What is a quantum computer?

A quantum computer is a computing device that uses quantum mechanics to perform certain calculations much faster than traditional computers.

Basic idea:

• Traditional computers use bits (0 or 1) to store data.
• Quantum computers use qubits, which can exist in multiple states simultaneously (superposition) and can be correlated (entanglement). 

Key features:

1. Superposition: A qubit can be 0 and 1 at the same time.
2. Entanglement: Qubits can be linked so the state of one influences another, enabling complex operations.
3. Parallelism: Quantum computers can explore many possibilities at once, which gives them potential speed for some problems. 

Limitations & current state:

Quantum computers are still largely experimental. They face issues like qubit stability, error correction, and scaling up to many qubits. 

Applications:

They hold promise in fields like cryptography, material science, optimisation, and simulation of quantum systems.