Quantum Mechanics

• small particles, small energy, in mini Kelvins

• light beam - reflect 1/2

• never 1/2 of photon - flips a coin and goes either way

Beam Splitter

• probability is not what we actual see in real world with laser
• 2nd beam splitter undos the action of the first one
• magnitude of complex number is gonna describe probability
• real part = amplitude (can be negative and can cancel out)

BB84

• a = value of the quantum bit
• b = basis of the quantum bit, bit string, sequence of bits
  • choice of the basis
• start with 1000 bits
• key is 500 bits
• 250 of the key is to estimate the error
• → discover if someone has your key
  • if someone is listening

Quantum bit

• 2D vector of complex numbers