice and Bob publicly disclose the basis they used for each qubit but not the specific values. They then compare a subset of their measurement bases to verify the integrity of the transmission and discard the corresponding qubits if they used different bases. This step, known as basis reconciliation, helps to detect any potential eavesdropping attempts by Eve, as her interception and retransmission of qubits would likely introduce errors in the measurement bases.
Once the integrity of the transmission is verified, Alice and Bob use the remaining qubits to generate a shared secret key by applying a procedure called information reconciliation. This process involves comparing subsets of their measurement results to identify and correct any discrepancies caused by noise or eavesdropping.
Finally, Alice and Bob perform privacy amplification to distill a shorter but secure key from their initial shared key, further reducing the information potentially accessible to Eve.
Quantum key distribution offers the advantage of providing unconditional security based on the fundamental principles of quantum mechanics, allowing for the distribution of cryptographic keys resistant to interception or decryption by classical or quantum adversaries. QKD has applications in secure communication, particularly in scenarios where the confidentiality of transmitted information is of utmost importance, such as in government communications, financial transactions, and data encryption.