A quantum computer is a computer that takes advantage of quantum mechanical phenomena to perform computations. Unlike classical computers, which encode information in binary bits that can either be 0s or 1s, quantum computers use quantum bits, or qubits, which can be in a superposition of one and zero simultaneously until its state is measured. This allows quantum computers to consider a large number of possible combinations simultaneously, which can be extremely advantageous for certain tasks where they could vastly outperform even our best supercomputers.

Quantum computers can be implemented in different ways, such as gate-based quantum computers, adiabatic quantum computers, and measurement-based quantum computers. Generating and managing qubits is a scientific and engineering challenge, and some companies, such as IBM, Google, and Rigetti Computing, use superconducting circuits cooled to temperatures colder than deep space, while others, like IonQ, trap individual atoms in electromagnetic fields on a silicon chip in ultra-high-vacuum chambers.

Quantum computers promise to power exciting advances in various fields, from materials science to pharmaceuticals research. They can revolutionize industries such as machine learning, chemistry, optimization, and molecular simulation. However, quantum computers are not expected to replace classical computers, as using a classical machine will still be the easiest and most economical solution for tackling most problems.

In summary, a quantum computer is a computer that uses quantum mechanical phenomena to perform computations, and it has the potential to solve problems that are too complex for classical computers. Quantum computers use qubits, which can be in a superposition of one and zero simultaneously until its state is measured, allowing them to consider a large number of possible combinations simultaneously. Quantum computers can revolutionize various industries, but they are not expected to replace classical computers.