Quantum computing

A quantum computer is a (real or theoretical) computer that uses quantum mechanical phenomena in an essential way: a quantum computer exploits superposed and entangled states and the (non-deterministic) outcomes of quantum measurements as features of its computation. Ordinary ("classical") computers operate, by contrast, using deterministic rules. Any classical computer can, in principle, be replicated using a (classical) mechanical device such as a Turing machine, with at most a constant-factor slowdown in time—unlike quantum computers, which are believed to require exponentially more resources to simulate classically. It is widely believed that a scalable quantum computer could perform some calculations exponentially faster than any classical computer. Theoretically, a large-scale quantum computer could break some widely used encryption schemes and aid physicists in performing physical simulations. However, current hardware implementations of quantum computation are largely experimental and only suitable for specialized tasks.

The basic unit of information in quantum computing, the qubit (or "quantum bit"), serves the same function as the bit in ordinary or "classical" computing.^[1] However, unlike a classical bit, which can be in one of two states (a binary), a qubit can exist in a superposition of its two "basis" states, a state that is in an abstract sense "between" the two basis states. When measuring a qubit, the result is a probabilistic output of a classical bit. If a quantum computer manipulates the qubit in a particular way, wave interference effects can amplify the desired measurement results. The design of quantum algorithms involves creating procedures that allow a quantum computer to perform calculations efficiently and quickly.

Quantum computers are not yet practical for real-world applications. Physically engineering high-quality qubits has proven to be challenging. If a physical qubit is not sufficiently isolated from its environment, it suffers from quantum decoherence, introducing noise into calculations. National governments have invested heavily in experimental research aimed at developing scalable qubits with longer coherence times and lower error rates. Example implementations include superconductors (which isolate an electrical current by eliminating electrical resistance) and ion traps (which confine a single atomic particle using electromagnetic fields). Researchers have demonstrated that certain quantum devices can outperform classical computers on narrowly defined tasks, a milestone referred to as quantum advantage (previously quantum supremacy), though these tasks are not necessarily useful for real-world applications.