Data is transformed into bits of 0s and 1s for transmission using polarised photons in Quantum Cryptography. The sender places photons into a specific quantum state, which the recipient then observes. Four different polarizations—90, 0, 45, and -45 degrees—and three different bases—rectilinear (horizontal or vertical), circular (left-circular or right-circular), and diagonal—can be used to detect a photon. Here, just the circular and rectilinear bases are employed. The receiver can tell the difference between polarisations at 0 and 90 degrees and at -45 and 45 degrees. Both observations cannot be made since, according to physical laws, a measurement kills the photon.
Therefore, in addition to receiving the signals, the receiver must also measure them in relation to the proper basis. Because if intercepted, a third party would be able to learn the secret key, this information cannot be shared between the sender and the receiver. Instead, the receiver picks one of the two options at random: either it can distinguish between polarisations of 0 and 90 or -45 and 45 degrees for each signal. The sender is then contacted by the receiver to discuss the type of measurement that was carried out on each bit in public. The sender verifies which kinds were accurate, and both the sender and the receiver discard the incorrectly measured bits.
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