##### By Natasha Poon

With the rise of quantum computers (powerful computers that use qubits, the basic unit of information in a quantum computer, rather than bits, which are used in regular computers), regular forms of cryptography such as RSA will be easy to decrypt. Governments are planning to converti encrypted RSA files to Post-Quantum cryptography, a form of encryption that is difficult to decrypt, by both classical and quantum computers.

#### Qubit States

In cryptography, characters of a message are encoded within qubits, and decoded by measuring the state of the qubit. If you measure the spin in one direction while the spin is in a perpendicular direction, this results in a 50/50 probability of changing the qubit’s state from one of two options to the other: up/down or left/right.

#### Background Information

One form of Post-Quantum cryptography is Quantum Key Distribution. Before understanding this, we first need to know a couple of key concepts of Quantum Computing! There are four different directions or states that a qubit can be in: 0 & 1, or + & -. To find out what direction the qubit is in, one must measure it in the corresponding axis of the qubit: either x-axis or z-axis. However, if one measures a qubit in the wrong direction that the qubit is in, the qubit will be in a quantum state of superposition – the ability of a qubit to be in two states simultaneously.

#### Situation

Let’s say Alice is sending a message to Bob using Quantum Key Distribution. But, an eavesdropper named Eve is intercepting the qubits in order to also receive the message! Alice would send qubits to Bob and tell Bob which axis (x or z) to measure the qubits in. If Eve intercepts the qubit and measures it in the wrong direction (x-axis instead of z-axis), she has a 50/50 chance of changing the spin of the particle. However, she has no way of knowing when she changes the spin. Since Alice and Bob know the correct axis to measure the qubits on, they will have different results. This lets Bob and Alice know that someone is eavesdropping on their conversation.

Eight bits represent characters in traditional computers. A single eight bit is represented by one qubit in quantum cryptography. Since there are so many characters to translate in a message, if Eve gets the spin correct for just one of the qubits, there is a high probability that she will get many of the other qubits wrong. This informs the senders of Eve’s interception.

#### Importance

I have been learning about quantum physics through UBCs Quantum Computing Masterclass. I think that quantum computers will make the world’s technology so much more powerful and advanced. However, there is still much more to research. Quantum computers will easily decode classical computer messages. So, I think it is a priority to research different methods of post-quantum cryptography. If the technology of quantum computers becomes too advanced for current methods of cryptography, all encrypted information is available to companies with a quantum computer.

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