Week 11 - my notes
Table of Contents
Week 11 - Quantum Key Distribuition: Part 2
Recap
The Key
- It is a series of classical bits
- It is used to encode and decode the party messages
- in QKD, the key is developed together
- QKD helps us construct and send a truly secure key
- Quantum measurement guarantees that we know if there was an eavesdropper listening
QKD protocol
Phase 1: Sending
- Sender randomly picks some classical bits
- Sender randomly chooses a basis (Z or X) to each bit.
- Sender encondes each classical bit in a qubit using the rules the party agreed on.
- Sender sends the qubits using a quantum channel
Phase 2: Receiving
- Receiver ramdonly picks bases
- Receiver measures the qubits with his/her chosen bases to get new quantum states
- Receiver decodes the states of his measured qubits into bits
Phase 3: Comparing
- Sender and receiver compare bases using the classical channel and eliminate the ones that their bases didn’t match up.
- Sender and receiver compare bits over the classical channel
Importance of second step in third phase
- If their bits are not the same, it means someone eveasdropped
- Remember, when we measure quantum states, they can change
QKD Part 2: Eve returns (imp smiling)
- Phase will be slightly different:
- Someone will intecerpt the qubits before Bob can measure them.
- When agreeing upon the bases and the table, we gotta consider Eve knowing them too
Eve’s work
- He intercepts the qubits sent
- Then, he chooses bases (at random) to figure out the bits (he is measuring)
- Of course, as Eve used some different basis than the sender used, his measurements contain differences from the previous ones.
- Even, to remain undetected, needs to send off those results to the receiver
Receiver’s work
- They choose random bases to figure out the bits
- They decode qubits back to bits using his random bases and the table previously agreed on
Comparison
- Sender and receiver compare their bases and cross off the ones tha don’t match
- Afterwards, they check to see if their bits are the same (and it is not!)
- After knowing there is an eavesdropper on the line, they need to go and find a more secure method of communicating
Other QKD protocols
- The QKD protocol we’ve been learning is AKA BB84 procotol. It was invented by Bennet and Brassard in 1984
B92 protocol
- Sender uses two states to enconde - \(|0\rangle\) \(|+\rangle\) - Notice that these two states are still in different bases (Z and X).
E91 protocol
- This protocol uses entenglement - Sender and receiver have one qubit each, and the two qubits are entangled. The key is generated using these shared, entangled qubits
QKD in the real world
- Many quantum algorithms & protocols are not yet usable, because they assume perfect, fault-tolerant hardware
- These are called long-term algorithms
- However, we can start implementing QKD right now!
Differences
- QKD protocols have multiple components, such as devices, medium, distance between senders, cost of implementation
- Therefore, different QKD protocols are used for different use cases.