Charles Bennett and Gilles Brassard came up with the BB84 protocol in 1984. It is one of the most important quantum key distribution (QKD) protocols. By sending quantum information stored in photons, it is a prepare-and-measure procedure that Alice and Bob can use to create a shared secret key. The safety of BB84 depends on basic ideas in quantum physics, especially the no-cloning theorem and the fact that measuring something changes a quantum state.
How Does the BB84 Protocol Work?
Step 1: Prepare and send photons (Alice):
Alice wants to send Bob a series of bits that will finally come together to make their shared secret key. She randomly selects one of two non-orthogonal bases to encode each bit she intends to send. In BB84, the two bases that are used most often are
- Rectilinear basis (+): Consists of horizontal (↔, representing a bit value of 0) and vertical (
, representing a bit value of 1) polarization states. These are often denoted as |0⟩ and |1⟩, respectively. - Diagonal basis (×): Consists of +45° (
, representing a bit value of 0) and -45° (
, representing a bit value of 1) polarization states.
- She also randomly selects a 0 or a 1 to encode each bit.
- So, Alice picks two random numbers for each bit: (1) the base (+ or ×) and (2) the bit value (0 or 1). Once she has a single photon with the right polarization, she sends it to Bob through a quantum path, which could be an optical wire or empty space.
Step 2: Measurement and reception of photons (Bob):
- Bob picks one of the same two bases (+ or ×) at random for each photon he receives in order to measure its polarization. He doesn’t know which base Alice used to get the photon ready.
- Bob writes down the result of the test and the method he used for each photon he got.
Step 3: Basis Reconciliation (Classical Communication):
- Alice and Bob talk over a public classical route, like the internet or a phone line, after Alice has sent a long enough string of photons.
- For each photon sent and received, Bob says what base he used to figure out the value. At this point, though, he doesn’t say what the test result was (the bit number he got).
- She then tells Bob, for each photon, whether he picked the right base (the one she used to make it) or the wrong one.
Step 4: Key Sifting:
Alice and Bob throw away all the photons for which Bob picked a different measurement base than the one Alice used to get ready. If they both used the same basis, the results of their measurements should be exactly linked. For example, if Alice sent a photon with horizontal polarization and Bob measured in the rectilinear basis, he should see horizontal polarization. The bits that are left make up a sorted key, which could be a shared secret.
Step 5: Error Checking and Eavesdropping Detection:
- As a way to see if Eve is listening in, Alice and Bob pick a random group of the sorted key bits and compare their values over the public classical channel.
- All of these bits should match if there was no listening going on and the broadcast was perfect. But because the quantum channel isn’t perfect and people might try to listen in, there might be some differences (errors), which are measured by the Quantum Bit Error Rate (QBER).
- Eve would have to guess what Alice used as a base if she had tried to catch the photons and measure their polarization. The uncertainty principle says that her test would change the state of the photon if she is wrong about what it is. There is a chance that Bob will get the wrong answer when he measures this upset photon again in the right base. This will cause mistakes in the shared key.
- When Alice and Bob look at the QBER, they can guess if someone was listening in and how much information she might have gotten. This means that they can be pretty sure that the key is safe if the mistake rate is below a certain level. They throw away the key and start over if the mistake rate is too high.
Step 6: Privacy Amplification (Optional but Common):
It’s possible that Eve could have learned a little something even if the QBER is below the cutoff. Privacy amplification is a common post-processing method that Alice and Bob can use on their sorted key to get a shorter, completely secret key that Eve doesn’t know much about. To do this, a highly universal hash function must be applied to the sorted key.
Step 7: Set up the secret key:
After these steps, Alice and Bob will have a shared secret key that no one else, not even Eve, should know. Classical symmetric encryption methods, like the one-time pad (OTP), can then be used to encrypt texts with this key. If you use an OTP with a secret key that is truly random and the same length as the message, it should be impossible to crack.
What is the Security of BB84 Protocol?
- The BB84 protocol is safe because if Eve tries to get information about the sent qubits, she will always cause a disturbance that Alice and Bob can see because of the higher error rate. Eve has to choose between information and disturbance: the more information she tries to get, the more likely it is that she will make errors that can be seen.
- BB84 is a quantum key sharing scheme, which is something you should keep in mind. The goal of this process is to create a secret key that can be used with standard encryption methods to send data safely. There is no data transfer built into the BB84 algorithm itself.
- Changes and changes have been suggested for the BB84 protocol to make it work better and be more resistant to noise and channel losses. Although the basic protocol assumes perfect conditions, photon sources, detectors, and the communication route must all be fixed in order for it to work in real life. Even with these problems, BB84 is still an important part of quantum cryptography and a key step toward safe communication in the quantum future.
Read More about What is Quantum Key Distribution (QKD)? How It Works
Read More about What is quantum cryptography? How It Works