What is gate-based quantum computing?
Gate-based quantum computing is a standard paradigm in which quantum algorithms are implemented through a sequence of quantum gates, each performing a controlled transformation on one or more qubits.
Classical logic gates:
Before addressing what makes quantum gates unique, let us recall that the concept of logic gates is not new. In fact, they are the fundamental building blocks of classical computing, operating on bits taking the values 0 and 1. Concretely, a logic gate is a Boolean transformation on the unit of information. It has one or more inputs and produces one or more outputs, and the exact number varies from one type of gate to another. One of the easiest logic gate is the NOT gate, which reverses the value of the input bit. Some logic gates are called « conditional » because the output value depends on the input combination. For example, the output of the AND gate is 1 if and only if both input bits are also 1. By combining different logic gates, one can construct circuits capable of performing more complex operations.
Quantum gates:
Quantum gates have a similar role but act on a more general unit of information: qubits. A notable difference between classical and quantum gates is that quantum gates are inherently reversible. Each gate corresponds to an invertible transformation that can restore the system to its initial state. Moreover, a remarkable mathematical result proves that a small set of quantum gates is sufficient to implement any quantum algorithm.
Geometric intuition:
The state of a bit can be either 0 or 1, which you can picture as the two endpoints of a line segment. The NOT logic gate acts like a switch that moves the bit from one end to the other. The state of a qubit, on the other hand, can be in any superposition of 0 and 1, for example, to be « a little » in state 0 and « a lot » in state 1. Such a state can be visualized as an arrow inside a sphere, pointing to a point on its surface, called the Bloch sphere. Quantum gates act as rotations of this arrow on the sphere, providing a helpful geometric intuition for how qubit states evolve under gate operations.