- What is quantum computation?
- A fundamentally new mode of information processing that can be performed only by harnessing physical phenomena unique to quantum mechanics.
- What is quantum mechanics?
- The deepest theory of physics; the framework within which all other current theories, except the general theory of relativity, are formulated. Some of its features are:
- Quantisation: which means that observable quantities do not vary continuously but come in discrete chunks or 'quanta'.
- Interference: which means that the outcome of a quantum process in general depends on all the possible histories of that process. For some important tasks, quantum computers can be made substantially more powerful than classical computers by using algorithms which exploit quantum interference.
- Entanglement: two spatially separated and non-interacting quantum systems that have interacted in the past may still have some locally inaccessible information in common – information which cannot be accessed in any experiment performed on either of them alone. Quantum teleportation and some important quantum cryptography protocols exploit entanglement.
- Uncertainty: knowing or measuring the value of one quantum observable (for instance, the position of a particle) implies an intrinsic uncertainty about the values of complementary observables (for instance, the momentum). This means in particular that obtaining some information about an unknown quantum system generally causes a disturbance to the quantum state of that system. The security of quantum cryptography relies on this tradeoff
- What's all this about parallel universes?
- If you only want to predict what quantum computers will be able to do,
you only need the equations of quantum mechanics. But if you want to
explain how they will do it, you need to understand that, if the
mathematics of quantum theory accurately reflect physical reality, the
computer you can see you can see and touch is only one tiny facet of a
far larger object, which is just as real even though its existence is
only detectable indirectly, through the computational work it does for
us. The best way to describe the structure of a quantum computer is
not at present clear, but in some respects it is like many computers
similar to the one we see, performing different but correlated
computations which affect each other through quantum interference.
In quantum computers, the effects of quantum interference are writ large. But the theory says that the whole of reality behaves in the same way. So the whole universe that we see around us is full of a subtle inner structure. This all-pervasive property is felt by some physicists to be best described as a multiverse, which contains many universes like ours, interacting only through quantum interference.
- What will quantum computers be good at?
- These are the most important applications currently known:
- Cryptography: perfectly secure communication.
- Searching, especially algorithmic searching (Grover's algorithm).
- Factorising large numbers very rapidly (Shor's algorithm).
- Simulating quantum-mechanical systems efficiently.
- At which institutions can I study quantum computing?
- Where can I find out more about quantum computation and related matters?
- Start at our Home Page or at our Introduction and Tutorials page.
Quantum Computation and Quantum Information by Michael Nielsen and Isaac Chuang. Cambridge University Press 2000.
The essential introductory textbook to the subject.
Minds, Machines and the Multiverse: The Quest for the Quantum Computer by Julian Brown. Simon and Schuster 2000.
A good non-technical overview of the current state of the field and its meteoric history. The whole of chapter one is freely available online from here.
Quantum Computing by Josef Gruska. McGraw Hill 1999.
A good technical introduction to the field.
See also The Fabric of Reality by David Deutsch.
For more detailed/technical information you can look at our literature list.
Last modified by David Deutsch
17 March 2001
17 March 2001