Hey Folks, it is been a while since I started my adventure in quantum computing at the Irish Center for High-End Computing and so far it is mind-blowing. Together with my colleague Sara, we started implementing an algorithm to compare strings using a quantum computing approach. So how does a quantum computer even work?

### The basics

A classical computer performs operations using classical bits, which can be either 0 or 1. With many bits, we can create binary code that can represent texts, computer operations or any other kind of data and can be processed by a classical computer.

A quantum computer uses quantum bits or short qubits. There are a number of physical objects that can be used as qubits, for example, a photon or electron. The object needs to be small enough for quantum physics to apply and it needs a spin that can be influenced with a pulse from the outside. A qubit can be both in state 𝛼|0> and in state 𝛽|1> at the same time before measurement. The coefficients (𝛼, 𝛽) represent the state’s amplitude and the absolute values of them squared are the probability of measuring those states.

So far so good but how does this enable us to perform interesting computations? For this, we have to take a look at multiple qubits interacting with each other. For example, if two qubits interact with each other we can create an entangled quantum state and put it in superposition, namely the state of 𝛼|00> + 𝛽|01> + γ|10> + 𝛿|11>.

The main difference between classical bits and quantum bits is the amount of information saved. While we could create the states 00, 01, 10 and 11 with two classical bits we still only have two bits of information, because which of the four numbers we have is determined by the value of the first and the second bit. In quantum computing, the state of the two qubits is determined via the four coefficients. You could say that two qubits hold the information worth of four classical bits. Saying this in a more general way we have classical information of two to the power of the number of qubits stored in our quantum states. Sadly we can not just measure a superposition, all we can measure are our qubits in their basis states |0> and |1>. So we have to manipulate our superposition in a way that the final result can be represented with a unique state expressed through the basis states of our qubits to be able to measure.

### The actual project

In our project, we encode non-binary strings in quantum states and compare them with each other to determine the degree of similarity. This kind of application is widely used in bioinformatics to match gen or protein patterns. More about the actual implementation and the string comparison method will soon be posted on Sara’s blog. So stay tuned and stay safe!

Master's degree student in chemistry at the University of Vienna. Interested in HPC, Dynamics and Drug Design.

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