Post-quantum cryptography is the science of encryption methods intended to defend against a quantum computer. Though they are not widely used yet due to instability and challenging technical design, quantum computers are gaining traction in the computing industry, especially as Google develops its own quantum computer. Rather than standard mathematical algorithms and bits, a quantum computer uses quantum mechanics and qubits, which are more changeable and flexible than bits. Quantum computing, then, uses concepts from quantum mechanics such as light particles and entanglement. Quantum mechanics is unpredictable and unreliable, and so too can be quantum computing.
In quantum computing, qubits do not hold only a value of 0 or 1 like bits do. In quantum physics, particles may not hold one value consistently. Quantum entanglement, the interdependency of particles upon each other, and the instability of qubit value both allow a quantum computer to compute many combinations of numbers at the same time, an advancement beyond standard encryption.
Encryption keys typically use a complex series of numbers (the more bits, the more complex) to make eavesdropping or decoding more difficult. If a quantum computer attempted to decode a standard encryption key, it would be much more successful. Hackers can spend a very long time attempting to guess number combinations; utilizing the shifting nature of quantum entanglement would allow them to guess combinations at an unheard-of rate.
Although quantum computing is neither very reliable nor widespread, it could develop much more quickly depending on the advances of Google and other large tech corporations. Security experts are concerned that the development of quantum computers will threaten standard cryptography. One possible method of defending against quantum computer attacks is quantum cryptography. Though it sounds very similar to post-quantum cryptography (and is very similar), it accomplishes a slightly different goal. Quantum cryptography secures data transmission by using photons to create a secret encryption key. Any attempt to eavesdrop or interfere with the communication causes a clear disturbance, allowing users to terminate a connection or identify an attempted hack. There are also more complex cryptographic techniques that could help prevent quantum computing attacks, including creating encryption keys with much longer sequences of bits, which could make it more difficult for a quantum computer to guess a combination of numbers.