EMZETT.Quantum Computers
In short: A computer that, instead of classic bits, uses so-called qubits, which, thanks to quantum-mechanical effects (superposition, entanglement), can take on several states at once.
In more detail: For certain problem classes (e.g. factoring large numbers, simulating molecules), quantum computers promise massive speed advantages over classical computers — but they’re not suited for everyday tasks and are currently still highly error-prone (decoherence). Relevant for IT security: sufficiently powerful quantum computers could break today’s asymmetric encryption schemes, which is why work is underway on “post-quantum-secure” cryptography.
In Depth
A classic bit is always unambiguously 0 or 1. A qubit, thanks to superposition, can instead be in a superposition of both states at once until it’s measured — only the measurement “decides” the concrete value. Combining several qubits via entanglement (a quantum-mechanical effect where the state of one qubit is inseparably linked to that of another, no matter how far apart they are spatially) allows exponentially many state combinations to be represented simultaneously for certain calculations — the theoretical origin of the possible speed advantage.
However, this advantage explicitly doesn’t apply to arbitrary calculations, only to problem classes for which special quantum algorithms exist — such as Shor’s algorithm for factoring large numbers into primes (the basis of many asymmetric encryption schemes) or Grover’s algorithm for accelerated searching through unsorted data. For everyday tasks like word processing or web browsing, a quantum computer brings no advantage whatsoever. Qubits are also extremely susceptible to interference (decoherence) — even minimal temperature fluctuations or electromagnetic interference can destroy the sensitive quantum state, which is why current quantum computers usually have to be cooled close to absolute zero and elaborately shielded against environmental influences. Particularly relevant for IT security: should a sufficiently powerful, error-corrected quantum computer ever be built, it could break today’s asymmetric encryption (e.g. RSA) in a practical amount of time — research into “post-quantum-secure” cryptography is therefore already underway today, to be able to switch to resistant schemes in time.
See also: Asymmetric Encryption